·About the Author Professor Clarence E Beanett has been head of the Department of Physics of the Unisersity of Maine since 1939 Previously he was a member of the faculties of Brown University and of the Massachusetts Institute of Technology He holds the PhD degree from Brown University A fellow of the American Association for the Advancement of Science and of the American Physical Society and member of the American Association of Physics Texchers, the Optical Society of America, the American Society for Engineering Education, Phi Beta Kappa, and Sigma Xi, he bas contributed atticles to scientific journals and is the author of the College Outlines Pbysses and Pbysses Problems, and of a standard textbook, First Year College Physics

·COLLEGE OUTLINE
SERIES
PHYSICS
WITHOUT
MATHEMATICS
CLARENCE E BENNETT
BARNES & NOBLE, INC.
Publishers

·Corturit 1049
By BARVES & NOBLE NC
All ngbe reserved
Reprinted 1959
Lc Cataloe Card Number 85-9311
PRINTED N N VVITED STATES OF AMERICA

·PREFACE That the nonscience student of today is interested in the phi 81- cal world about him, and realizes the importance of scientific knowledge, need not be questioned The many orientation and survey courses offered in our colleges and universities bear witness to this fact It 18 doubtful, however, if the problem of introducing genous but nonprofessional science students to the field of physics has yet been adequately solved The author of the present tert has come to feel that the ansrer does not lie so much in eliminating most of the technical parts of the subject as it does in divorc ing the subject matter of physics as much as possible from anthmetic, for it is the numercal put of phs <1c8 at which the nonscience student shudders with the usual comment that he never could see" mathematies Furthermore, there 19 & eur prising amount of material in a college physics course which need nat be treated aumerically Experience has shown the author that a one semester "nonarithmetical' coure, without the omagjon of otherwise technical vocabulary, is not only possible but practicable for these students who will probably not go on in phy anec It has been found that a real appreciation of the subject, not to be confused with a falec sense of comprehension can be des cloped in this manner To be sure, the arithmetic may not be completely eliminated but it can be minimized almost to the vanishing point of course thye reults in a corresponding sacrifice in the tersenece and the rigor which properly characterize the general college phy erre course, but the need not be as crdone Although the language of physic$ 14, to a certain extent, the language of mathematics because the physiciet appreciates thre abbreviated and indis. pensable ymbolic form of corrpeajon it does not follow that the student who lack mathematical bent cannot appreciate a large part of that fundamental science which plays such an important part in the world in which he lives Morros er, the phuct should feel ad obligation to help his non-cintific tolleague obtai ·vi PREFICE clearer picture of the sorld of physics and should be willing to forego thc niecties of mathematical language when necraan Such a feeling has motit ated the preparation of this book In this text the order of des clopment of the topics 13 a logical one designed to emphasize the unity of the subject matter The relationships between each topic and the ones preceding it arc carcfully indicated so that the entire subject is des eloped from fundamental consideratione Technical vocabulary, however, 19 not avoided It is emphasized, to encourage correct usage of technical terms Every technical term 19 italicized and defined when it is first used Only the anthmetic 19 minimized It is the writer's opinion, based upon experience, that such a presentation can be made to interest tlie serious nonscience student without resort to the superlatives and the spectacular phases of pseudo science It is definitely expected that a course based on this text will be a lecture course and that considerable attention will be given to lecture demonstrations Phy arcs, unlike many subjecta, lends itself to the lecture type of presentation because of the almost unlimited amount of demonstration equipment and visual aid material which is available The lecturer till find innumerable opportunities to expand upon the material included in this text according to his available equipment These pages are not in. tended to replace the epohen word, but rather to serve as a guide for reading assignments and as an outline of subject matter To facilitate the administration of such a course of one semester in length, the material is subdivided into chapters each of which may be considered roughly 83 A seekly assignment Questions are listed after each chapter and general review questions are inscrted at places appropriate for hour quizzes The latter are of the "multiple choice" type, they can all be answered on the basis of this text, but they have intentionally been made difficult enough to require a genuine understanding of the points covered The student is cautioned against rapid and careless reading of this text In the absence of arthmetic, Fords hate to carry an extra burden, and they must be read carefully, even slowly, for their full meaning Time must be allowed for reflection CEB Orono Maine ·CONTENTS tion CHAPTER PAGE I INTRODUCTION Scientific Method Fundamental Conccpts, Measurements, and Basic Units 1 II HISTORICAL CONSIDERATIONS Ancient and Medieval Physics, The New Awakening in Physics, Classical Physics, Modern Physics 12 III MECHANICAL CONSIDERATIONS Forces and Motion, Vector Nature of Force, Equilibrium, Laws of Mo- 26 IV MECHANICAL CONSIDERATIONS (Continucd) Work, Energy, and Friction 42 V ELASTIC CONSIDERATIONS Clasticity Vibrations, and Fluids 54 VI TAVES AND SOUND 66 VII MATERIAL CONSIDERATIONS Constitution of Matter, Properties of Gascs, Surface Effects 79 VIII THERMAL CONSIDERATIONS The Nature of Heat, Thermometry, Expansion, Calorimetry, Change of Statc 93 I THIERVAL CONSIDERATIONS (Continued) The Nature of Heat Transfer, Quantum Theory, and Certain Other Philosophical Considerations 107 ELECTRICAL CONSIDERATIONS Static Electricity, Chargee, Potential Capists 118 VI VACSETISY 130 VII CLECTRICAL CONSIDERATIONS (Continued) Current Flectricity 11 ·111 CO TETS CHATTER TAGB 148 XIII ELECTROVIC CONSIDERATIONS AND ATOVIC PRENOS! EVA Introduction to Vodem Phyescs, Electrical Discharges in Gases, Electronica, . Raye, Radioactivity Nuclear Physics XIV OPTICAL COXSIDERATIONS Photometry, Lans of Geometric Optics, Optical Instruments V OPTICAL COWBIDERATIOıs (Continued) Physical Optics Dispersion Spectroscopy Interference, Dif fraction Polarization APPENDIX Supplementary Review Questions INDEX 160 178 191 197 ·CHAPTER I INTRODUCTION SCIENTIFIC METHOD FUNDAMEYTAL CONCEPTS, MEASUREMENTS AND BASIC UNITS To many, the science of phy 103 19 a profound, highly math matical, and exceedingly difficult subject to be undertaken 1 those gifted with special mathematical abilities and to be avoidi by all others Although it is true that physses is probably t1 most highly organized branch of science today and does requi profound mathematical anal; -9 for its complete comprehensi0 It is alea true that it is so closely related to almost every activit of every day lie that some familiarity with its laws and concen should be had by every person who lays claim to being educato Morcover, contrary to popular belel, a real insight into the natu: of the phy gral world can be obtained without the use of high! mathematical language Naturally the professional phycot unwilling of not unable, to drepense with such & useful tool concisc analytical expression as is afforded exclusncly by math matics, but the lay reader ppcepssing 4 reasonablc degree scientific curiosity should not be deceu cd into thinking that t! wonders of this fundamental science are forever denied inumi with an ability to reason logically, he is but willing to underta! the cultus ation of a technical 1 ocabulary Of course, this is not trval undertaking and will require concentration Scope of Physics A bre consideration of such matters : force, woth, motion, energy, fuid phenomena, wasty, round, hes clectnety, magnetism, radio, electrons, lexecs, prisms, optic: instruments, color, and polarized light 19, in general, the scope this studs These topics are to be considered, however, not as cenes of unrelated things, but rather as a part of a single subjet platetrs, the study of the plis cical world in which we luc A the«e topics are characteried by technical terms, and one of 11 purpose of this etudy is to deselop a proper appreciation of ·2 INTRODUCTION terms by learning their correct definitions as well as to acquire A familiarity with certain fundamental relationships which experience has shown to exist betireen them in this way only can an appreciation of physics be obtained, when account is taken of the high degree of organization that the science has achieved Definition of Terms Most of the terms used in physics are defined terms that is to say, they take on their precise technical meaning merely because of common agreement. It will therefore be necessary at the very outset to recognize that the defining of new terms is not just a preliminary but is an indispensable condition for the succces of our hole program It is perhaps not an exaggeration to say that an introduction to physic9 13 largely & study of one concept after another, each of such is carefully defined in terms of concepts more fundamental than itself Thus A structure of related concepts is developed, based upon certain ones which are considered fundamental and incapable of definition in terms of anything more basic The full significance of this statement is not easily realized, but when the student finds that all the concepts of mechanics involve only three fundamental concepts-length, mass, and time--and that all the rest can be derived from these, he begins to appreciate the unity of physica A consideration of these three fundamental concepts is to be the first detailed study in this text, but before this is undertahen, scveral brond aspects of the situation will be considered Inductive vs Deductive Reasoning The foregoing references to the logical development of this subject from fundnmental considerations to more compler concepts may possibly conve the impression that physics eremplifics purely deductive tersouing On the other hand, the very expression scientific method' implies inductive reasoning" It should be pointed out, however, that inductive reasoning characterizes the research phases of a science After & body of information has been dis covered and properly organized, it is often passed on most effectively to future generations of students by deductie processes Deductive reasoning was des cloped to a high degree by the Greek philosophers, but deductive reasoning alone was unable to advance the study of physics beyond certain limits False premises lead to wrong conclusions in spite of correct reasoning Thus Aristotle, the famous Greek philosopher, reasoned that heavy Sec chapter II for hıstancal summary of developments ja pbysics ·SCIENTIFIC METHOD 3 bodies fall faster than light ones, and this conclusion was actually accepted as true for hundreds of years simply because of the authority behind it It was not until the time of Galileo (1564- 1612) that the matter was tested by experiment and found to be fal e Galileo is often called the father of experimental physics because he was one of the first to advocate the basing of conclu- Sions upon experimental fact rather than upon deductive reason ing alone The Scientific Method The scientific method, as initiated by Galileo 19 inductive and always involves the several following distinct steps In the first place obseri ations are made Then a hypothesis is postulated in terms of which the observed facts are consistent A successful hypothesis becomes the basis of a theory which not only "explains" but suggests further observation in the form of experimentation Finally a successful theory leads to the establishment of a scientific law Thus the scientific method is based upon observation and inductive reasoning It was by such a procedure that the laws of motion of the heavenly bodies were discovered " Attention is called particularly to the role played by observation in the process Reasoning alone, no matter hon correctly it is done, 19 not sufficient to eatablish a scientific Inn or principle Observable facts are facts regardless of the methods used to explain them Thus physics is essentially an experimental science, yet the student need not follow the same path as the pioneer in order to understand what the pioncer has accomplished In this text the student will not be required to discover for hinsell Nature of Explanations. A word regarding the nature of explanation is perhaps in order at this point An explanation of any thing is always a relative matter It is the making of a thing sccm rcasonable to a person in terms of his past esperiences Dife ferent degrees of appreciation have to be recognized, and what constitutes an explanation of something to an advanced student of a subject is likely to be quite unsatisfactory, if not unintelligible, to a beginner Thus, as already pointed out, mathematics 19 e*cential for the explanation of physical phenomens for the more advanced etudent of the ecience, but to the uninitiated explanations in terms of mathematics often become un atref&c. tors To the beginner, such matters as vocabulars are more important than mathematical relationships, although some of ·4 INTRODUCTION concepts are admittedly mathematical by their very nature Fortunately for this study, honeser, there is ample matenal for qualitative, or descriptive, considerations in clementary physics to cnablc the beginner to get a fairly good idea of the naturc of the subject Although the explanations and definitions in this text appcar verbose of compared with the standards of the professional physicist, they are nevertheless adequate for our present purpose Quantitative Observations Measurement. Since the scientific method not only calls for an ability to reason but also for careful observations, it is obvious that measurement plays an important role in physics Observations must be quantitative as well as qualitative Thus the question "hot much" must always be anesercd before it is safe to base theoretical conclusions on any observation This docs not mean that the present reader must concern himself unduly with arithmetical details, but it does mean that the experimental physicist must give as much attention to his tools, which are all forms of measuring instruments, as the theoretical physicist gives to the laws and processes of mathematical analysis and logic The distinction between experimental and theoretical (or mathematical) physicist is often made, but at 19 important to note that the theoretical physicist 18 limited to the material furnished by the experimentalist Direct vs Indirect Measurements Measurements of all kinds have to be made, but a remarkable thing about physics 19 the fact that not all measurements have to be made directly in fact, many of the so-called measurements in physics, such as the determination of the size of an atom, or the measurement of the velocity of light, etc, are indirect measurements This again is the result of the high degree of organization of the subject The lans of physics and their relationships require that certain things must be so if certain other things are true Thus two simple measurements of length yield a value of the area of a rectangle Şimlarly, a measurement of length and one of time suffice to determine speed, to an accuracy comparable with that of the two measurements Furthermore, a determination of length, one of time, and one of mass enable the physicist to measure the kinetic energy of a body, or its momentum, or any one of many quantities to be defined later In brief, no more than these three kinds of direct measurement are required to determine any mechanical characteristic of a physical body This is merely another way of stating our ·MEASURF VEYT 5 earlier assertion, viz , that all the concepts of mechanics are derived concepts expressible in terms of three fundamental ones length, mass, and time Thus se see that the direct measurements in physics, at least in the field of mechanics, can be restricted to measurements of these three fundamental things Nature of Measurements If we stop to consider, we realize that a measurement, such as a measurement of length, for example, 18 nothing but a comparison with a standard, which itself 19 nothing but a unit determined by common agreement Thus & table is said to be three feet long merely because its length coincides exactly with that of a standard yardstick held along at Similarly, the length of & rug or 8 room, may be determined with respect to a yardstich, a foot rule, a measuring tape, or for that matter any convenient unit of length Therefore length is some thing which can be measured in terms of unita, but which otherwie has no significance except as it constitutes a basic concept for the development of other concepts In other words, a basic concept such as length cannot be defined in terms of anything simpler, but a measurement of it can be made in terms of a standardized unit Similarly, mass and time are quantities which are measured by comparison with standard units of these quantities It should therefore be quite clear that the first task in the study of phy«ics 19 to understand the manner of specification of the units in terms of which length, mas, and time are measured, for everything else seems to depend upon these units Incidentally, it should be noted that it is only by custom that these three particular concepts are taken as basic for the development of the entire subject Theoretically any three concepts could have been used, but these three gre the most natural, and probably the simplest, development Without further delay we shall proceed to the specification of the generally accepted units of length, mase, and time, with the understanding that all the concepts of physics are to be expressible in terms of units, each of which will be related in some way to the units of these three quantities Complicating the situation somewhat, ne find that agreement is not universal in these matters and that at least two independent systems of units must be considered, the so-called metric stem and the English system of units The English standard of length as the yard, dis- ·INTRODL CTION tance between two scratches on a certain bar carefully kept in London This length is subdivisible into three feet or turty-six inches The worhing unit in this system is taken as the foot The metric standard of length is the international meter, defined as the distance bets een two scratches on a certain platinum iridium bar preserved near Paris, France Although this unit was originally intended to represent a certain fraction of the earth's circumference continued mca-urements of the latter quantity hanc disclosed errors in the original determination but this does not in any way affect the agrecment by thich this particular length 19 accepted as the standard Secondary standards, which are rep. licas of the original, are kept in every country The one for the country is preserved at the National Bureau of Standards in Washington, DC Incidentally the American standard of length is the American yard, which although approximately equal in length to the Enghsh yard, 13 legally defined as 3600/3937 of the international meter One one hundredth part of the mcter is the centimeter, which has become the working standard of length in scientific work Thus the metric and English units of length are connected by the approximate relationship that one inch cquals 254 centimeters Thıs 19 worth remembering It is also worth noting that international commissions are constantly at work maintaining and improving the legal status of units for purpoves of trade Indeed, a large portion of the program of the National Burcau of Standards is des oted to just such matters Units of Mass To understand next how the unit of mass is defined, one must first be sure that the concept of mass is clear It is not such a familiar concept as length Mass is defined as a measure of inertia, where inertia 15, by definition simply a certain property of matter, by virtue of which a body tends to resist any change in its motion The state of rest is considered a special case of motion, namely zero motion It will be pointed out later that * force is necessary to change the motion of a body, whence it fol lows that inertia is a property involving force and motion For the purpose of this discussion, however, a force is to be thought of simply as a push or a pull tending to change the motion of a body Weight Not the Same as Mass One of the commonest forces to act upon a body is the pull of gravity, that is to say, the attraction of the earth All bodies on the earth are under such an iniu ·WEIGHT AND NASS ence at all times The pull of gravity on a body is technically called its weight See fig 11 Thue a man is said to weigh 150 pounds because the earth attracts him with a force of 150 pounds This illustration also calls attention to the fact that the common and familiar unit of force is the pound In the metric system the unit of force is the dyne, roughly equivalent to the weight of a thousandth part of a postage stamp The pound is sometimes used as a unit of mass, a concept quite different from weight, and confusion arises from the use of the same unit to cxpress two different concepts Such difficulty can be avoided only by painstak Fig 11 The weight ing care in the use of technical language of a body can be de- See pp 37-38 for a more complete discussion termined by a spring balance of this matter It is obvious that a body without mass would be weightless (indeed it would not exist), yet mass and weight are different concepts Maes, hon ever, 19 considered by the physicist to be the more basic concept of the two, and the unit of mass 18 arbitrarily defined A certain piece of platinum, having rust thus characterustu by definition, is preserved in London along with the English standard yard, and its mass is defined as a pound mass See fig 12 The United States pound is legally defined as a certain fraction of the mass of the international kilogram (sce below) The standard pound of weight is now defined as the pull of gravity at a specific place (sea level and latitude 45°) on this one-pound 1 lb. mac It is a fact that the pull of gravity on a body (its seight) varies slightly from point to point on the earth's surface, be- Fig 12 The standard pound ing greater at the poles than at masa 1 preuered under a glass the equator because at the poles cover sa London it is nearest to the center of the carth But at sea level and in latitude 45° (as London) the tandard art of mass and the standard unit or force are thus simultaneouely defined in terms of a specific picce of platinum In a similar manner, another particular piece of platjaum pre- ·8 INTRODUCTION seri cd ncar Paris is said to have a mass of one kilogram One onethousandth of this mass is called a gram, & unit shich is generally accepted as the working unit of mass in the metric syatem, although the kilogram is the fundamental standard The metric system is a decimal system and has been accepted almost universally in scientific work because of its convenience Engineers, hon ever, both in Great Britain and the United States lean toward the English or the so-called British Engineering system of unite, on the ground that it is more familiar to most people This difference might well be used to illustrate an important distinction between science and engineering Whereas science deals for the most part with truths systematically organized into a bods of knowledge, with logic carried right down to the matter of units, engineering deals more with the applications of science toward commercially practicable ends Compromises are frequently made with accepted customs and usages by practical people Of course in the last analysis, there is much overlapping, and no one should attempt to distinguish too sharply between science and engineering Unit of Time The last in this discussion of the fundamental units 19 the unit of time Whereas time is often defined as a measure of duration, duration can only be defined as a measure of time Thus time is a basic concept, not to be defined but rather to be measured in units It has become almost unu ersal practice to measure time by the use of clochs which have been calibrated to read in terms of the time required for the earth to rotate about its axis in its orbit about the sun The mean solar Fig 1.3 Intervals of day is the accepted umt, of which the time are measured by clocks mean colar second is & 1/86400 part The length of the mean solar day is found by averaging over the whole year This unit is used in both metric and English systems, and thus does not share in the confusion created by the other units Sce fig 13 Systems of Units true, thes, have vuued the vas units in two systems The particular metric system in which length is measured in centimetere, mass in grams, and time in seconds is called the CGS (centimeter, gram, second) system 10 19 18 ·MEASURING INSTRUNETS 9 At the present time an attempt is being made to popularize the use of a metric system called the M.KS system in which the meter is actually used, not just defined, as the fundamental unit of length, and the kilogram is actually used instead of the gam as the fundamental unit of miss This system is a compromise bets een scientists and engineers, and its ments lie chiefly in the electrical units which come later in this study Of course, the adoption of this system will be a gradual process In this text all metric measurements will be expressed in CGS units Where English units are used, lengths will always be expressed in feet, force (not mass) in pounds (see chapter III for further considerations of mass and force), and time in seconds In terms of such units the physicist records the direct measurements upon which his conclusions are reached and tested Measuring Instruments It is also seen that, in the last analysis, direct measurements boil down to scale readings of 1 דיזייני" 3 Fig 14 Inter als of length are ordinarily measured by foot rules, Jan.tiche meter stich: etc length, clock readings of time, and pointer readings of masa See figs 14 and 15 Such pointer readings of mass are obtained with a balance by the uc of vluch the tendency of a body placed in one pan, to be pulled downward by gravity, is balanced by supplying an equat alent amount of standardized mass in an ad- Joining dan connected to the former by a bar resting on a kofe Fig 13 Mars is mesured by the beam balance ·10 INTRODUCTION edge at its center in the course of his work the physiciet has in. sented ingenious devices to facilitate thcsc measurements, in cluding the serier and the micrometer calipers (ace fige 16 and hg 16 The vernier caliper is weed to make accurate measurements of relatively short lengths 17), the measuring microscope specialized forms of mass balances, and intricate clock work mechanisms Fundamentally, however, scale readings of length, mass, and time constitute all the direct quantitative observations Fig 17 The macrometer caliper is used to make accurate measurements of very short lengthg Relative Value of Quantitative Considerations Now, lest the beginning student should get the impression that this discussion of fundamental units appears to emphasize the numerical aspect of the subject in spite of promises that this book would minimize quantitative considerations, he should be reminded that much of the beauty of the science of physics to the physiciat lies in its high degree of organization and the discipline which its strict definitions provide In studying such a subject, even if the quantitative or numerical aspects are not pursued to the limit it is well worth noting the importance of getting first things first Only thus does one get the correct onentation prop- ·SUMMARY 11 erly to appreciate the more complicated, and perhaps more fascinating aspects of the story which is to be unfolded in the subsequent chapters of this book Summary In summarizing this chapter we see that physice 13 to be considered as a logically organized body of concepts to be understood in terms which are uniquely defined By inductive reasoning based upon observation, conclusions are drawn regarding these concepts These conclusions must be subjected to tests of further experimentation and observation Although inductive reasoning is paramount to the research phases of the science as true today as ever since the earliest times-deductive methods are used to advantage in passing the results on from one generation to another In this chapter the importance of measurement has also been cmphasized but only to serve as material upon which to build The physicist uses measuring instruments as well as instruments of the mind in making his observations, and expresses his results in terms of suitable units, of which there are a few more basic than the rest Future chapters will deal with some of the results of measurement and although measurement naturally sucgests arithmetic, the latter will be minimized so as not to interfere with descrip tions of physical concepts Before going on immediately to a detailed drecussion of phs cical concepte, however, attention will be gyon in the next chapter to the place of physics in our civilization by a brief treatment of the history of the science of phy cca Such a histoncal summary will constitute something of an over-all look at the field before a detailed consideration of the various parts is undertaken QUESTIONS 1 What is meant by the "Scientific Vethod '? 2 Distinguish between the meter, sard, foot, centimeter, mil. limeter 3 For what purpose is the verdier caliper veed? 4 What is the difference betseen "inductie" and 'deductive" reacoding?

·CHAPTER II HISTORICAL CONSIDERATIONS ANCIENT AND MEDICVAL PHYSICS THE NEW 111 ALEVING IN PHYSICS CLASSICAL PITI SICS MODERN POISICS In a descriptic survey of phy 109 it is appropriate to consider before getting absorbed in the detailed concepts of the subject matters of a broad nature, such as historical development. By such a procedure the student is provided an opportunity not only to appreciate how the science has grown but also to es nlunte its importance in and to, modern ciuilization After all this en scientific age and it is indeed a matter of some concern to the serious student to find out just that physics has contributed to his world This point becomes all the more significant when it i noted that physics is one of the oldest and onc of the most highly organized of all the sciences Certainls the contribution of physics to the cultural and eco nomic welfare of mankind has been very great indeed in time of botlı peace and war throughout the ages It is the purpose of the present chapter to outline some of the important des clop ments in the science and to asociate them with persons Too often the student of science or technology overlooks the fact that physics is just as much concerned with people as it is with things Discovenes of natural laws are made by persons whose own lives as well as those of others are affected by them It is therefore appropriate that this historical outline bc developed around the names and dates of famous physicists The student should try to associate the dates with world cvents with which he may be familiar outside the realm of science Possibly come connections betneca events of great social import and discoveries in science will be noted that have not previously attracted his attention Four Periods of Physics It seems natural to consider this historical survey in four main subdivisions which we shall reier to as periods of physics, as follows Ancient and Medieval Physics 12 ·ANCIENT AND MEDIEVAL PERIOD 13 (roughly 3000 BC to 1500 AD) The New Aualening in Physics (1500 to 1700), Classical Physics (1700 to approximately 1890), Vodern Physics (1890 to the present time) These dates can be considered only as very approximate The first and last of the e periods might be subdivided further A distinction could well be made between the ancient and the medieval periods but the contributions made in the latter period, which might be said to extend from 500 AD to 1500 AD, were so few as to make such a distinction hardly worth hile Moreover, the advances made in the so called modern period have been co numerous that a breah around 1925 micht ccem justifiable Perhaps the advent of the atomic bomb in 1915 will make historians of the future consider this year the beginning of a new era lloncier the present is not the time to et aluate the progress of contemporary physics and so it has secmed best in this surtey to group all tl e accomplish ments since 1890 into a single so called modern period First Period Ancient and Medieval Physics (3000 DC- 1500 AD) Records indicate that the ancient Babyloniana and reptians as cork as 3000 BC vcrc familiar with come of the (undamental principles of physics, especially tho e dealing with the measurement of land There is also cu lence that the begin nings of astronomy were comes hat understood by these early people But nothing en definite in a scientific ecnsc, Focme to be come from this Of course crcromize the eng neering achiciement represented by the piramids of Eart, but the astronomy was more the sort of thing nox known as astrolom It also eccras certain that the Chinese nere familiar with the magnetic compass carlier than 1000 BC, but here again the recor is ATC crs scants The GRFEK PART OF TIE First PERIOD From approximatels 700 BC to 150 AD very defimte adi ances in physics ere made by the Greek Thales (6107-516 BC), a rather shadon figure, appears to hai c recognized certain aspects of what to!ai e known as static clectricits Historical scientists al o credit lim and a froup Ascociated with him known as the Ionian School with the crustallization of the ven that Fire Trater, and Earth were fundamental subatance This certainls represented a reco2011- able beginning of scientific thought Other early names encountered in tc records are Pythagoras (550-500 nC) and Derocritus (460-370 BC) The formet je ·14 TIISTORI OF PHYSICS membered as the founder of the Pythagorcan school of philosophers, his most important contribution was probably the Pythagorcan theorem of geometry, which states that the fiy potenuse of a right triangle cquals the square root of the sum of the squares of the sides Democritus is credited with postulating an atomistic view of matter Although an atomistic view is held to. day, Democntus is not considered its father since there appears to have been little of any scientific basis for his postulate It is more appropriately rcferred to as idle speculation than as a scien. tific postulate Next we come to Plato (4272-317 Bc) and his pupil Aristotle (384-322 BC) Plato was undoubtedly one of the greatest of the Greek philosophers, yet his contributions to physics were relatively meager His famous pupil Anstotle, however, was the great organizer of scientific knots ledge of his time He is credited with having written the first textbook in physics He also contributed so much to the science of physics that for many centuries his influence was paramount In spite of his greatness, honever, we criticize Aristotle today on the ground that his famous thcores were not based primarily upon experimental evidence Although he frequently refers to the value of observed facts in the development of a theory, he seems not to have followed his own aduce Tery conscientiously Much of his work is based upon abstract argument not substantiated by obscrvation It is often stated that Aristotle was more concerned with explaining "why" things were than how' nature behaved Modern science, of course, is based on the latter point of view with considerable emphasis upon quantitative measurements made as nearly exact as possible By present-day standards Aristotle was certainly the armchair' type of philosopher whose conclusions were more deductive than inductie Although observation is not entirely missing, it was more of the passa e curiosity type not accompanied by purposeful experimentation Neverthelese Aristotle s name was unques tionably one of the greatest, if not the grcatest, in the Grech period of science Another Greek of importance was Euclid (4502-374 BC) Although he made contributions to geometric optics, his fame je much greater in the field of mathematics than in physice Finally, in the Greek period, we encounter Archimedes (2877- 212 BC) of the Alexandrian school On the practical side Archi- ·ANCIENT AND MEDIEVAL PERIOD 15 medes is perhaps the best known of all the Greek physicists Today he would be called an engineer or an engineering physiciet by virtue of his many applications of physical principles to wartime as well as peacetime pursuite Yet there are also those who would call him the founder of mathematical physics because of his apt and accurate applications of mathematics His work in hydrostatics alone would make him one of the most famous of the ancients, but he made potable contributions to opties, mechanics hydraulics, and other fields of physics as well Unlike Aristotle, honever, he interested himself in practical matters, or, as would be pointed out today, he kept his feet on the ground In summarizing the Greek period as a whole it seems farr to state that the entire period was characterized by deductive rather than inductive reasoning, that observation was for the most part euperficial rather than critical, that there was more passive curiosity than active experimentation and more armchair specu lation than fact finding Today one cannot help wondering why this was all so Yet it must be remembered that experimentation in physics is a manual procedure and that the social status of the free born Greek citizen above the slave forbade manual labor Whercas the impact of science on society 19 ordinarılı acknowledged, we note here a definite impact of societs on ecience, for it is undoubtedly true that had manual actruts not been frowned upon in this period, more advancement would have been made by the Grechs Instead, the science of phy ice pro ceeded just so far and no farther in a period of several hundred years compared with the enormous advances made in just a fen years in our time THE LATTER PART OF THE FIRST PERIOD Although the first period continued until about 1500 AD, practically no advance in physics was made between 50 BC and 1550 A D This period covers the rice and fall of the Roman Empire, coinciding with the decline of Grech culture generall, and the ou crrunning of the ancient western world by the barbanane The Romans gradualk abeorbed Grech culture, but by COO AD all Furope had been depra ed of the opportunits to avail steell of it The Romane strony not scientifically inclined Practicalk all Grech manuscripts et to Arabia Inn 79, Grech science was thue presened for the terits by the Araba who themechi ee added very little to it did, hones cr, introduce to science the so-called Arabic opet ·16 HISTORY OF PHYSICS numbers Alhazen (9657-71039) produced a work on optics to be sure, but generally speaking Grech science was not improved upon to any appreciable extent by its translation into Arabic It was still based upon the authority of Aristotle Between 700 and 1100 AD a beginning was made toward a revival of learning in Europe Large universities de eloped under the shelter of the church Trade spread, and both Grech and Arabian manuscripts gradually found their way back to Europe The crusades also assisted in this process Since the church had survived the Roman state and become all powerful, it was natural that the revival of learning, and of science in particular, should tahe place under its influence Many of the scientific manuscripts were translated from the ongnal Greek into Latın by the monks in the monasteries These scholars, like the Greeks, also scorncd manual labor and so the science which they passed on to the world was the orginal Aristotelian sersion unadulterated by any experimentation of their ona Thus the authority of Aristotle was carefully preserved in this period the scholars stuck to the Grock and Roman numbers, leaving the Arabic numbers to trade and so even the Arabian influence was minimized This is another reason why it becomes possible to state that for approximately 1500 years there were practically no advances over Grech science It way certuly a period of scientific stagnation By the year 1500 or so science had just about returned to the stntus which it had occupied 1500 years earlier Nescrtheless the church had re established science in the various large universities, directly under its control It is easy to sce, honcier, that church dominntion flavored it to suit itself The doctrines of Aristotle came to have the power of law behind them Truth was not discoverable, by that time, it was dictated by the church, and it becamen crime of the first order even to question the church sponsored views of Aristotle, to say nothing of suggesting that experimentation might be a better way to establish the truth Naturally science degenerated under this ag atem, in which it became so interwoven with ecclesiastical doctrine that scientific facts could not be separated from religious dictates The time was not quite ripe ior t'ne new tra winth Bus to duten ahoat 150 Second Period The New Awakening in Physics (1500- 1700) As time elapsed and trade de eloped wealth increased This led to the development by wealthy merchants of univer- ·REVAISSANCE PERIOD 17 eities for study and research outside the church This movement nurtured a gromag dresatisfaction with authonty in science By 1550 doubters of Aristotle appeared with experimental proof of their viens That is to say, doubters began to express themselves openly As a matter of fact there had probably always been doubters but they had been successfully suppressed Tho cen tunes earlier Roger Bacon (12149-1294) had taught that belief should be based on obeers ation and experimentation rather than on authority, but such expressions resulted in his spending practically the last third of his life in prison Truls he was a man nio sed at least two centuries ahead of his time Also, in Italy, there as Leonardo da Vincz (1452-1519) But sa spite of the great accomplishments of this man in pyactical enery field of the arts and sciences, he was hardly known in lsis time and his influence tras practically as He left his works in manuscript form and they have come to be appreciated only in recent times Now it is realized that he actually was one of the great scientzats of all time The period of Det awakening really commences with Copermars (1473-1513), Gables (356*1612), Tycho Brahe (1516- 1601), Kepler (1571--1630), and Gubert (1540-1603), who all paved the way for the great Isaac Newton (1642-1727) Copos nicus deseloped the heliocentnc theory of the unsverse Galileo, Tycho Brahe, and Kepler established the fundamental dess of modern celestial mechanice, based upon observation first and theorizing afterward, thus revolutionizing scientific thought Galko is particular stressed the idea of controlled fixerimentation to such a degree that todas he is recognized as the father of the modern scientific method based upon inductive rather than deductne learning Bowes er, be, 89 nell as some of the others, paid the price of propagating revolutionary Ideas by spending periods of his life in prison Golleo carried obsensation to the quantitatste stage by making Accurate measurements He truls emphased the how' (erca the how much") 24 contrasted with the shy" of Inzetotic By his quantitative obeer stions on falling boses and other mechan. ical motiona, seejated by intruments of his own avention to improve the accuract of his meseurements, he laid the ſounds. tson for the dracorenes of Venton, Tho was bora the year hr diod llis sork as bot limited to the Geld of mechanic, lor.

·18 HISTORY OF PHYSICS ever His contributions to other fields, including optics, were numcrous Sir Isaac Newton is usually considered the greatest scientific genius the world has produced He crystallized the scientific thought of his time into a few fundamental statements now ac cepted as lars of nature These include three famous laws of motion and the law of gravitation in the field of mechanics alone In addition, he invented the calculus and contributed greatly to the field of optics His role was primarily that of a co ordinator of information or a systematizer of koos ledge He formulated the over-all pattern by which scientific knowledge was to be organized in the great classical period that was to follow his time and which has not yet become outmoded, even today, except perhaps in the restricted field of atomic physica By 1690 much of mechanics was known and fitted into the Newtonian pattern Neutons contemporaries include Huygens (1629-1695), who proposed the ware theory of light in opposition to Newton, and other famous physicists including Boyle (1627–1691), well known in connection with gas laws and IIoohe (1635–1703), whose work in clasticity is famous Pascal (1623-1662) remembered for his law of fluid preseure Thus the second great period in physics ends around 1700 or $0, when the ideas of Galileo, Bacon, and others, who were irked by the doctrines of Aristotle forced upon them by the church, finally tooh hold and opened up a sast new era It was certainly a period of New Awakening and it paved the way for the physics of the eighteenth and nineteenth centuries, during which time new discoveries, although made in rapid succession, seemed to do hardly more than confirm the broad structure of the science laid down by Newton Third Period Eighteenth and Nineteenth Century Classical or Newtonian Period (1700–1890) It will not be feasible in this historical survey to enumerate all the achievements of the cighteenth and nineteenth centuries The science of physics was really gathering momentum by this time and becoming very complicated The science was getting large enough for the various subdivisions to be significant Advances were made in the fields of mechanica, heat, light, and electricity, as if each branch were more or lees independent, but the work of Newton provided the ·CLASSICAL PERIOD 19 means of integrating all this knos ledge In a way the period can well be described as one in which the fundamental views of Newton gradually came to be appreciated and established A wealth of quantitative material was gathered, but it all seemed to fit together in a most remarkable way into the Mewtonian pattern Indeed, the success of Newtonian physics was so great that by the end of this period it almost seemed that the end of physics Tas in sight Almost esery thing seemned settled, hence the designation classical period' Rather than attempting to develop this period in a chronological order as in the case of the two preceding periods, we shall consider the various branches of the science separately and do hardly more than list the outstanding names, dates, and events in this period Commencing with mechanics, we note the work of Bernoulli (1700-1782) in hydrodynamics and gas theorv, D Alem bert (17172–1783), Eule (1707-1783), Lagrange (1736–1813), and Laplace (1749-1827) in theoretical mechanics In the field of heat, the period 1600 to 1800 saw the develop ment of thermometers and temperature scales by Galileo, Fahren hent (1686–1736), and others It also saw the introduction of the concepts of latent and specific heats by Black (1728-1709) and the development of the steam engine by Ta't (1736-1819) The nineteenth century cat the concepts of heat fitted into the Me. tonian picture by the work of Rumford (1753-1814) Joule (1818- 1889), and Rowland (1818-1901) These men established the vien that beat was merely a form of energy Carnot (1796-1832), Vayer (1814–1878), Helmholtz (1821-1891) Kelmn (1824- 1907), Claustus (1822-1888), and others established the fundamental laws of thermodynamics, in which the basic concept of energy seried to unify the concepts of heat mth those of mochanica Finally tre note the work of Gibbs (1839–1903) in Chemical thermodynamics and later 12 statistical mechan es closely related to the subject of heat Also in the field of light, mans advances were made in the two centuries, and aguin the unfving influence of the Vewtonun methods tras ser apparent in the latter onc Galilco had attempted to measure the clocity of light but had found it too frent for accurate determination Roemet (1611-1710) and Bral. ley (1693-1762) made more succesful determinations, arrivo ·20 JEISTORY OF PIISSICS at values not too far different from that accepted today-around 180 000 miles per second Foucault (1819-1868) and Fitzcau (1819-1896) refined the mea-urements around 1850, but the final result waited for Michelson (1852-1931) Huygen 8 kave postulates scre rcvi cd by the work of Young (1773-1829) and Fresnel (1788–1827) Malus (1775-1812) dic. cosered tlic phenomenon of polarization of light by reflection This work as extended by Brouster (1781-1868) Fraunhofer (1787-1826), Kirchhoff (1821–1887), and Bunsen (1811-1899) laid the foundation for modern spectroscory The crowning climax of this period in the field of light on the strol e of finality given to the age-old question of the nature of light, wlicther corpuscular or wavelihc, by the great daruell (1831–1879), whose electromagnetic marc theory of light satis fied all partics to the controversy As a matter of fact, lawell 18 considered by many to be the greatest theoretical physicist of the nineteenth century as a result of his extraordinary accomplishments in the ficlds of light and electricity The last field to be considered in this third period of physics is that of clectricity, or electricity and magnetism, for it was found in this period that these two fields ti pre really one Probably inore attention was given to electricity than to any other branch of physics during the eighteenth century Gray (1670- 1736), Du Fay (1698-1739), Franklin (1706-1790), Cavendish (1731–1810), and Coulomb (1736–1806) did significant work in clectrostatics Galvanı (1737-1798) and Volta (1745-1827) were pioneers in current electricity In the latter part of the period the outstanding contributors were Faraday (1791-1867), Henry (1797-1878), and Maruell (1831-1879), although they had many contemporaries, including Ampere (1775–1836), Theatstone (1802–1875), Lenz (1804-1865), Kelvin (1824–1907), Kirchhoff (1824–1887), and Hert: (1857–1894) Faraday was the outstanding experimentalist of his time, and Maxwell, as atated earlier, was probably the most outstanding theoretical physicist of the period in this field, as in all the others, the period ended with a great feeling of satisfaction by physicists that the science of ply Fur var firally gunned and reary wall integrated Remarkable generalization had been accomplished by the theoretical physicists especially Maxwell, utilizing the general methods of Newton and the highly successful concept energy It was in ·YODER PERIQ 21 deed the classical period in phyeres Sa complete was the picture and co satisfied were the physicists in their accomplishments that by 1890 A gencral feeling became widespread, although it is diffi cult to determine its precise origio that physics was about done, that probably no startling new discoveries would ever be made, and that probably future generations of physicists would have to contcnt themsches with mcrcly extending the accuracy of hoown information, perhaps to the next decimul place That this was not the cgee is the story of modern physics wluch period started just about 1800 whes physicists were jolted rather abruptly out of their lethargy by several aotenorthy dıscoveries including the clectron, the I ray, and radioactivity The Fourth Period Modern Physics (1890 to the Present) The great generalizations and carrelations in theoretical physics and il e refinements of measurement in experimental physics during the mineteenth century, especially the latter part certainly placed the science in a strategic posstion with respect to the tech non cal and industnal activity just commencing The cconomic world was surels destined to feel the impact of it ladecd not even today has industry come anywhere near to exhausting the applentions of classical physics to its Deeds, even though the fruits of a still more recnt period the modern period hase come to occupy front place on the technological scene Of course me refer to electronice telovalon Atomic energy, ete, all of which cre unheard of much belore 1890 But it is still classical or Nets tonian physcs which properly constitutes the major part of college program in phs res today, especially for those students anticipating a career in engineering In fact this text Alo ll be largely classical in its scope The so called newer phyfice which is the material covered in the fourth period cannot prop cris bc understood and es iluated wthout a substantial bach ground in classical pha sice Although the achievements of the nineteenth century kere out standing the imirteton must not be grea that perfection been attained Tiere were gaps here and there, and there some certain contencies For crample the electromsmetir sem theory of light as wonderful but it required the poete's a medium the luminlerous ether, for the water to kare onc could find any tangible esidence for the erstence *** medium Also the studs of heat radiation *** got oli ·22 HISTORY OF PHYSICS Furthermore, recent discoveries in electricity, such as the photoelectric effect by which the action of light produces an electric current, were inconsistent with Maxwell's theory of light Yet all such difficulties were considered to be of very minor importance compared with the over-all success of the general Nen tonian pattern It was felt that sooner or later they would all be straightened out, like a jigsaw puzzle complete cxcept for the last few pieces The fact is, however, that inetead of this happening the difficulties became enormously magnified as plig sicists concentrated their attention on them It finally took a whole nen point of view, such as is utilized in the quantum theory and in relativity, to strughten things out But this did not happen until a series of brand-new discoveries were made, including the 1solation of the electron, radioactivity, I rays, and the concept of the electrical structure of matter, all of which were likewise er. plained by the new viewpoint Crookes (1832–1919) had discovered cathode rays in 1878, but J J Thomson (1856–1940) explained their nature as negatzvely charged electrical particles, called electrons This was between 1890 and 1897 and is usually referred to as the beginning of the electronic era, although Stoney (1826-1911) had suggested the name "electron" as early as 1874 Lorentz (1853 1928) formulated an electron theory of matter in 1895 in which vibrating electrons accounted for the electromagnetic radiations predicted earlier by Maxwell and discovered in 1888 by Hertz (1857-1894) Also in 1895 Roentgen (1845-1923) discovered X rays, and in 1896 Becquerel (1852–1908) discovered radioactivity These were followed in close succession about 1898 by the isolation of polonium and radium by the Curies, P Cure (1859–1906) and M. Curie (1867-1931) The year 1900 is markcd by the introduction of the quantum theory by Planch (1858–1947) This, follon cd by the theory of relativity in 1905 by Einstein (1879– ), set the stage for many advances because of the new way these tso theories provided for viewing natural phenomena These theories struck at the fundamantalahibonly of my suy and nuuded them to the esttion of several problems left over from the preceding period Tho quantum theory explained heat radiation and the photoelectric effect almost immediately The theory of relativity went farther ·MODERN PERIOD 23 than Newtonian mechanics to explain certain phenomens associated with matter moving with extraordinanly large velocity, such as electrons, atoms, and molecules It also provided an explanation for the failure of the Michelson Morely experiment (1887) to determine the drift speed of the earth through the luminiferous ether Although these theories were very reluctantly received, they have nevertheless gradually become established in physics, in spite of the fact that they have so altered scientific thinking that many of the older physicists of today have confessed their inability to reconcile some of the postulates with classical physics or even with common sense Indeed, much of the newer theoretical physics is abstract mathematies, but most of the younger physicists have come to accept the results of these two theones as reasonable From about 1911 much interest has been focused upon the general field of physics known as atomic phyaics therford (1871-1937), with the help of many socjetants over a period of tears, finally established the concept of the nuclear atom Bohr (1885 devised a model of the atom in 1913 which was patterned after the solar system and its planets This so called planctary atom, conseting of a positively charged nucleus encircled by negatively charged electrons, has been very widely publicized Although today this picture has been more or less replaced by abstract mathematical representation in the mind of the theoretical physicist, many of the Bohr features are still useful, especially in elementary explanations of atomic phe- Domena To the uninitiated this last statement may seem strange, rars ing as it probably does the question of how a picture can be accepted if it is not correct. This is an example, however, of the modern point of view in physics, and its relation to common sense Quantum and relativistic phy-103 have prepared the scienufic mind to appreciate how complex nature really is, and how far the world of atoms and electrone is removed from that of compmon sense anymas It reminds ve that explanations are only relntrue to the students background To an elementary student something can be a perfectly satisfactor explanation which would not be appropnate for the ads anced student. In other words, there can be any number of ways of explaining anything It is not a question of which is the correct one, it is rather 1 ·24 JUISTORY OF PHYSICS question of which is the better one for the purpose Thus perhaps it is clear hon the newer physics hae scemed so confusing to the older generation Many names might be mentioned in connection with atomic phy eyes, but in this brief summary only & fex eccm appropriate II H Bragg (1862-1912), Aston (1877 ), CTR Tuson (1869 ), and Mullihan (1868 ) are all names which attained prominence before 1920 Of course no distinction can be made here between atomic and electronic physics, and furthermore these lists are very far from complete During the 1920s further advances were made in electronica, spectroscopy, and nuclear phyics, although research in all fields increased greatly after the First World War A H Compton (1892 ) practically removed the last traces of doubt of the quantum theory in 1923 by his experiments with X raya De Broglie (1892 ) introduced the concepts of wale mechanics in 1924 Hersenberg (1901 ), Drrac (1902 ), And Schrödinger (1887 ) did much to develop this field between 1921 and 1926 Darsson (1881 ) in this country And GP Thomson (1892 ) in England produced experi mental evidence for the wave nature of the electron in 1927 and 1928 In 1932, Chadurch (1891 ) discovered the neutron, a fundamental particle which carries no electrical charge, and Anderson (1905 ) discovered the positive electron sometimes called the positron Lawrence (1901 ) invented the cyclotron in this same year Joliot (1900 ) and his wife Irene Curie-Joliot (1897 ) discovered artificial radioactivity in 1934 Fermi (1901 ) and others produced artificial radioactivity by neutron capture Hahn (1879– ) in 1938 discovered uranium fission, which is the basis of the atom bomb of 1945 Thus we have touched upon some of the high points of physics right up to the present time As stated before, the list of names and dates is far from complete, nor is it intended that the elementary student should understand all of the above, but enough has been included to maintain the continuity Research today, however, is not a matter for individual physicists This is the age of group research, and in many of the projects noted above only the leader or leaders have been mentioned Hundreds of ·SUVVARY 25 others have contributed who may never receive full recognition for their work It has been the alm of this chapter to outline the advances in physics from almost prehistone times up to the pre-ent It should be obs ious that these advances have been made at an accelerated rate except for the dark period between 50 BC and 1500 AD Today, physics is progressing at such a rapid rate that we can only guess what will be discovered and what the consequences will be in the future Certainly the atomic bomb has indicated how closely the affairs of this world are related to the developments in the physics laboratory, and it can only be hoped that man can learn to get along with his &ind as fast as he can discover the secrets of nature, lest things get out of control and he annibilate himself In a way this chapter has been something of a summary of the whole field of physics but one developed chronologically rather than logically Following chapters will deal with much of this material in amplified form QUESTIONS 1 How can the Greek period in physics bę brefly characterized? 2 Why were Aristotle's viens 80 widely accepted as late as the fifteenth century ? 3 What is meant by the statement that the Arabians left Greek science in cold storage ? 4 Why is Galileo considered the father of modern science? 5 11 hy is the classical period often referred to as the Newtonian period in physics ? 6 il hat broad fields characterize modem physics ?

·24 HISTORY OF PISICS question of which is the better one for the purpose Thus perlinpe it is clear how the newer physics has scemed so confusing to the older generation Many names might be mentioned in connection with atomic physice, but in this brief summary only a few secm appropriate TV II Bragg (1862-1942), Aston (1877 ), CTR Filson (1869 ), and Mallıhan (1868 ) are all names which attained prominence before 1920 Of course no distinction can be made here between atomic and clectronic physics, and furthermore these lists are very far from complete During the 1920's further advances were made in electronica, spectroscopy, and nuclear physics, although research in all fields incrcased greatly after the First World War A H Compton (1892 ) practically removed the last traces of doubt of the quantum theory in 1923 by his experiments with X raya De Broglie (1892 ) introduced the concepts of warc mechanics in 1924 Heisenberg (1901– ), Dirac (1902 ), and Schrödinger (1887 ) did much to develop this field between 1921 and 1926 Davisson (1891 ) in this country and GP Thomson (1892 ) in England produced experi mental evidence for the wave nature of the electron in 1927 and 1928 In 1932, Chadurch (1891 ) discovered the ncutron, a fundamental particle which carries no electrical charge, and Anderson (1905 ) discovered the positive electron, sometimes called the positron Lawrence (1901– ) insented the cyclo tron in this same year Joliot (1900 ) and his wife Irene Curie-Joliot (1897 ) discovered artificial radioactivity in 1934 Ferme (1901 ) and others produced artificial radioactivity by neutron capture Hahn (1879 ) in 1938 dis. covered uranium fission, which is the basis of the atom bomb of 1945 Thus we have touched upon some of the high points of physics right up to the present time As stated before, the list of names and dates is far from complete, nor is it intended that the elementary student should understand all of the above, but enough has been included to maintain the continuity Research today, horever, is not a matter for individual physicists This is the age of group research, and in many of the projects noted above only the leader or leaders have been mentioned Hundreds of ·COMPOSITIOY OF FORCES 27 tant point that when two forces of equal magaitude are added together, ie, act upon the same body, the result is not necessarily a force with twice the magnitude of one, but one which N N . E S Fug 31 No effect in easterly direction Fig 3.2 No net effect may even be zero, depending upon the directions involved The result is a doubled force only when the two forces point in the same direction See fig 33 When the directions of two forces of equal magnitude happen to make an angle of 120' (an angle of 120° is one third of a Fig 3,3 Effect doubled whole circle) it can be shown, both by calculation and by experiment, that the resultant force—that is the force which by itself would produce the same effect as the other two combined-has exactly the same magnitude as each of F1 R R 909 20% -Fi F2 Fig 34 R has same Fog 35 Magnitude magnitude as Fand F. of R 19 approximately 14 that of F, or F, them (no more, nor less) although of course the resultant direction is quite different from either See fig 34 Another special case 18 the one in which two such forces make an angle of 90° — ie a right angle-with each other Here is a force of about 50% greater magnitude than each, which points in a direction lying halfway between the two See fig 35 Composition and Resolution of Forces The students! be interested in knop. ho- thans ---- datormind 11 ·28 MECHAVICS F2 a matter of experience, of course, that they are true, but the fact that they can be determined sithout resort to experiment, except to check the answer, is sery important The simplest way to determine a resultant force is to repreacnt cach force graphically by an arrow drawn to scale, so that its length indicates in a relative way the magnitude R of the force, and so that it points in the prescribed direction II, non a drawing 19 made with the tail end of one arron joined Fi to the head end of the other, each pointing Fig 36 Vector addition in its proper direction, the straight line drawn from the tail cnd of the latter to the head of the former will indicate by its length and direction (using the same scale) the magnitude and direction of the resultant See fig 36 This is obviously the diagonal of a parallelogram made by the pairs of the two arrows as sides Such arrows are called vectors and force 13 a 1 cctor quantity That is to say it is a quantity which requires the specification of direction as well as of magnitude for its complete determination Vector quantities or which there are many in physics, can be added (compounded) by such a graphical method as just de scribed and of course, the procedure us not limited to just two vectors There are also rules of mathematical physics by which vectors may be subtracted multiplied divided, etc, but it is not necessary for us to consider these here Thus we see that forces may be compounded to produce resultant forces A question then naturally arises May not any force be considered to be such a resultant of perhaps several subsidiary forces may not a force have certain FN components into which it may be broken down for convenience The an. swer is obuous, and for practical con. siderations, very important A certain force pointing in a northeasterly direc. Fig 37 Fmay be thought of tion for example may be considered to as the resultant of F. and F. be the resultant oi a force in a northerly direction combined with & force of the proper magnitude, according to the graphical pro cedure just outlined pointing in an easterly direction, even though these two forces may not be apparent Sce fig 37 The latter two N F le w E F 0 $ ·COMPOSITION OF FORCESS 29 forces are card to be components of the orginal one A force may thus be broken down into any number of components The projected component of a force in the same direction as the force 13 the entire force itself, while the projected component of a force at a night angle to itself is zero See fir 3.8 N N F F has components 1a Dortberly and easterly directions The component of F in the easterly direc trop us F itself Fig 38 The component of F in the northerly directron is zero It should be clearly borne in mind that it is frequently incon venient to exert a forcc in the right direction to get its full benefit For example, it is probably more convenient to tow a barge through a canal by pulling a rope obliquely from the shore than to ton it directly behind another boat See fig 39 Again, it 19 probably much more convenient for a tall person to pull a sled by a rope which makes an appreciable angle with the ground, although the full benefit of the pull would be realized only if the pull were made horizontally In these cases one is satisfied with Fig 39 Towing a boat by pulling at an angle to the direction of motion of the boat only one component of the force exerted On the other hand, in the case of a sailboat, it becomes possible to sail almost directly into the sind onls because the soil can be set at such an angle that the force of the wind perpendicular to it has a component w the duration the desired motion of the boat The romander of the winds force is dissipated indirectly by means of the keel of the boat See fig 3 10 ·30 MECHANICS on Sa 1 aga nst Keel 2 In these simple ca es se have developed the vicw that not all of a force need be utilized Lest this create the false impression that the components of a Wind Mot on force are always small as compared with the result. ant, other cascs should be Component in Drect on of Mot on considered When the angle between two forces of Component of Wind cqual magnitude, acting at & point, exceeds 120° the Component D ss pated magnitude of the resultant force is less than that of either force alone See fig 311 As greater and the wind blows obliquely upon a sail the greater angles are considsail appreciates only the component per-ered the corresponding rependicular to itself. je the wind can sultants decrease more and puch of a sail only perpendicularly more in magnitude In the limit, when two such forces act in opposite directions (180° apart), as in the example considered early in the chapter, the resultant is zero Also, when the angle R between two forces is almost 180°, a small force acting as f F2 the resultant has components along the prescribed direc- Fig 311 Case of R whose magditude is much smaller than that of comtions of much greater magpouents F, or F, nitude than the force itself This is easily tested by experiment If a heavy package in the shape of a cube is lifted by & cord wrapped about the package, 120° F1 AF2 F2 F1 FA F. and F, each equal to balf the weight of the box, F, and F, each equal to the whole weight of the box Fig 3 12 F, and F. each greater than the weight of the bor ·EQUILIBRIUN OF FORCES 31 the cord is quite likely to break when the cord is wrapped qute tightly On the other hand, if the cord 13 so loose as to make only a small angle at the point where the cord is lifted, it is not broken so easily When the cord makes an angle of nearly 180° at the point where it is lifted, as in the case of the tightly wrapped cord, V Fig 313 Tension is tennis net approaches infinity as the alack 19 almost talen up the force developed in the direction of the cord becomes infinitely great See fig 3 12 This is also why no force whatever is sufficrently great to take absolutely all the slach out of a tennis net if it is applied along the direction of the net See fig 3 13 Equilibrium of Forces First Condition of Equilibrium А consideration of the case where the resultant force is zero when a northward component 18 compounded with a southward component of equal magnitude also clears up another difficulty commonly encountered by the beginning student It is sometimes hard to understand, if we accept the definition of a force as a push ou pull tending to change the motion of a body, how a body can be at rest on a table, ie, be under the influence of no force, when we know that the force of gravity is always acting The answer is, of course, that the table also acts on the body, and that the resultant of this force acting upward, compounded with the weight of the body acting downward, is zero, so that the net tendency to move is zero See fig 314 If the table were not there to support it the body would move downward Thus a body at rest on a table Fig 3 14 Push of table represents a condition called equilibrium on body balances pull of gravity on body for equi It is characterized in part by the fact librium that the vector sum of all the component forces 13 zero If, for practical purposes, we should restrict our consideration of components to components pointing upward, downward, toward the nght, and toward the left, we may say Р W ·32 MECHANICS that for equilibrium to exist all components upward must be balanced by all components downward, and all components toward the right must be balanced by all components toward the leit Scc fig 3 15 This is what is meant by the statement that the 1 cctor eum must be zero The realization of this makes it possible for the engineer to calculate in advance the forces which will act upon the various members of bridges and other structures The Fug 315 First condition basic physical principle behind such of equihbrium requires that calculations is simply thus principle of forces up balance forces equilibrium down and that forces to right balance forces to left This principle is also utılızed in the determination of mass by the beans balance as previously indicated, but here & second condition of equilibrium is also involved See fig 3 16 A little reflection shows that there are two general types of motion into which a body may be set by the action of a force These are translatory motron, defined as motion in which every straight line in a body remains unchanged in direction through- Fug 316 Beam balance out the motion, and rotary motion, defined as a motion of a body such that every point of the body rotates in a circle about some axis In the case of the latter, the effectiveness of the force is found to be directly proportional to the distance beth cen the axis and the line of action of the force-1e, it is doubled, tripled or halved, etc as this distance is doubled, tripled, or haled, respectively This distance is known technically as the lever arm of the force See fig 3 17 If the line of action passes directly through the axis, no rotating tendency is produced because the ·SECOND CONDITIOY OF EQUILIBRIUM 33 resulting leser arm is zero Thus the tendency to rotate depends upon the two quantities, force and lever arm, which give rise to a concept called torque It is defined as the product of a force and its lever arm, with respect to a gven axis Torque is some 10 cm 20 cm F2 F1 50 g ams 25 g ams Fig 317 50 grams at 10 cm lever arm balances 2 grams at 20 cm lever arm F, tends to prod ce counterclockwise motion about the fulcrum O F, tends to produce clock Risc motion about the fulcrum o times called moment of force Clochuise and countercloch wise rotations are thus capable of being produced by the action of clockwise and counterclockwise torques Second Condition of Equilibrium. The formal statement of the second condition of equilbrium is that the sum of all torques about any axis must be zero, le, all clockwise torques must be balanced by counterclockwise torques In the use of the beam balance the pull of gravity on a body placed in one pan acting with a certain lever arm with respect to the fulcrum is balanced by the pull of gravity (weight) on standard masses placed on the other pan acting with its lever arm Notel if the lever arm of one pan is doubled, only half the force is necessary to maintuin the constancy of the torque, ie, to produce a certain tendency toward rotation This is what 19 meant by the statement that a force is made more effective as regards rotational motion by increasing its les er arm with respect to some axis which serves as a fulcrum Thus machines such as levers and other mechanical contrivances can be devised to facilitate certain tasks, but a more complete discussion of these matters will be reserved until such concepts as work and energy have been descloped Need for Study of Motion In the discussion of forces up to the present point the vector aspect, in particular, has becn con sidered It 19 obvious, however, that a complete appreciation of tlie force concept cannot be had until the vocabulary of motion is developed, for the tendency to change motion 19 an inher ·34 NECIJANICS part of its definition Motion, on the other hand, is a meaning. less concept until the concept of position is established Position and Displacement. Position is a relative matter In the three-dimensional world in bich we live, bodies are beat located with respect to certain landmarks by lay ing off specified distances in three mutually per- Up pendicular directions from the landmark tahcn as an origin See fig 318 Thus so many paces to the north (or south), then so many to the east (or w west), and so many feet up (or Co ord note Axes down) from a guen point will suffice to locate any other point in space with respect to this Down point A change in position in- Fig 318 Three-dimensional 10 volves & displaccment from the cation of point P (north-south eastfirst to the second position This west up-down) is obviously a directional matter See fig 3 19 The concept of change of position in a certain direction is called displacement, and is a vector quantity like force This means that in technical considerations a distinction must be made betseen distance and displacement For example, S o B Fig 3 19 Displacement from A to B with respect to the ongin D on a straight line a car left in a parking lot may later be found ten feet to the right of its original location It is therefore correct to say that the car has undergone a displacement of ten feet to the right, although in so doing it may have been driven a distance of several miles, out of the parking grounds and all over the city, later being returned to the lot and parked ten feet to the right of the original location Displacement, then, is a technical concept and one which is fundamental in the study of motion Velocity. The next concept to be considered is that of velocity, the time-rate of displacement, and again the fundamental natures of length and time are brought out When a displacement takes place in a certain length of time, the ratio of the ·VELOCITY AND ACCELERATION 35 displacement divided by the time is called the average velocity of the body undergoing the displacement Thus a car traveling twenty miles northerly in one hour is said to have an average velocity of twenty miles per hour northerly Velocity 19 therefore also a vector quantity and is subject to the laws of vector com. bination That the direction of a velocity is important is brought out by the example of the ros boat crossing a flowing stream in which the current is appreciable It is obvious that, to reach a specified point on the opposite shore in a given length of time the direction in which the rover points his boat 18 fully as im portant as the speed with which he rous Speed is defined as the magnitude of velocity, ie, just the amount of it, not considering direction In this consideration the matter of constancy of velocity has not yet entered Although a car may make a displacement of twenty miles in a given direction in one hour and thus maintain an average velocity of twenty miles per hour, it is common knowledge that the actual rate may be a vanable quantity Thus a distinction must be made between average velocity and instantaneous velocity The latter quantity implies a measurement made over an infinitesimally small interval of time In a millionth of a second for example at is inconceivable that the velocity of a body can change very much Therefore the average velocity over such a small time interval is accepted as approximately the actual instantaneous velocity If this interval is not sufficiently short for a given purpose then it is agreed that an interval of a billionth of a second should be considered and so on It is thus seen that instantaneous velocity is really an abstract concept in spite of the fact that it has become a common everyday term in this auto mobile age Strictly speaking the speedometer of an automobile does not measure velocity but only the magnitude of it or speed and the device is therefore correctly named since it does not indicate direction Acceleration A name is also given to the time rate of change of velocity It is acceleration This is a vector concept too because a change in one direction is not the same as a chance in any other direction Of course there are various ways in which a velocity may change Either the magnitude or the direction may change or they may both undergo a change simultaneously Although the magnitude change-correctly speaking this is ·36 VECIIANICS change of speed—18 a common experience, we must not overlook the fact that a change in direction, cren with a constant speed, constitutes a perfectly good acceleration Thus a car traveling in a circular path experiences an acceleration although the speed may not change The direction of this accelcration is toward the center of the circle With this very brief introduction to kinematics the student, if he is disposed toward arithmetic, can already apprecinte certain numerical relationships, although as has been stated the e are of secondary importance in this study Thus the product of a constant velocity and the time during which it continues represents the total displacement, a quintity which might not otherwise have been specified in some problem Similarly if a velocity chances uniformly, ie if motion is uniformly accelerated motion, the foregoing relationship between velocity, time and displacement holds, providing average velocity is used in terms of initial and final values the average of any two quantities is one half their sum Thus a car starting from rest (initial velocity zero) and acquiring a final velocity of 60 miles per hour (88 ft per second) has an average velocity of 30 miles per hour (44 ft per second) In ten seconds it will have been displaced 140 ft if the acceleration is uniform Falling Bodies We now come to our first consideration of a physical phenomenon It is a matter of phyical observation that every unsupported body falls in a straight line with constant acceleration, except for the fact that air resistance alters very high speeds Sec fig 3 20 This means that in a vacuum all unsupported bodies have a constant downward acceleration which has been measured as approximately 32 ft per second per second (ie, a gain in velocity of 32 ft per second each cecond) In metric units this amounts to 980 cm per second each second Thus the pull of gravity (neight) alone changes the motion of a body by a con stant amount each second, ie, at a constant rate This fact has not always been known, and its discovery was a remarkable scientific achievement The acceleration is not readily measured by direct experiment because a body falls 16 ft in the first second from rest and 48 ft in the succeeding second, and a second is a rather short interval of time for precise direct measurements As has been said before, however, physicists starting with Galileo ·NEWTOV'S LAYS OF MOTION 37 have devıcd indirect means of measuring very large and cry small quantities, so that today the acceleration of gravity is known with a high degree of accuracy As we have seen (p 7), at vanes slightly from place to place on the earth's surface This means al o that the weight of a body is not constant but varies as the acceleration of gravity varies Sce also p 38 The fact that the pull of the earth gres all unsupported bodics the same acceleration was not recognized prior to the time of Galileo Aristotle had taught that heavy bodies necessarily fall faster than light ones, and even when Galileo demonstrated the fallacy in Aristotle s conclusion by dropping bodies from the top of the leaning tower of Prsa, all were not convinced Eren in Galilco s time it was not altogether clear that the motion which & body acquired was the result of the force acting upon it Newtons Laws of Motion The Fig 3 20 In at exhausted Whole matter of force and motion was tube & mctallic coin and bits finally summarized by Sir Isaac New of confetti fall at the same rate showing that air resistton (1612-1727) in the form of threc ance is what ordinarly makes generalızations now known as laws these objects fall at different of motion In the scientific sense rates The acceleration of gravity is the same 20 a they represent a discovery which has Vacuum for all objects at a stood the test of experiment ever since given place on the earth a Newton's time In simple language surface they may be expressed as follows 1) Every body continues at rest or in a state of uniform notion unless a force acts upon it 2) If a force acts upon a body, the body experiences an acceleration in the direction of the force and proportional in amount to at as well as inversely proportional to the mass of the body 3) A«ucrated with every force there is an equal and oppositely directed reaction force ·38 MECHANICS Significance of the First Law. Let us consider now the significance of these brief but comprehensııe statements which have come to be accepted as natural laws The first one makes it clear that no force whatever is required to keep a body moving at a constant speed in a straight line, but rather that a force is necessary to stop such a motion, or indeed to alter it in any way in other words it implies that all bodies exhibit a property knonn as inertia Moreover, a condition of rest is to be thought of as merely a special case of uniform motion Thus we see that except for the necessity of overcoming friction and air resistance no force at all is required to keep an automobile going at 50 miles per hour over a level, straight road Perhaps the drivers of 'free- Whecling' cars have come to appreciate this point, if only to 1 limited extent Momentum The common expression that a car is sometimes carried along by its momentum is a popular i ersion of thus important law The expression is technically correct of the correct defimtion of momentum is understood, as the product of the mass and the velocity of the body concerned Momentum is another vector quantity and is expressable in terms of arrows Newton often referred to momentum as quantity of motion, thus emphasizing the view that matter in motion is more meaningful than motion in the abstract The first law also provides the theorctical basis for the conditions of equilibrium already discussed Significance of the Second Law The second law explains llow motion is changed by the application of a force In other words it indicates that force is necessary to overcome inertia A mathematical statement Mould be that force equals mass multiplied by acceleration This is why on p 7 the statement was made that the weight of a body varies slightly from place to place on the earth s «urface with the acceleration of gravity Thus we sec that muss is not the same as weight, and that the unit of force (dyne) is equal to the unit of mass (gram) multiplıcd by the unit of acceleration (cm per sec per sec ) This is also why the unit pound should not be used for weight and mass (see p 7) The second law also brings out very clearly that the amount of effort required to change a motion depends proportionally upon the acceleration involved The reason for powerful motors in automobiles is to provide acceleration ability rather than to cpable them to travel fast What is popularly called "pickup" ·THIRD LAW OF MOTION 39 requires the application of force This means that if tuo bodies of the same mass were to be acted upon by different forces, the one acted upon by the larger force Bould experience the greater acceleration Furthermore, the same force acting upon different masses would produce a greater acceleration of the Jeug maseve of the two bodies Thus the new fast trains are made less massive than the older ones and require much less time to stop and to come up to speed again at stations by the application of forces presumedly equivalent to those utilized in the more massive trains The second law also explains centripetal force, the force necessary to Fig 321 A body traveling keep a body traveling at constant in & circular path with conspeed in & circular path, and its reacstant speed has an acceleration directed toward the cen tion (see third law below), centrifu ter due to continuous gal force, both of which are involved change in the direction of the when cars skid around curves at exvelocity cessive speeds See fig 321 In this case the acceleration produced by a force is usually the result of change of direction only The centripetal force acts on the revolving body whereas the centrifugal force is exerted by the revolving body on something else Significance of the Third Law The third law expresses the observable fact that a force never exists alone It is impossible to exert a force without a reaction force A little reflection cona F1 F2 Fig 322 A tug of war is not possible unless F. opposes F. and vice versa vinces one that a one-man tug of war is an impossibility One cannot exert a force without opposition See fig 322 The most powerful automobile in the world is helpless in a snow bank where no traction is provided for the wheels 1e, when the snow is too slippery to develop enough frictional force to push the car out Actually it is the push of the ground that makes the car start Nature of a Scientific Law It must be remembered that these three laws are accepted today as scientific truths The ·40 MECHANICS represent discou cries in the scientific sense, ie, according to the scientific method already described Accepting them as fundamental truths, it is possible to explain all the known phenomena in mechanics Of course it is possible to set up a logical framework of explanations of these phenomena in terms of other assumed truths, but experience has shown that these assumptions are not only most reasonable and natural—based upon experience --but that they are relatively simple Thus the physicist today does not attempt to cxplain Nen ton's laws of motion It is sufficient to explain all else in terms of them, and the fact that clas. sical physics can be so explained is one of the marvels of science itself To be sure, not all of the modern atomic physics has fitted so well into this beautiful pattern, but at least in an approumate way we can say that these laws of Newton's constitute one of the most important Inndmarks in the science of physics Today more inclusne theories must be considered but not to the exclusion of Newton's lans Newton's Law of Gravitation At this point we mention another all-important contnbution by Newton, the law of grautational attraction In connection with his lass of motion just discussed, it becomes necessary to associate the acceleration of gravity with the force called weight That is to say, come force of attraction between the earth and an unsupported body is neres. sary to account for the acceleration which the body has toward the earth Neu ton discovered that by postulating universal gruva tational attraction between all bodies in the universe he could, with the aid of his laws of motion, explain the motions of the heavenly bodies and so concluded that such a postulate was justia fiable This is now an accepted lax It states that every body in the universe attracts every other body with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them It is referred to as the inverse square lan of gravitational attraction The proportionality factor, now known as the gravitational constant, was evaluated quite accurately by Lord Cavendish in the latter part of the eighteenth century in England and more recently by P R Heyl of the National Bureau of Standards in Washington, using large, heavy balls and very delicate methods of measuring slight attractne forces By this time, the point must be clear that one of the goals of ·LAW OF GRATTATION 41 physics is ultimately to be able to explain et cry tluay in terms of the simplest assumptions, and express these explanations in the most comprehensive fashion Thus Newton's lats—three brief statements--and the law of universal gravitation express a wealth of information to those who understand them Of course it must always be remembered that physics, like all science, never attempts to explain the ultimate'why' of anything but rather the "how" of at Another such comprehensive and all inclusive statement, or law, is the las of the conservation of energy, which we shall consider presently, but first the concepts of work and energy must be developed The next chapter 1] deal with these concepts QUESTIONS 1 What is meant by a vector quantity? 2 How would the arrow used to represent a south wind differ from one representing a north wind? A west wind? 3 How would an arrow representing an upward pull of 30 lbs differ from one representing an upward pull of 50 lba ? 4 If three forces act on an object how would you proceed to find the resultant force? 5 A wire supporting a picture is attached to two points rather close together How does it happen that if the points to which the wire is attached are rather far apart, the wire might break? 6 If a heavy box is dragged at a uniform rate along a horizontal walk and then up an inclined walk, in which case is the greater force probably required? How 19 this explained? 7 Does the speedometer on a car register speed or velocity? 8 Can a car traveling at a constant speed have an acceleration? 9 If a spring balance reads 15 lbs when a given object is placed on it will it read more or less as it is carried to higher altitudes? Will it read more or less at the north pole? at the equator? 10 Will an equal arm balance which is balanced at the top of a mountain be balanced or unbalanced at sea level? 11 State Newton's three laws of motion 12 Why does the physicist not attempt to prove Newton's laws of motion? 13 What is the difference between centripetal force and centrifugal force? 14 Distinguish betn een kinematics and dynamics 15 What is momentum?

·CHAPTER IV MECHANICAL CONSIDERATIONS (Continued) 50 lbs WORK, EVERGY, AND FRICTION The Concept of Work. Work is a concept which, in physics, does not have quite the same meaning as in popular usage, there It seems to imply physical exertion In fact, it is difficult to tell just what is meant by the term "work" in a popular sense, in view of the many arguments over what constitutes and what does not constitute work It 18 a technical term here, and as such has to be used in a very exacting manner When a body is displaced, a force may or may not be acting upon it depending on whether or not the body is accelerated If a force is acting on a body when the latter is displaced and if the direction of the displacement is the same as the direction of the force, then uorh 13 said to be done on the body If the direction of tlic force should be perpendicular to the direction of the displacement or if there is no displacement, then, technically speaking, there is no work inft involved Thus work is an abstract concept 50 lbs defined as the product of force multiplied by the displacement of the point of application Fog 41 Fifty foot of the force in the direction of the force Sec pounds of work done fig 41 This is the same as the product of the in lifting fifty pounds displacement multiplied by the component of one foot vertically upward the force in the direction of the displacement In either event, the two quantities which are multiplied to yield work must point in the same direction, and furthermore, it is necessary (in order for work to be done) that a displacement take place From the foregoing it is clear that holding up a wall or a ceiling all day long involves no work at all in the technical sense because no motion takes place, although the process may produce con- 42 ·WORK AND ENERGY 43 siderable muscular fatigue Morcover, there would be no work involved at all in "pushing" a heavy trunk at constant speed along a perfectly smooth, frictionless (if such were possible) floor, OnDg to the fact that, in the absence of inction, the only forces acting on the trunk (10, the pull of gravity and the upward push of the floor) act at right angles to the displacement See fig 42 . Fig 42 If floor is frictionless no work is done moving trunk along at constant speed since no force is required This means, of course, that it is imposable to push anything (at constant specd) along a frictionless surface Although this frictionless condition can never be wholly attained, it can be ap. provimated as on slippery wax, or ice, etc, under which circumstances the force required is actually very small Work, then, is a very special concept, whose importance may not even yet be appreciated It is measured in foot-pounds or dyne-centimeters One dyne-centimeter is called an erg It will be recalled that the dyne is a very small unit of force-namely, that required to give a gram of mass an acceleration of onc centimeter per second per second Energy-Kinetic and Potential. The capacity to do work is also an important concept This is called energy Any body capable of doing work is said to be endowed with energy, of which man varieties have been classified There is energy due to motion called hinetic energy Sec fig 43 This is the type of energy possessed M by a moving automobile, as indicated by the damage which would result from a head on collision with another Fig 43 Kinetic energy 19 car It can be shown that kinetic possesed by the car if it has energy is expressible as one-half the & velocity product of the mass and the sccond poner of the velocity, but in the present development this relationship will not need to be emphasized A boulder on the top of a cliff al o has onor, bonosios them ·MECHANICS m boulder, if released is capable of doing work as it crashes down the side of the cliff Sec fig 44 This is called potential energy In the grandfather clock, the eights are raised and potential energy 19 ghen to the cloch by the person who winds it As this energy released the weights gradually fall, the mechanism is kept 11 motion, and work is done Coal, fuel oil, and dynamite, as well as coiled springs and atomic bombs, also possess potential energy because they are all capable of doing work Thus this abstract capacity to do work becomes a very fundamental notion h Indeed, it is a unique concept as me shall non see Significance of the Energy Concept As one contemplates the concept of energy Fig 44 Boulder has potential energy with one realizes that it is in all reality a most respect to lower les el comprehensive one, for the entire physical world can be thought of as a world of energy Potential energy stored up in the earth in the form of cool, oil etc by the sun-the source of all energy on this planet--is gradually transformed into the kinetic energy of motion, the chemical energy of plants and explosives, the energy of radio, electricity, light, sound, etc Indeed, each branch of physics may properly be said to be merely a different manifestation of energy Conservation of Energy The concept of energy further provides a possible approach to the whole study of physics because of a very important principle This is the principle which states that all the energy in the universe as conserved, or that this uni verse can nerther gain nor lose any of its capacity to do work This capacity can be transformed from one kind to another but not lost The implication is that energy takes on a sort of mate rialistic significance such that the various branches of physics merely represent different manifestations of it Nowhere can the store of energy be increased without a corresponding sacrifice elsewhere It must be remembered however, that energy really has no such material existence, certainly not in this discussion It is just a useful concept in terms of which certain explanations become possible, and about which the conservation principle pro vides the basis for a broad generalization The generalization is of ·POWER 45 far reaching consequences and is on a par with Newton's lans of motion in order of importance Needleag to say, it provides a cry useful method of analyais for the theorctical phyºicist as well as the practical man who designs machinery, for machines do work, even if the amount of work donc by a machine is often less important than the rate at which it is done Power The rate at which work is donc is called porer A large machine can be said to be more powerful than a small one merely because it can do the same amount of work in less time than the small one It 19 to be noted that the power of a person, a mechanical device, or an animal should not in any way be confused with the amount of work capable of being done A race horse, for example, pulling a light buggy at a high rate of speed may do errctly the same amount of work in a given length of trae as a draft hore pulling a heavy load rather slowly The commonly accepted unit of power is the horsepower, equal to 550 ft-lbs per sccond or 33 000 ft-lbs per minute The significance of the power concept is clearly indicated by the present-day methods of paying for labor, ie, by the hour or by the job Everyone realizes that for manual labor a man 19 more valuable than a boy and hence 19 paid more per hour On the other hand, a boy working half as fast and receiving half as much pay per hour as the man may accomplish the same total amount of work and acquire the same total pay, but of course in twice the time Some jobs however, call for a faster rate of ex penditure of energy than a man can provide, whereupon & more powerful device is required Machines A machine is defined as a device for transmitting and (or) multiplying force By the use of a machine a large resisting force can often be overcome by the application of a much smaller effort force In view of the preceding discussion it is now easy to see how this is possible The amount of work required to be done on the machine is never less than the amount done by the machine—Indeed it is always somewhat greater, because of friction--but, Eince work is the product of a force and a displacement, force can be reduced if the displacement 19 made correspondingly larger See fig 45 Thus the force can be halved if the displaccment is doubled because the product of the two remains the game With a black and tackle for instance, it may be necessary ·46 MECHANICS out ten fect of rope to lift a load two feet See fig 46 Therefore, except for friction, it should be necessary in this case to exert only one-fifth as much force on the rope as would be re- Fig 4.5 Work here is force F times 013- placement DW=FXDA smaller F acting through a larger D could produce the same answer quired to lift the load directly in this manner mechanical advantagc, defined as the ratio of the force overcome to the force appled, is acquired Sometimes a change in direction is the only advantage gained by a machine (sec fig 47), but in all cases the product of the effort force multiplied by the distance through which its point of application is moved equals the product of the Fig 46 A block and tackle with an advantage of 5 times Fig 47 Only an advantage of direction 19 gained by this pulley resistance force (including the resistance of friction) multiplied by its displacement, as a given amount of work is done In the absence of friction the mechanical advantage can be determined merely by considering the relative distances through which the forces are exerted In fig 46 fixe feet of rope must be ·MACHINES 47 pulled out of the tackle for every foot through which the lord is lifted This ratio, here equal to the number of strands connected to the movable pulley, 19 called the theorctical mechanical advantage as contrasted with the actual mechanical advantage, which is the ratio of the forces involved The ratio of the actual to the theoretical advantage 19 the efficiency of the machine For many purposes only the theoretical advantage 13 specified since this is readily determined by a consideration of the design of the device In other words it is usually possible to figure out how far a certain point must be moved by an applied force compared to the distance covered by the resisting force without even operating the machine Other Examples of Machines Another example of a machine is the ordinary lever See fig 48 It is obvious from the above discussion of the pulley that by means of a lever a large force can be overcome by the application of a smaller one because the smaller one acts through & larger distance The so called law of the lever, which 13 readily de Fg 48 The lever 18 & machine rived from the law of conservation of energy, states that the force applied, multiplied by its lever arm (previously defined), equals the force overcome, multiplied by its lever arm With a lever, very little work is required to overcome friction, ie, the efficiency 19 relatively high, because the small contact surface between fulcrum and lever does not allow the fnction force to operate through a large distance The inclined plane also operates upon the same principle as the lever Fig 49 The inclined plane and the pulley See fig 49 With this A small force f acting through device, advantage is taken of the fact ad stance D replaces a larger that the hypotenuse of a right triforce W Acting through & emaller distance H angle, ie, the longest side of the triangle exceeds the lengths of the other sides according to the size of the angles made between it and them Thus a heavy load can be raised from the floor to a table by sliding it up a plank stretched from floor to table, making a so called inclined plane The length of the incline being greater * ·48 DIECHANICS the elevation of the table, less force (except for friction) 13 required to push the body along the direction of the incline than to lift it directly Even if additional work is required to overcome the added friction introduced by length ening the path over which the body 13 moved, the inclined plane often makes possible thc raising of loads that would otherwise be imposable because of limitations such as are 2 imposed by human strength A modification of the inclined plane is the ordinary screw, a thread of which can be imagined to be unvound so as to produce a wedge Fig 4 10 The screw is really shaped figure which is nothing but & series of inclined planes of an inclined plane Sec fig 4 10 A circumference 2nr and pitch P relatively small force may suffice to produce a complete twist of the screw, causing it to advance, by a distance known technically as the pitch, against considerable opposition Similarly a relatively weak force applied to the blunt edge of a wedge can produce a large force perpendicular to the face and thus huge logs can be readily split open by forcing wedges into them The wedge is simply an inclined plane In each of the machines listed above (and many more could be + Friction The force of friction 18 a very important force in physics Although the engineer is often engaged in attempts to minimize its effects, as by the use of lubricants, to increase the efficiency of a machine, it is nevertheless an important and desırable factor in the daily lives of all Imagine hon one would start even the most powerful or most expensive car upon a frictionless surface Coefficient of Friction The force of friction is characterized by the fact that it always acts along the boundary surface between two bodies It is thus a tangential force and its magnitude is always proportional to the force pressing the two surfaces together See fig 411 The pull F due to the weight of the falling body just balances, and is therefore a measure of the force f of ·COEFFICIENT OF FRICTION friction when the body is caused to more at constant rałe The proportionality factor is known as the coefficient of fnction, bich 19 technically defined, for any pair of surfaces, as the ratio N ng الشند Fig 4 11 Friction force f 19 proportional to N, F =constant (coefficient of friction) of the tangential force of fnction divided by the perpendicular force pressing the two together The force of friction does not, as is commonly thought, depend upon the area of the surface in contact. The coefficient is of course different for different paus of surfaces Summary In the last two chapters we have seen how force and motion are intimately related We have considered the sector nature of force, a characteristic which is chared by many quantaties in physics The vocabulary of motion has been developed to the point where it becomes possible to understand Newton's fundamental laws of motion The concepts of work and energy have been developed and the very fundamental principle of the con fer ation of energy is thus given meaning We also have seen hos mechanical devices can be used to multiply force, and thus a better understanding of the operation of machines is had work, energy, force, and motion comprise that portion of physics commonly called mechanics Even with this rather brief survey of these topics, limited almost to vocabulary considerations, the student should acquire a better appreciation of this phase of the phyical world in which he lives Note In addition to questions after each chapter, sets of "Rever Questions" art mtrotated etter groups of chapters These are of the "multiple choice" type each question 13 pre- Eented with several answers, only one of which is correct, ·60 SIECISICS QUESTIOXS 1 Il a loud is to be roleed to a joint 10 ft highci thao i would you prefer to carn it up a icrtical ladder ab inclined plane? Which requires the gocater force'l casc will more work be required? 11ht? 2 llos much work is required to push a 50-1b body 10 a horizontal frictionlea plane at a constant clocity 3 bine is meant by cnern? Why is it important? 4 11hs are coal and gasoline said to contain energi? 5 Explain hos a force of 20 lbs can mosc a 200-1b inclinc which rice I ft in 10 ft. if friction is G 11t is poser? Is it possible for a so-called motor and a 'horecposer motor to do the eme as work? 7 11 hat is meant by the mechanical adı antage of a mr 8 1 hy is something wuch as hard to cut placed near of a pair of scissors? 9 Must a leser always be uscd so as to gain ads antage or is a reduction in force sometimes advantageous? . REVIEW QUESTIONS ($ce page 199 for One) CHATTENS I, II, III, AND IV 1 A hypothesis is 1 a necessary step in inductive reasoning, the same thing as a scientific theory , 3 al mais successful, 4 nei cr successful, (5) pot alloned in plıysics 2 Galileo I was the father of inductive science, 2 alnavs reasoned deductus els , 3 as a Greek scientist, 4 taught that heavy bodies fall faster than light ones, 5 lived in the nineteenth century 3 Physies is concerned with all of the following ercopt i force, 2 motion, 3 logic, 4 sophistry, 5 relativity 4 The hilogram is the basic unit of 1 mass, 2 length, 3 time, 4 momentum 5 acceleration 5 Measurements in physics are 1 always preciee, 2 usually indirect, 3 necessary to check theoretical speculation, 4 always made with micrometer calipers, 5 never made with micrometer calipers 6 The Grech period in physics is conspicuous because 1 of rallen 2 Aristotle favored inductive reason { ·REVIEW QUESTIONS 51 () () 10g 3 the Greeks delighted in manual labor 4 Arabic numbers had replaced Roman numerals 5 of deductive reasoning 7 Physics is primanly 1 an experimental science 2 & study in logic 3 an earth science A applied mathematics 5 engineering 8 Vodern phy aics completely displaces Newtonjan physics 2 is nothing but Einstein e relativity 3 started about 1800 4 explains why light is prop agated 5 is more inclusive than Mewtonian physics 9 The British standard of length 13 1 the foot 2 the yard 3 the inch 4 the centimeter 5 the meter 20 A force 1 always keeps & body going in a straight line 2 can never point upward 3 is a ve tor quan tity 4 19 necessary to give a body constant motion 5 15 a synonym for power 11 The meter is 1 a unit of inertia 2 equal to three feet 3 a fraction of the earth s rotational period 4 ten times & decimeter 5 100 millimeters 12 A force of 10 lbs exerted on a lever 10 ft from the fulcrum will overcome a load at 2 ft from the same fulcrum of 1 50 lbs 2 50 grams 3 200 ft. Ibs 4 200 lbg 5 100 lbs 13 The resultant of tno forces of equal magnitude making an angle of 120° with each other is 1 a force of the same magnitude 2 a velocity of the same magnitude 3 a force whose magnitude 13 equal to the square root of the sum of the squares of the two forces 4 zero 5 a force at night angles to either one 14 A velocity may change 1 only in two ways 2 in direction only 3 in magnitude and direction 4 only vertically 5 only by Newton 8 law 15 The acceleration of gravity 19 1 32 cm per eec per sec 2 32 ft per ecc 3 32 ft per sec per ecc 4 32 cm per sec 5 zero 16 Galileo dropped a large body and a small to ly & multaneously from the top of the lean ng tower of NA 3t ha pieces of metal ·() 52 REVIETY QUESTIOVS 17 Momentum is 1 mass times force, 2 force times velocity 3 force times distance, 4 mass times velocity, 5 mass times acceleration 18 The Cavendish experiment refers to 1 the determination of centripetal force, 2 the consertation of momentum in a collision, 3 the gravitational attraction between bodies, 4 the constancy of the stretch of a spring to the force acting on it, 5 the determination of the acceleration of gravity by dropping a ball upon a pan pivoted to make pencil marks on a rotating drum 19 A freely falling body falls in the first second 1 32 ft , 2 32 in , 3 980 cm 4 16 ft ,5 64 ft 20 To keep a 10 lb body traveling with constant speed of 5 ft per sec in a straight line requires a force of 1 50 lbs, 2 10 lbs, 3 20 lbs , 4 2 lbs , 5 0 lbs 21 Newtons second law of motion leads to the conclusion that I forces appear as twins, 2 all bodies are attracted toward the center of the earth, 3 force is mass times acceleration, 4 & body at rest continues at rest unless a force acts, 5 weight is the force of gravity 22 Maas is the quantitative measure of (1) inertia, 2 gravity, 3 weight, 4 momentum 5 displacement 23 Velocity has 1 force, 2 magnitude 3 displacement, 4 mass, 5 weight 24 Galileo lived at about the time of 1 Aristotle, 2 Democritus 3 Plato, 4 Newton, 5 Maxwell 25 A body traveling in a circle with constant speed 1 is accelerated 2 has constant velocity, 3 is not accelerated, 4 does not move, 5 is not affected by gravity 26 If body A is projected horizontally at the same instant body B 13 dropped from the same point and if air resistance is neglected 1 B will reach the ground first 2 A will reach the ground first, 3 Awill go farther than B, 4 Asill have a smaller displacement than B, 5 the displacement of each is () () zero 27 Acceleration is 1 the time-rate of displacement, 2 the rate at which distance is covered, 3 the timerate of change of velocity, 4 the time-rate of ·REVIEW CESTIOS 53 () change of speed, 5 the time rate of change of force 28 Work is measured in 1 ft-lbs, 2 horecponer, 3 grams, 4 ft-lbs per scc, 5 lba 29 Tork is the product of force and distance 1 regardless of direction, 2 neier, 3 when the two are mutually perpendicular, 4 when the two point in the same direction 5 only in the absence of inction 30 The work done in moving a 10 lb body 5 ſt hon zontally on a frictionless surface 13 1 10 ft-lbs, 2 500 ft-lbs 3 zero 4 50 ft-lbs , 5 500 lb ft 31 The work done when a 50 lb body is lifted 10 ft vertically is 1 500 ft-lbs, 2 zero, 3 500 joules, 4 250 ft-lbs, 5 32 ft lbs 32 The capacity to do work is called i power, 2 en ergy 3 mechanical advantage, 4 momentum, 5 efficiency 33 A2 horsepower machine pecessanly 1 can do thice as much work., 2 works twice as fast 3 erpends twice as much energy 4 expends four times as much energy 5 works just as fast as a 1 horsepower machine 31 A 138 lb man climbing a flight of stairs 20 ft high in 20 seconds displays approximately 1 125 HP, 2 one-hall H.P 3 one quarter HP 4 138 ft-lbs per minute 5 one HP 35 If the velocity of a body is doubled i the kinetic energy is quadrupled 2 the kinetic energy 13 halved 3 the potential energy is doubled, 4 the potential energy 19 halved, 5 the Linctic energy 19 unchanged 36 By the use of an inclined plane to raise a heavy body up onto an elevated bench 1 more work is probably done 2 less work is probably done, 3 more force is required, 4 the same work 18 probably done 5 the same force is probably required than without the use of the inclined plane 37 The mechanical advantage of any machine 13 1 the ratio of the force os ercome to the force applied, 2 the same as the efficiency of the device, 3 the ratio of the force appbed to the force overcome, 4 always two, 5 always zero () C) ·monic motion 56 ELASTICITY, VIBRATIONS, FLLIDS tion is called simple harmonic motion See fig 5.3 The E tance of this kind of motion lies in the fact that it is pro whenever & distortion of an elastic body is released an resultant vibration takes place in a str line There are more complicated tv vibrations, of course, but all straight-lo brations of small amplitude are simple monic to a first approximation Characteristics of Vibratory Mo A simple harmonic vibration, being : odic motion, is often specified by it quency, defined technically as the num complete vibrations ss hich take place i second of time Thus if a boy on A £ Tag 5.3 Strip of iron board jumps up and down two time an a vise Another exsecond, his frequency of vibration ample of simple har. vibrations per second This 13 equn ale saying that one-half second is require one vibration The length of time required for a single vibr 19 known as the period of vibration The period, then, is th ciprocal of the frequency, ie, it is unity divided by the quency These two concepts, frequency and period, proud means by which vibratory motions are usually specified in a g titative manner The amplitude of a vibration is the maxi displacement measured from the equilibrium position, and 1 to be confused with the total to-and-frosting, which is tric amplitude Resonance All bodies have natural periods, or frequen of vibration depending upon their masses, their geometric c acteristics, and the manner in which they are set into vibra Because of this fact the phenomenon of resonance is import A body is found to be into vibration very readily when af acts upon it periodically with the body natural frequency child readily learns how to "pump" a swing, ie, he learns he can give the swing a vibration of considerable amplitude i properly times his impulses in accord with the natural perio t'ne swing T'ne diver on t'ne springboarà also learns how to's advantage of the natural frequency of the diving board to incre his initial velocity Resonance effects are sometimes undesirable and are then t ·SIMPLE PE DI LUV SWS 57 avoided Frequently it is observed that at certain speeds & car vibrates more readily than at other speeds This is due to a coincidence of the vibration frequency produced by irregularitics in the road and that produced by the rotating motor Tro vibra tous with the same frequency are said to be in phase with each other sf both start out together Resonadce occurs when two 1 lar vibrations are in phase with each other and does not occur when they are out of phase The Sornple Pendulum Another vibratory motion closely resembling simple harmonic motion is the motion of the simple pendulum This is simply a small bob suspended by a relatively long flexible cord and set into to. and fro molion See fig 54 The interesting characteristics of pendulum motion were supposedly discovered in the scientific sense by Galleo He found that the period of vibration of a pendulum depends upon its length and the acceleration of gravity at the place where ić 18 located Although this 19 not a direct proportionality the relation ship is relatively simple He also discovered that F8 54 The 8 mple pendu the period is the same whatever the amplitude of tum the swing providing it is not very large The pen dulum of the grandfather clock although not technically a rim ple pendulum illustrates these characteristics The student can readily infer and correctly so that one method of determining the acceleration of gravity at & giren place is to measure (rather precisely of course) the period of a simple pendulum Thus method is employed by the United States Coast and Geodetre Survey which keeps the scientific world posted with such infor mation at specified locations on the earth surface The Study of Liquids a Branch of Elasticity Before go ing on to a further consideration of vibrations and their effects on matter as a consequence of its elastic properties a strictly logical presentation of the phenomena of physics requires that He stop here to allow for the study of another group of phenomena which result from considerations of elasticity namely liquid phenomena Elasticity was defined as the ability of matter to recover from distortion If one were to list various types of distortion such a list would include emple atretch Isnear compresion or er all compression or expansion and twists or shears See fig 55 T ·58 LLAS RICITY, VIBRATIO\S, FLUIDS last of these types, the shear, in which the force is applied tangentially to the surface, plays an important role in physics 10 that it provides a means of classifying matter Matter is readily classified into two main divisions called solids and fluids according to whether or not resistance 13 offered to a 17 Shear Over al Compc จ Smple Stretch Fig 5w Various types of distortions. shearing type of force Although it is all relative and a sharp distinction is not possible in cieri case, the general classification 13 useful The significance of this is that fluid will not withstand a transverse force Also, it is impossible to tw a column of fluid For convcnience, fluids are subdivided into two groups, liquuls and gascs, depending upon hether or not they display a free surface Thus a jar can be only half full of water, but with a gas it is different If the jar contains nothing but air it is always com- Ar Fig 50 A container partly full of a liquid displays a free surface but a gas always completely fills its container pletely full, whether it contains a little or a lot, because the air displays no free surface See fig 56 Now it is clear that the study of liquids at rest-hydrostaticsexemplified by water in free ponds, open columns, wells, test ·DEX SIT 59 tubes, etc , invokes only the action of gravity upon the liquid as a whole, ie, a simple external force, and does not involve internal forces of the shearing type, because only forces perpendicular to the surface of a liquid can be applied Thus a free liquid surface is called level because it is perpendicular to the plumb line, which 18, strictly speaking, merely the direction of the pull of gravity Density The study of fluids requires special vocabulary The concept of density, although not restricted to fluid considerations, is all important to the logical development of this branch of physics because it is one of those concepts in terms of which others are derived The lay man is more or less familiar with the term density and generally speaking, uses it to differentiate between more and less massive substances For example, a marked distinction between lead and aluminum in terms of mass 13 readily appreciated, but the technical definition of density as mass per unit volume is not such a common notion See 57 Density B с Fig 57 A pound of (A) lead (B) aluminum (C) feathers illustrating d fferences in density is defined as the ratio of the mass of a body divided by its volume (grams per cubic centimeter) je it is mass per unit volume The distinction between weight and mass again comes up Although density is correctly expressible in pounds per cubic foot (mass pounds) it is not correct to refer to it as weight per unit volume in spite of loose practice on the part of many tech nical people Not that weight per unit volume is an undesirable concept, it simply is not density Sometimes this is referred to as weight density Again we note that strictly logical thinking often requires painfully precise definitions and careful usage, without which false conclusions may be reached ·60 ELASTICITY, VIBRATIONS, FLUIDS Specific Gravity. The term "specific gravity" is also a term heard quite frequently today car owners are often concerned with the specific gravity readings of their storage batteries What does the terma mean? Technically the specific granty of a sub stance is defined as the ratio of the density of the substance divaded by the density of water It provides a way of expressing the relative mass of a certain volume of the substance with respect to the same volume of water Specific gravity, then, is just a number Fluid Pressure The concept of fluid pressure, already mentioned, will be considered next. Because no shearing force can be exerted on a fluid, it follows that the only force which can be exerted on a fluid must be directed perpendicularly against its surface Recall the reference to the sailboat in the chapter on forces Thus the walls of a flud container push against the fluid, but only perpendicularly Furthermore, the total force so im pressed obviously depends upon the total area upon which it acts In terms of the perpendicular force per unit area, such a total force is given by multiplying it by the number of units of area This force per unit area perpendicular to the surface is designated as flurd pressure Thus it makes sense to state that the pressuie in a given water mam is 80 lbs per square inch, meaning that upon each and every square inch of surface 80 lbs is acting regardless of the size of the pipe An interesting consequence of this is the not so easily realized conclusion that a force of a more fraction of a pound may suffice to stop a leak if the cross-section area of the leak is only a small fraction of a square inch Thus a very careful distinction must be made between the concepts of pressure and total force if one is to reason intelligently about these matters It can be further shown that pressure at a point below the surface of a given flud depends upon the depth of the fluid This of course makes it very easy to calculate the pressure of water at the base of a standpipe, for example Moreover, since the pressure at a given place thus depends only on the depth, it follows that in two open vessels communicating with each other at the bottom and containing the same liquid, the level of the liquid in one will be the same as in the other This is because the pressure at the bottom where they communicate with each other cannot have to different values, as would be required if the levels were different Thus liquids are said to seek their own level under these circum ·BLOYAS 01 <tances Sec fig 5.8 For this reason people often find it necessary to install cellar pumps a hou-es sf the water ind in the land about the houece 29 higher than the basement foors, www. Fig 58. A liquid "arels its own level" in communicating resu kels regardles of the shape of the containers, 2 Buoyancy It is a common experience that bodies with specific gramaties leas in vaagnitude than unity, ie, with fractional specific gratites, float rather than sink in water Sec fig 59 This brings out an important concept called buoyancy, with which the fundamental principle of Archimedes 13 concerned When a lods is emersed either wholly or partly in a furd, obviously a certain amount of fluid 39 diplaced From a conexderation of the pressure which the fluid exerts on the hody at folloss that Wood Iron Water Toont Hosts a water Ita de sity 29 less than that ol water Mercury Iros Also floats 33 mercury Fig 59 the net eficct of the pressure forces 18 & resultant force upward *hakiroda torony nord the fose, if the weight of a body as less than the weight of the fluid which it would deplace if submerged, the body will float in the ffurd, and sice versa, if it 13 heavier than the same volume of the fuld it w:]] sink Archimedes' Principle 13 the formal statement of this thoroughly tested cocclusion It states that all bodies com- ·62 ELASTICITY, VIBRATIONS, FLUIDS pletely or partially immersed in a fluid find themselves buoy ed up by a force equal to the weight of the fluid displaced The principle explains the action of the floating type of hy drometer universally used in garages to determine the specific gravity of automobile batteries A bob either floats up to a certain mark or it does not, depending upon the specific gravity of the solution in which it is floated Sec fig 5 10 Thus the degree of electrical charge in the battery is determined because it depends upon the specific gravity of the solution Gases According to the definition already given, gases are fluids and so it should, and does, follow that the foregoing applies to the Fig 510 A byocean of air which surrounds the earth and drometer to meas constitutes our atmosphere Air has mass and the specific therefore deneity Balloons are buoyed up in gravity of liquid air in accordance with Archimedes' principle just as ships are buoyed up in water See fig 5 11 By changing the ballast a balloon can be made to rise or fall at wall just as a submarine can be made to rise or sink in water ure ICC Balloon Shp Fig 511 Balloons are buoyed up by the air just as ships are buoyed up by Fater Atmosphere Pressure and the Barometer The earth's surface is at the base of the atmosphere and hence a hydrostatic ·ATMOSPRERIC PRESSURE 63 preasure exists due to the reight of the air This so-called at mosphenc precaure amounts to approximately 15 ibe per square inch and is readily measured by the sanous types of barometers available today One can appreciate the magnitude of atmospheric pressure by the following experiment A slender glass tube come three feet long 18 filled with liquid mercury The top is then clo cd by holding ones thumb over the exposed end If now the tube is inverted and the end which is closed by the thumb is immersed in a dish of mercur, it is found Approx 30 nchas upon removing the thumb that the mer cury does not all run out of the tube Sec fig 5 12 A column some 30 inches high remains in the tube with a vacuum above Mercury 3t Thug at 39 epen thet the preseure at the Fig 512 Barometne bottom which can be thought of as the Columna place where a tube of mercury communicates with a vessel of air (the whole atmosphere) 19 just that which exists at a point 30 inches belos the surface of mercury, hguid whose deasity 29 betn een thirteen and fourteco times that of water Thirty suches of mercury corresponds to approximately thirty four leet of nater, and so lift pumps can race water up out of a well not more than thirtz four {ect This is becavee the action of the nump 19 merely to remove the ass from the pipe above the water until the atmospheric pressure forces thc water up to take its placc Measurement of Altitude The preasure of the atmosphere varies slightly from time to time almost continuously due to variations in meteorological conditions Many predictions of weather are b3eed to a large extent upon barometric pressure Altitude can also be measured by the barometer because the pressure decreases with altitude This 13 obuous in view of the fact that at high altitudes the air above, given point weighs les3 than that above a point at lower altitude there beibg less of it 22 the former case The aviator utilizes this praciple to measure huis elevation The height of Mt Katahdın, the highest mountain in Name, was first measured by observing the drop in a mercury barometer as it was cared from the base to the summit of th mountain ·64 ELASTICITY, VIBRATIONS, FLUIDS Pascal's Principle. Going back to the discussion of liquid pressure, another interesting consequence follows from fundamental considerations If two vertical tubes of different croeg cection are joined together and are partially filled with a liquids as to leave two open surfaces of different sizes at approximatek the same level, then the application of additional pressure to one 114 Fig 513 Pascal's Principle A small force f act- Llein halance larger cross ators, barber chairs, etc side is transmitted without loss to the other See fig 5 13 This is known as Pascal's Principle This principle is utilized in the operation of many practical devices, including the hydraulic brakes on the modern automobile As a consequence of the pressure transmission without loss, the total force on the small surface in fig 5 13 is proportional to the total force on the large surface in just the same ratio as the areas themselves Thus a 10-1b force on a 1-inch area is multiplied to 100 lbs on a 10 inch area because of Pascal's principle, the force per unit ares being the same in each cace Applications of this principle should be obvious The hydraulic pre-s acts because of its two cylinders of unequal area connected together In the case of the automobile brake pedal, which creates a certain pressure in a tube of brake fluid, the pressure is transmitted through the pipe lide to the brake drums in the wheels, where it acts upon pistons of larger area to create large b' forces The hydraulic elevator and the hydraulic lift at modern filling stations represent further applications of the same principle ·SUMMARY 65 Summary In this chapter we have seen that matter displays elasticity This property accounts for vibratory motions such as are exemplified by springs of all kinds Simple harmonic motion is the simplest type of vibratory motion Matter is divided into two classes, solid and fluid, according to its elastic characteristics Liquids and gases are both fluids Fluids dieplay ecveral unique properties such as buoyancy, pressure transmissibility, etc Archimedes' principle gives the relation between buoy ancy and fluid displacement The atmosphere is a fluid which exerts a pressure on all bodies This is measured by the barometric column, which is also utilized in measuring altitude Pascal's principle accounts for the operation of many machines such as elevators and hydraulic automobile brakes Thus it is seen that the study of fluids is a very important part of the study of physics QUESTIONS 1 What is meant by elasticity ? 2 Would you say rubber is more or less elastic than steel? Why? 3 Describe the operation of the spring balance 4 Distinguish between stress and strain 5 What is meant by the period of a vibration? 6 What is the amplitude of vibration? 7 How does the period of vibration of an automobile change when an additional load is placed on it? 8 Describe how the acceleration of gravity might be meas. ured by means of a simple pendulum 9 That is the technical distinction between a solid and a fluid? 10 Why is water said to seek its own level? 11 How does the "cuction" pump really work? 12 Is a 'suction" pump suitable for pumping water from a level 37 feet below the surface of the ground? Why? Explain 13 Is the reading of a barometer affected by carrying the instrument up a mountain providing the temperature is left constant? 14 What is Pascal s principle? Give an illustration 15 Why does an ol leak in one brake of a hydraht brakeequipped automobile ruin the braking action of all four brakes?

·CHAPTER VI WAVES AND SOUND All the topics thus far discussed under the general heading of elasticity, including the topic of fluids at rest, are commonly grouped under the heading Statics of Elasticity-a branch of the Mechanics of Deformable Bodies 11 hen deformations are prop% gated through matter, so-called elastic u aves are produced and their study is properly referred to as the study of Dynamics of Elasticity Fundamental Wave Concepts It is a common observation that deformations may be transmitted through material media A small distortion, such as a localized compression or a trans verse shear, readily passes along the whole length of a long coiled spring See fig 61 Moreover, everyone realizes that a 0000000000000000000 Fig 61 A compression will travel along a coiled spring stone dropped into a still pond produces first a depression in the water and then & succession of circular mounds and depres sions which spread out and radiate over the entire surface of the pond, moving outward from the central point of disturbance with a definite speed See fig 62 This phenomenon 19 called wave Stone Fig 63 Cross section of the exceler mounds and dem pressions propagated from the point where & stone into a pond The medium just brates About 1 position 66 ·WAVE CHARACTERISTICS motion Technically, uave motion is defined as the propagation of deformations through a deformable medium It is usually a periodic disturbance and is produced by & vibrating source The medium is set into a succession of vibrations, each specific part moving back and forth about its own point of equilibrium, but the disturbance travels It should be noted carefully that the medium itself does not travel Indeed, it is more accurate to say that only energy travels Wave Characteristics. Waves have numerous characterstics The speed of a wave is a very definite matter By means of a stop watch one can readily time the passage of a given wave across a known distance and therefore determine the velocity of the wave as it radiates out from its origin The distance be. tween two successive wave crests, or between two successive wave troughs, or more generally between two successive similar configurations, is referred to as the length of the wave or the uave length See fig 63 A traveling train of waves also has a frequency, le, the pumaber of crests that pass a given point per second Obviously the following relation holds the distance between two successive crests (wave length) multiplied by the number of crests per second (frequency) just exactly equals the velocity of the wave Wave length A Fig 63 Velocity equals wave length times frequency motion This is probably the most fundamental of all wave relationshups It is also obvious that the frequency of the wave motion is just the same as the frequency of the disturbance producing it For example, a vibrating reed sets up a wave motion in the medium in which it is located, such as air, with a frequency just equal to the number of times it makes a complete vibration per second It is also a fact that the length of the wave thus produced depends upon the elastic characteristics of the medium, such as the density and the stiffness, etc This is usually expressed ·68 VAVES AND SOUND in terms of the velocity, which, as has just been shown, depends directly upon the frequency and the wave length Another fundamental concept is the amplitude of a mare, de fined as the maximum displacement from the equilibrium po sition See fig 63 again Types of Waves Waves can be classified according to type There are transverse waves such as the ripples just mentioned when a stone is dropped into a still pond These are characterized by the fact that the displacements of the medium are always perpendicular to the direction of propagation (the surface of the water moves up and down as the wave progresses horizontally) There are also longitudinal waves in which the displacements are always back and forth parallel to the direction of propagation Taves such as sound waves in air are of this type Coropressions and rarefactions of the medium pres along in a direction parallel to themselves These are the two most common types and the ones to which elementary discussions are usually limited, but tor sional, circular, spherical, and ellipsoidal waves, just to mention A few, are considered in more advanced treatises Representation of Waves The waves produced by pure simple harmonic disturbances are technically known as sine waves, and are readily represented by the familiar "wavy line' А t Fug 64 All waves can be represented diagrammatı cally by the simple wave picture Although it should be remembered that saves do not have to be as simple as these, nevertheless waves in general can be repre sented diagrammatically by the simple wave picture See fig 64 Interference of Waves The trave picture affords, among other things, a simple means of representing the superposition of waves, a common phenomenon 11 hen tno nates are superimposed (as for example two water waves) the result depends upon conditions If the two similar waves of equal amplitude arc in phase, ie, of crest coincides with crest and trough with trough, the result is a reinforced ware of twiec the amplitude of either ·03 INT2PZPECE ASD BEATS On the other band, the reult es complete culi5eation of both f the two waves happen to be just out of phase, 14., if the rest of one coincides with the trough of the other There may also be con- 1 film Rest tart steny Fix 6.5 Constructive and destructure interference O 277 The waves in part 3 represent the resultan's ch three n 1 ad 2 ditions in between complete reinforcement aal complete cofiation This general phenomenon is referred to 2* interferenc", 2:// includes both constructie and destructure types See fiz. 6.5 Beats As may be inferred, interference plecam2 2 very important role in the study of wales Wratto cc=******* waves of lightly different frequencies are euperimposed the rest is an alternation of constructive and destructrie te larte.com АДЛАЛАА Fig 66 Beats are produced by two vares of alt the am frequency coming into phare with each other Eary 09 12 a while known as the beat phenomenon, who-e frequent exactis equal to the difference between the two ongoal frequencies bee fiz eh This is easily appreciated in the case of sound waves tr, seríkrar tro tuning forks of nearly, but not exactly, the game free to my ·70 WAVES AND SOUND Reflection of Waves Waves are reflected at boundars betrs een media Just as a tennis ball can be bounced up from the ground, ie, reflected, so can water saves be reflected from an obstruction back into the medium in which they originated A complete analysis of the phenomenon is not possible here, but suffice at to say that it involves chicfly a further study of the clastic properties of matter Refraction of Waves The changing of speed as a wave passes from one medium to another is technically known as refraction See fig 67 This is another important concept which we shall run across later, particularly in the study of optics V V2 Aant Rare Dense Fig 67 Refract:on of waves Waves may undergo a change in velocity-e be re fracted as they pass from one medium to another Going from a rare medium to a denser one they are slowed up and the wave length is decreased Standing Waves 11 hen a nave traveling in one direction in & guien medium interferes with a wave of the same length travel ing with the same speed but in the opposite direction, an interesting situation develops The result is a wave motion of a very unique type Every portion of the resulting vibration passes through the equilibrium position at the same instant There are certain points in the vibrating medium which are always in equilibrium and other points where the disturbance is a maxi mum The pattern is called a standing wave pattern because the waves do not progress Thes are stationary The points of mini mum disturbance are called nodal points or nodes, and the points of maximum disturbance are called antınodes or loops See fig 68 This condition is realized when a wave passing along a medium meets its own reflection coming back with the same speed and wave length from a bound iry Obviously resonance effects are readily appreciated under these circumstances when the phase ·OVERTONES 71 relationships are favorable Ossing to the existence of the natural subration frequencies already mentioned, standing waves are readily set up in bodies Thus it is natural for a violin stnng fired at either end to vibrate in a standing wase pattern with a loop 1 ܬ L Fig 68 The nodes and loops of standing waves produced by two waves of the same length and amplitude but traveling in opposite directions at the same tume at the center and with nodes at the ends Similarly it is easy to make a column of air in a pipe, open at each end, assume a standing wave pattern with loops at either end and a node at the center Overtones As a matter of fact, there are many natural modes of vibration for a string or an air column or for any body at all Whereas it is natural for a string, such a violin strine fixed at each end to vibrate with a loop at the center ie single segment, it is also natural for it to vibrate in two segments with a third node at the center Moreover, vibrations in three or more segments are possible Indeed, it is possible for such a ag a Fg 69 A string vibrating in (1) one segment (2) two segments (3) three segments (4) four segmentse in ats fundamental and 1st 2nd and 3rd overtone frequencies string or air column to vibrate in any integral number of RCA* ments See fig 69 These various modes are referred to not tone vibrations with respect to the single segment type, culty the fundamental mode The length of the wave of the first elit tone in question is obviously just twice the length of the si ·74 WAVES AND SOUND frequency is decreased and the wave length is increased as the source and the observer recede from each other In the case of sound, the result is a lowering of pitch in the latter case and a rise in pitch in the former This principle, when applied to light waves, gives the astronomer the remarkable power of determour Observer F1g 6 13 Doppler's Prociple As the fork moves to the nght with the velocity v the waves are crowded and appea to arrive at 0 with a greater frequency than otherwise ing the speeds of certain stars and planets when their motion 1 directed toward or an ay from the earth It is obvious that tel. scopic means alone are insufficient to supply this information except when the motion is across the sky, se, perpendicular to the line of sight Sound as a Wave Phenomenon Numerous references hav been made to acoustical phenomena in this discussion of waves This is because sound is one of the most common of all the war phenomena Sound is nothing but a wave motion in ma (usually air) of a purely mechanical type Although it is thought to be restricted to those elastic vibrations capable o stimulating the sense of hearing, there is no particular reasot for such a restriction In a more general sense, the term "sound applies to any and all elastic waves in matter Today the field a supersonics is recognized This is a field in which vibrations about the audible range are considered The human car is restricted, 11 its ability to recognize sounds, to a range of frequencies' approximately 20 and 20 000 vibrations per second Thus an al dible sound 13 recognized when anything vibrates with a suitat intensity and frequency within these limits in a medium capable of supporting the vibrations and propagating them Pulses of all suffice to produce sounds regardless of how they are produced, ·CHARACTERISTICS OP SOUNDS 75 whether by an intermittent blast as through holes in a rotating disk or by the vibration of reeda, bells, air columns, strings, or mer may readily be seen to vibrate of the whole device is mounted in a transparent exhau-ted glass container The velocity of sound in air is approximately 1100 feet per second Characteristics of Sounds Sounds are recognized by three characteristics loudness, pitch, and quality LOUDNESS Loudnesg 19 a function of the intensity of the disturbance The volume control on a radio governs the loudness of the sounds emitted and, if the set is properly designed, affects to other characteristic PITCH Pitch is the auditory effect of the frequency of the sound The human ear is capable of detecting slight changes in pitch Pitch is a characteristic by which high notes are dıstıngrushed from low notes on a musical instrument A range of frequencies in which the frequency is doubled is called an octave Thus we have several octaves on the standard piano keyboard such as, for example, the range from middle C approximately (256 vibrations per second) to upper C approximately (512 vibrations per second), which constitutes a single octave Actually this scale is an old one on the modern piano this range is from about 261 to 523 vibrations per second It is the pitch of a note which 18 affected by relative approach or recession of source and observer according to Doppler's principle already mentioned It is sometimes difficult to distinguish between the effect of pitch and loudness upon the human ear The lower limit of loudness capable of being detected by the average ear, or the threshold of hearing, vantes considerably with the pitch of the note, as has been shown in recent years by experiments at the Bell Telephone Laboratories in New York City, a higher intereity being required for the lower frequencies than for the higher ones There is a slight discrepancy between the musician's standard of pitch and that of the scientist Whereas 256 vibrations per second has been recognized as middle C by scientific workers, the musicians favor 440 as A above middle C This is somewhat different from the 427 which would be required on the scientific scale, and also means that the musician's middle C 18 261 ·Vi 76 WAVES AND SOUND tions per second instead of 256 As a matter of fact, it 13 fair's generally agreed that 440 for A is universal musical practice today QUALITY The quality of a sound is determined by the exist ence of overtones This means that if it were not for the difference in overtone patterns, a given note would sound just the same when played on the piano, the violin, the cornet, or any other musical instrument Stripped of their overtones, the came notes played on these several instruments would be indistinguishable and furthermore such pure notes would lack all semblance of quality Thus the only reason why a genuine Stradivarius violin may produce more pleasing notes than an imitation instrument 15 the difference in overtone patterns resulting from the elastic prop erties of the materials used in construction and from the construc tion itself Tuning forhs usually produce very pure notes se brations consisting almost entirely of fundamental vibration modes completely free from overtones and N for this reason they are often used in the laboratories for standards of pitch Standing Waves in Pipes Vi N brating air columns have the in Open Closed teresting characteristic that when Fug 614 The fundamental produced in pipes closed at one end wave length of an open p pe 19 twice the length of the pipe and open at the other they lack all whereas the fundamental wale the overtoncs corresponding to the length of a p pe closed at one odd integers considering the fun end 19 four times the length of damental as the zero overton Pipes open at both ends are not so restncted in overtone possibilities See fig 6 14 Thus it is fair to say that so called open organ pipes are richer in quality than so called closed orgap pipes See fig 6 15 Analysis of Sounds The quality of a musical note can readily be determined by the use of instruments which are able of detecting all the various components in a complicated sound The trained ear can of course do this to a certain extent as is evidenced by the manner in which the various instruments can be detected in a symphony Orchestra but electrical apparatus ha M the p pe ·BUNNARY 77 - Antinode Node Fig 6 15 Ap open pipe (one open at each end) can have twice as many harmonic overtones us a closed pipe L I. as many been developed which can do this more satisfactorily than the average ear Summary We have seen in this chapter that the whole physical side of music is in reality a branch of physics We have considered the subject of sound as a part of the more general bubject of vibrations and their associated waves as fundamental concepts in this descriptive survey of physics It has been found that these concepts are just as logically organized and interrelated as the concepts of force, mass, energy, etc, discussed in earlier chapters Again it has been shown that physics 19 a highly organized and logically developed science, to understand which the student must acquire a special vocabulary and use it with utrcost precision QUESTIONS 1 What is meant by sound? 2 Does the frequency of a sound influence the speed at which the sound travels? Do the high-frequency notes of a distant orchestra reach you before the low-frequency notes of the same chords?

·78 TAVES AND SOUND 3 Is there any part of a gong which remains stationary while It is sounding? 4 Note some of the factors that influence the quality of a sound 5 Which one of the three factors-pitch, loudness, and quality-enables one to distinguish one orchestral instrument from another? 6 What happens when tro sound waves of the same frequency meet out of phase? What is the result if they meet in phase? 7. What happens lien two sound waves of slightly different frequencies are combined? 8 As a violinist tightens a string on his instrument is he increasing or decreasing the pitch? 9 Does the loudness of the sound from a tuning fork increase when the fork is set on a table top? Why? 10 Explain the formation of an echo 11 A fork produces a frequency of 1000 vibrations per second What could be the frequencies of a second fork which, combined with the first, would give a beat frequency of 5 beats per second? 12 How may sound waves be produced? 13 How is sound energy transferred? 14 After a filash of lightning, thunder may not be heard for several seconde Why? 15 How does the sound from an open organ pipe differ from that from a closed pipe? 16 A whistle is blown and after 3 seconds an echo 19 reflected from a chiff How far away is the chiff ? 17 Are sound waves of the order of magnitude of inches, feet, miles, or millimeters in length? 18 The acoustics of a room filled with people are better than those of an empty room Explain 19 Can there be too much absorption of sound in a room or auditorium from the point of view of good acoustics? 20 What is meant by the threshold of hearing? Does this threshold vary for different frequencies? The ear is most sensitive to what range of frequencies? 21 Define or state what is meant by "save motion" and "alastusty" 22 What is meant by the terms "standing wave," "node," "antinode"? 23 TVhat are "beats"?

·CHAPTER VII MATERIAL CONSIDERATIONS CONSTITUTIOV OF MATTER PROPERTIES OF GASES SURFACE EFFECTS One cannot proceed very far today in the study of the physical world, ie, the study of physics, without a consideration of the structure of matter Although, to some, this appears to be the realm of the chemist, it is well known to the initiated that there is no sharp boundary between the fields of chemistry and physics This is due in large measure to the fact that the chemist and the physicist working together have, in the last 50 to 75 years, made considerable progress in their attempts to unravel the mysteries of matter Only by the use of the physicist's tools, such as the spectroscope, the X ray, the electron tube, the mass spectrograph, the high-voltage generator, and others too specialized to mention here, has the chemistry of matter advanced to its present stage Chemistry has also furthered the knowledge of many physical properties of matter Physics and chemistry together, therefore, have developed theories of the structure of matter around the concepts of molecules, atoms, electrons, protons, and numerous other particles in a continuous state of motion Molecules and Atoms. Present-day belief is based on the assumption that all bodies are built up out of molecules which are defined as the smallest known units of given chemical sub stances It furthermore seems evident that molecules themselves are built up out of smaller component units called atoms In . 11 + units. As recently as fifty years ago science accepted the view that the atom was an indivisible entity-hence the name 'atom – but since that time knowledge pertaining to the structure of matter has mcreased enormously It seenis certain today that the atom 79 ·80 NATTER, GASES has an internal structure of its otra Starting with researches during the very last part of the last century and running all through the present one, the view has become scientifically substantiated that atoms are made up to a large extent of charges of electricity A very consistent theory was presented in 1913 by the Danish physicist Bohr, who pictured the atom Fig 71 Bohrs concep as a miniature solar system with units tion of the hydrogen atom cons sting of a negatively of negative electricity revolving about charged electron resolving a central, predominantly positively in an elliptical orbit about charged nucleus in a more or less ellipa rositas cly charged nu clcus (proton) tical orbit, as the planets revoke about the sun See fig 71 Although this theory is considered outmoded today, many of its features are still retained by contemporary physicista Positive and Negative Charge The terms positive and negative as used in connection with electricity merely indicate a distinction between two opposite types of clectrical charge as determined by the forces which they exert upon other charges, either attractive or repulsive When, for example, a glass rod is rubbed with a piece of silk, or when a piece of hard rubber is rubbed with fur, each of these objects acquires the peculiar property of attracting small bits of paper or other very light objects * + + + + + + + Fur Hard Rubber Hard rubber rubbed with fur becomes charged thus Fig 72 Stk Glass Glasg rubbed with alk becomes cbarged thus to itself This strange forec 23 called an electrical force and is attributed to the existence of clectrical charge The glass and the silk act differently, however, and a similar difference is noted between the rubber and the fur, as though one of each pair ex- ·ELECTROY, PROTOY 81 hibited an exces and the other a deficiency of this stuff called electricity The distinction was ongnally indicated by supposing tno kinds of electricity called positive and negative juet for convenience See fig 72 But since most electrical manifestations are readily accounted for by supposing the existence of merely a single kind of electricity and considering excesses of it as producing one effect and deficiencies of it as producing the opposite effect, it is not necessary to suppose that two kinds of clectricity really exist The details of these considerations, hoscer, will have to wait for & later chapter on electricity Building Blocks of Matter Electron, Proton, Neutron, Positron. The smallest unit of so called negative electricity known to exist is the electron (negative) The smallest positive charge is called the proton Bohr's theory suggested that for the simplest known substance, namely hydrogen, the atom consiste of simply a proton serving as a nucleus and an electron revolving about it, just as the moon revolves about the earth, in a fixed orbit See fig 71 The other atoms are similarly constituted but are more complicated, the number of electrons in the various or bits and the constitution of the nucleus varying as the atom in creases in complexity from helium, the second substance in the system, to uranium and plutonium, the most complicated This picture is somewhat out of date already in view of the tremendous rate at which research is being carried on in the universities and other scientific laboratories, but some of its features are still retained by those who require mental pictures for explanationa since the newest theories are mostly mathematical in nature Re cent discoveries include the neutron, a neutral particle, and the positively charged electron, the positron, so that the building blocks of the atoms are no longer restricted to two in number as was thought for so many years since 1890 or thereabouts At present it seems quite certain that even the nuclei of atoms have structure, so that whereas only a few years ago the atom was considered the ultimately small and indivisible unit of matter its structure and the components of its components now concern the physicist and chemist Such considerations, however, along with the release of atomie energy and the atomic bomb of 1945 must wait for a later chapter Brownian Motion Thus we see that matter is made up of molecules, atoms, electrons, etc , but more important still for the ·82 MATTER, GASES immediate discussion is the fact that these particles are in a state of incessant motion We live in a kinetic world, ze, a world of motion An indication of this is given by the observation of the so called Brownian motion This is the continuous agtation which one can see by examining, under the microscope, a sample of water in which fine particles of pulverized carbon are held Feld of Vew Mcroscope Fig 7.3 Particles viewed under the microscope are observed to be in a state of agitation The phenomenon 19 called Brownian motion in cuspension because they wall not dissolse Tiny specks of car ebon will be seen under these circumstances to be darting about in a most haphazard manner The phenomenon was first observed by the English botanist Brown, after whom it was named Set fig 73 It is explained by bombardment of the carbon particle by molecules of the water Au they atrihe they give rise to the irregular, haphazard motion of the particles which is observabl through the microscope The molecules themselves are much to small to be visible, being only a few hundred millionths of: centimeter in size They would have to be many thousands o times larger to be visible through even the most powerful micro scope available Kinetic Theory of Matter This simple postulate has grow into a theory of sizable proportions commonly known as th hinetic theory To a first approximation it is assumed that mole cules are small spherical particles which, by virtue of their un ternal energy, are bouncing around very rapidly In the gaseou ·$3 BQYLES LAH state the motion 18 Very pronounced and the molecules de play a wide range of velocities Molecules of a gyen substance display a characteristic asersge specd For nitrogen this is approximately 50 000 cm per second, or nearly one-third of a mile per second, under normal conditions of pressure and temperaturt The poperties of gases vary noticeably with temperature and prescure States of Matter, Matter ordharily exists in three states gaceous, lqud, and solid From the viewpoint of the kinetic theory, the distinction between these states is based upon the proximity of the molecules and the corresponding restrictions placed upon their freedom to move about They enjoy the toot freedom in the gaseous state, but in the solid state they are hemmed sa sa closely and are acted upon so strongly by intermolecular forces that they are forced to vibrate with very small amplitudes about their equilibrium positions The transitions from state to state are effected by rather complicated energy transformations Discussion 19 usually limited to the gaseous state in elementary courses in physics because this is fundamentally nat only the simplest state but the one best understood at the present time Theory and experiment check very closely for this state For this reason the theory 18 often referred to as the kinetic theory of gases Boyle's Law and the General Gas Law One notable success of this theory is its explanation of Boyle's Law This is the law discovered by Robert Boyle in 1662 or thereabouts, V which states that a giver mass of gas Va at constant temperature can undergo Pi 2. decrease in solume only by & corresponding increase in pressure, and Fig 74 As the volume of vice verse See big 74 As a conse a gas 19 decreased its pres quence, whatever the pressure or volbure is increased and vice lze of gas the product of the pressure versa if the temperature and and the volume of a given amount is mass are Icłt constant always the same An extension of thy to include the effect of texperature is grea by Charles's Law, and the combined effect of all three factors-mpreegure, volume, and temperatures the General Gas Law, which states that the volume multiplied by the presure PAT P2 ·84 MATTER, GASES and divided by the absolute temperature is always constant for & given mass of gas Absolute temperature will be defined later Specifically this law means that if one factor is kept constant the variation in the other two is predictable At constant pressure the volume is increased directly as the temperature is raised At constant Iolume the pressure and temperature increase together At constant temperature, we have Boyle's law itself Although these results follow from experiment, it is a remarkable fact that they can be derived mathematically from the kinetic theory of gases Avogadro's Law and Number. This theory also verifies the postulate put forward by the Italian chemist Avogadro (1776- 1856) who followed Boyle (1627-1691), that under the same conditions of temperature and pressure there is always the same number of molecules in a given volume of gas The number of molecules in a cubic centimeter of gas at normal atmospheric pressure and at the temperature of melting ice (these conditions are referred to as normal conditions in scientific work) is enormous It is the number obtained by moving the decimal point in the number 27 nineteen places to the right This is comparable with the fact that the diameter of a molecule is only 2 or 3 hundred millionths of a centimeter If one can imagine small spheres packed in a cubical container one centimeter to the side (less than one-half an inch) with each sphere only a few hundred millionths of a centimeter 10 size, they would be far from crowded In fact, there would he ever so much more space be. tacen the spheres than that ocupy by them The situation has been compared, for simplicity, to a gymnasium approximately 100 ft by 50 ft by 15 ft high containing only 50 basketballs Certainly the hall would not be said to be crowded with basketballs, even if they were all in the air at one time This is the approximate manner in which air molecules, for example, are pached into every cubic centimeter of air about us Approximate Nature of the Gas Laws Upon close examination it is found that Boyle's law and the general gas lay are only approximate lang Precise experimentation has shown that there are slight deviations from these laws in all observable cases Moreover, the deviations become more and more noticeable as we pass from simple gases like hydrogen and helium to gases of greater chemical complexity, and as we pass to extreme conditions of pressure and temperature ·THE IDEAL GAS 85 Concept of the Ideal Gas The scientist has found an ingenious way to cope with this situation Because all gascs obey the gas laws approximately and because the deviation in no casc is relatively very large, the concept of the ideal gas has been invented An ideal gas is defined as one which obeya these laws absolutely Real gaces are treated according to their degree of deviation from the ideal gas law This seems to be a good way of handling the situation because of the complexity which would arise of each case sere considered independently Morcover, the concept has been of inestimable value in organizing the known facts about cases in a general way Much of the research work today is concerned with the individual details, however, now that at is felt that the main portions of the picture are essentially correct The Nature of Ideal Concepts in Science. This procedure exemplifies a characteristic of the modern scientist Starting with an oversimplified postulate, he continually correcta and revises it as the experimentation progresses, fimg m the details as he proceeds In some cases, strangely enough, the dettuls have not fitted in so smoothly as they were expected to, and often major discoveries regarding main points have thus been made This has been particularly characteristic of the so-called modern physics Whereas around 1890 it was thought that all major discoveries in physics had been made and there was nothing left except the job of filling in details by precision-type measurements, quite the opposite has actually been the case In repeated instances the polishing off, so to speak, has uncovered vast areas of unexplored terntory, the developments of which ot only have added materially to the store of knowledge, but also have changed entirely the whole point of view Indeed, we live in an age when people like Einstein, working with the most abstract types of mathematics, have evolved view points of physics in many instances so different from the views held only two generation, ago, that it is sometimes hard to believe that phygicg has not become pure philosophy Many of the concepts of modern physics are purely mathematical in nature and are not capable of translation into physical pictures Nevertheless, much of the old romains, and it is the belief of some that many of the newer ideas are incapable of being intelligently understood until tlio shortcomings of the older and more natural views are apprecinteil It ·86 MATTER, GASES must never be forgotten, however, that the physics of the prai tical world, the world of machines and men, 18 much the car today as ever Gas Pressure on the Kinetic Theory. Going back to t! kinetic picture again, we readily see how the concept of preseu is explained Just as a continuous stream of machine gun bulle will cause a target to act as if it were being subjected to a stead force (see fig 75), so does the continuous bombardment of t! walls of gaseous containers by bouncing molecules produce a pe: bu Fig 75 Continuous machine gun fire develops a steady pressure on a target pendicularly directed force which, taken over a unit of are becomes a pressure in the same sense as the term was original defined Moreover, at 19 clear how this pressure should increa if the volume of & container should be reduced The molecu would be crowded more closely together so that the frequen and therefore the force of their collisions would be increas Absolute Zero of Temperature Suggested by the Kine Theory. An additional postulate of the kinetic theory is tl the temperature of a gas is directly related to the molecular : tivity, whence it is also clear how the pressure increases wi temperature This suggests a lowest possible temperature int world as that temperature for which molecular activity ceas It will be necessary to discuss this point more fully in a la chapter on heat, but suffice it to say here that this is the so-cal abeolute zero of temperature It is 459 Fahrenheit degrees bel zero, or minus 273° Centigrade Effects of Molecular Forces No discussion of molecu considerations would be complete without mention of some ·87 CAPILLARY ATTRACTO he effects of molecular fortes One of these the phenomenon ( curlace tension Molecules display forces of attrartion between hemsels es Forces between like molecules are called cohcane orces and those between unlike molecules are called adhonic orces It is the former which hold bodies together and the latter which make different bodice stick to each other at the euriatc oi liquid these forces produce an ctiect suggesting the exitence Gloss Tube Glass Tube Wate Merony Fig 76 Water sticks to clean glasy but merrury tends to pull away from glass Fix 7.7 A ctect Leedle 30 be flosted on the rur- Jace of water of a membrane stretched over the liquid o As to make the sur face of a given quantity of liquid always as emal as possible The phenomenon is usually referred to as surface tension In certain instances the adhesion forces between so gubatances exceed the conexion forces tending to hold the given substance together For example, water stichs very scadily to a cheppically cleaa surface of glasSee fig 76 On the other hand a drop of mercury placed on a plate of clean glass tends to avoid the glass snd pulls itself up into a hemispherical droplet Drops of mater on a greasy glass aurface do much the game thing Capillary Attraction This same phenomenon also causea Water to na-e up in small capillary tubes (sec fis 76) producing the many so-called capillary effects euch as those which keen Bak frame running out of a fountam pert, keep the moreture in the $ru), eta The opposite effect--1e, the repulsion of water from greasy surfaces is utilized in water-proofing garments, tente etc It is because of this same effect that insects can walk on the surface of liquids It 14 also possible to float 4 steel needle on the surface of water in spite of the fact that eteel 13 epveral times denser daan water See fig 77 It is merely necessary that the ·88 MATTER GASES needle be coated with a very thin film of oil or grease such as it may always have unless specially cleansed by chemical methods The formation of drops of all kinds is also due to surface tension Diffusion Other examples of molecular phenomena are dif fusion and osmosis If two liquids are brought into contact w cach other in a tube even if care is taken not to mix them to gether before long it will be found that one has penetrated into the other This is called diffusion Gases diffuse very readily go is evident by the fact that the presence of a volatile substance can be detected by its odor throughout a whole room very soon after it has been released in one corner of it Osmosis Certain substances act like one way gates for cer tain fluids A sugar solution can penetrate in one direction only through the tissues of some fruits and vegetables Everyone 19 familiar with the fact that certain fruits then soaked in water take up the water even to the point where they swell up because of the pressure built up by the water The water does not come out but only goes into the fruit through its surrounding mem brane This phenomenon is called osmosis The pressure devel oped is referred to as osmotic pressure Osmosis accounts for the rise of moisture up into high trees through their cellular walls to nounch them The growth of plant life is very dependent upon the existence of osmotic pressure Summary Summarizing the material of this chapter we have ceen that all matter is made up of small units such as molecules atoms electrons etc We have considered the highly successful hinetic theory of the structure of matter emphasizing those as pects which apply to the gaseous state Vanous other molecula phenomena have also been treated so that up to this point in out description of physics we have dealt with the mechanical elastic and material phases of the subject We proceed next to the study of heat and thermometry which we shall consider from the view point of energy QUESTIONS 1 Hon many molecules are there in a cubic centimeter of a gas under normal conditions? 2 What is some of the evidence in favor of the molecular theory?

·QUESTIONS 89 3 A bubble of air becomes larger as it rises ID & container of water Explain 4 How does one explain the rapid permeation of an odor throughout a room? 5 For what is the physicist Bohr famous? 6 Distinguish between the electron and the proton 7 What is Boyle's law and what are its limitation ? 8 Does a body whose density is appreciably greater than that of water necessanly sink in water? 9 Distingush between capillanty and osmosis 10 Explain how rough water is made calmer by pounng oil on at REVIEW QUESTIONS (804 page 100 for enters ) CHAPTERS V, VI, AVD VII 1 Elasticity is & property of matter by virtue of which i liquids are distinguishable from gases, 2 Archimedes principle holds, 3 bodies recover from deformations, 4 open pipes have more over tones than closed ones, 5 pressure is dependent upon density 2 Simple harmonic motion is characterized by the fact that i the motion is periodic, 2 the acceleration is proportional to the displacement from the - 3 en atom mnt on the 1 3 displacement 2 the maximum displacement from the zero position 3 synonymous with the frequency of vibration 4 the number of vibrations per second, 5 twice the maximum displacement 4 Within the elastic limit 1 stress is always zero 1 - n () 5 closely with elastic phenomena 1 Hoche 2 Boyle, 3 Archimedes 4 Newton 5 Helmholtz 6 Ivory soap floats in water because 1 all matter h19 2 all matter has density 3 the density of ory mass ET of ·90 REVIEW QUESTIONS 7 When a body is partially or wholly immersed in a liquid 1 it always sinks, 2 it is buoy ed up by a force equal to its own weight, 3 st 13 buoy ed up by a force equal to the weight of the liquid dieplaced, 4 it is buoy ed up because of Archimedes' principle and always floats, 5 it is buoyed up by a force equal to the weight of water which it would displace 8 If the mass of a body vibrating at the end of a vertical spring should be increased 1 the frequency of the vibrations would be increased , 2 the period of the vibrations would be increased 3 the amplitude would be decreased (4) the displacement of the body would be decreased, 5 the resonance frequency would be unaltered 9 Chladni figures are 1 wave patterns produced on a vibra stretched cord 2 always longitudina vibrations, 3 always present in sound waves 4 patterns produced in vibrating plates, 5 the same as antinodes 10 When a wave travels from one medium to another of different clastic properties 1 the wave length is unaltered 2 the frequency is changed 3 the velocity is changed 4 beats are established, 5 sounds are produced 11 hundt s apparatus was demonstrated in the lecture to show 1 vibrations in plates, 2 vibrations on cords, 3 beats, 4 overtones 5 standing waves in air columns 12 Sound is a wave phenomenon of the following type 1 longitudinal 2 transverse, 3 torsional, 4 circular, 5 elliptical 13 Standing traves are always 1 transverse, 2 longitudinal, 3 torsional 4 the result of interference, 5 the result of refraction 14 Kundt's apparatus is used to measure 1 the velocity of sound, 2 the velocity of any trave motion, 3 the elasticity of a vibrating medium 4 the number of overtones in a rod, 5 a harmonic series 15 Wave length of a wave is 1 the same as the frequency 2 the velocity of the wave divided by the frequency, 3 the product of velocity and frequency, 4 always zero in air, 5 the distance from a crest to the next trough ·91 ! C C) volume at constant temperature the pressure REVIEW QUESTIONS 16 sults 2 constructive interference always results, 3 refraction is demonstrated, 4 the phaec difference 18 always zero, 5 standing saves are produced 17 In air columns with one end open and the other end closed the wave length of the fundamental mode of vibration is 1 four times the length of the column 2 twice the length of the column 3 alaaye zero 4 indeterminate, 5 the distance between tno nodes 18 A node 18 a point in a wave pattern where the dis turbance is 1 a maximum, 2 A minimum 3 a rarefaction, 4 a condensation 5 alu ay varying 19 Sound waves I do not travel in & vacuum 2 travel best in a vacuum, 3 travel with a velocity of 186 000 miles per second 4 are transverse waves, 5 are always capable of being heard 20 Patch is 1 the number of vibrations per second, 21 ence of overtones 4 the pitch 5 the frequency of the hroton 22 The nu forks fork 2 is independent of the frequency of either fork 3 is exactly equal to the difference in fre quency between them 4 is approximately equal to the difference in frequency between them 5 is al 23 Atoms are 1 indivisible 2 invisible truly spherical 4 always stationary 5 composed of molecules 24 A long glass tube of mercury it its open end inn mersed in a dish of mercury can be used to demon strate the fact that 1 molecules are small 2 the atmosphere is made up of nitrogen and ovygen 3 the atmosphere exerts a pressure 4 Boyle s law 1$ true 5 preseure is the result of molecular bombard ment 25 In order to compreag & gas to one half its initial ( ways audible ·HEAT 94 Nature of Heat-Early View vs. Rumford's Interpretatio By the ancients, heat was thought of as a fud of some kind the addition of which to a body made it hotter, and the subtrac tion of which from a body made it colder This so-called calorific fluid was a concept which seemed to explain many of the ob servable thermal phenomens, but it had one serious dran bach All attempts to determine the density of the supposedly material substance failed About 1800 the American-born Benjamin Thom son, later Count Rumford, offered a new interpretation Engsged in the manufacture of cannon, in which one operation was boring Rumford observed that although chips were removed there was A noticeable rise in temperature This was not consistent wit the calorific fluid theory because the removal of chips should, or the theory, be accompanied by a loss of heat along with the chip removed Furthermore, he observed that there was more best generated with a dull tool than with a sharp one, although the latter removed more chips He sensed the fact that the amount of heat generated was directly related to the amount of energy cxpended rather than to the number of chips or the amount of material removed Of course one must work harder with a dull drill than with a sharp one to bore out the same amount of mate rial Heat a Form of Energy Thus was conceived the idea that heat is really nothing more nor less than energy itself Later er periments by the Englishman Joule and the American Rowland disclosed the direct equivalence of these tho This view is al o consistent with the modern kinetic theory of matter, already die cussed, in terms of thich heat is found to be just that energy associated with the random motion of molecules On this bacis friction develops heat simply because of, and in an amount equal to, the increase in the molecular energy associated with th haphazard, random molecular motion involved when to su stances are rubbed together Indeed, on the kinetic theory, heat defined simply as the total energy associated with this random haphazard molecular motion Moreover, temperature, althouch qualitatively definable as a property which determines the direc tion in which heat will flow (It always flors of its own accord ? from high-temperature regions toward loner-temperature re gions), fits into the kinetic picture as the average kinetic energy per molecule associated sith the translatory motion f ·THERMOMETRI 95 Thermometry. Before we can pursue farther this matter heat and temperature it 19 Deceeary that we understand how mperature 19 measured. This phase of the subject is called farm Water Watet Fig 8.1 'The warm water feels hot to the hand previously placed in d water but cold to the band previously placed in hot water, showing temperature 18 e relative matter hermometry Of course, we might rely upon our sense of warmth 1 cold, but it can be readily shown that such determinations very unreliable If one were to place a hand in warm water fter it had been in hot water the warm water would feel cold, ucreas the same hand in warm water would seem hot after has. been placed in cold water See fig 81 This shows how rela. the matter of temperature 13 In order to make a more redetermination of temperature we look to the various -operties of matter which are found to be tered by the application of heat and note that all matter usually expands when 1 Thermal Expansion Utilized in Tem- Measurements Liquids, generally eaking, expand more noticeably than colids, I gases expand still more The liquid subance mercury expands very uniformly ove: considerable range of temperature and so 'es us with a suitable thermometric subance which, in the form of a fine thread in glass capillary tube, cealed to an expansion Fig 8.2 Mercury in , constitutes a useful thermometer See glass thermometer 82 All temperature readings are relative The graduations on a hermometer scale are made to agree with conventional standon the basis of which certain temperatures are accepted 29 ed points The two most conspicuous fixed points are the tem- ·Freez ng 0 96 HEAT perature of melting ice and the temperature at which water box under normal atmospheric pressure, ie, 76 cm of mercury o 14 7 lbs per aq in It is merely a matter of common agreeme that these are the temperatures which are basic to all thermo metric work Centigrade vs Fahrenheit Temperature Scales We hare two relatively common scales of temperature the Centigrade scale, used chiefly in scientific work Cent grade Fahrenhet and the Fahrenheit scale, which is Bol ng 100- -212 perhaps more familiar to the layman. Point of Water On the Centigrade scale the melti point of ice, which is the same as the freezing point of water is arbitrarils called 0° C, and the boiling point 1 arbitrarily called 100° C On the Fahr -32 enheit scale, these are called 32° F. Pont of Water and 212° F, respectively See fig 83 Fug 8.3 The Centigrade It is further noted that the Centigrade and Fahrenheit temperature scale is subdivided into 100 equal Ecales compared degrees between freezing and boiling temperatures, whereas the Fahrenheit scale is subdivided into 180 equal degrees over the same region, each Fahrenheit degree being 5/9 as large as a Centigrade degree Converting from One Scale to the Other it is obviously a simple matter to change a temperature reading on one scale to the corresponding reading on the other one since the Centigrade degree is 9/5 as large as the Fahrenheit degree and the freezing point of water on the Fahrenheit scale is already 32 degrees above zero Thus the familiar temperature 68° F is 36 Fahrenheit de grees above the freezing point, and co corresponds to only 20 Cen tigrade degrees above the freezing point, or to just 20° C Inc dentally the temperature reading of 40° below zero happens to be the same on each ecale Coefficient of Thermal Expansion Whereas different solids and liquids expand quite differently, all gases expand by approxi mately the same amount The fractional expansion of any portion of a substance per degree of temperature change is defined as the coefficient of thermal expansion of the substance Expansion 10 one dimension only, ie, of length, is called linear expansion We may also consider surface and volume expansion The coefficient ·ABSOLUTE ZERO 97 f volume espansion for a substance is acecssanly approximately hree times the size of the linear coefficient Expansion of Gases. The volume coefficient is approximately he same for all gases and is very nearly equal to the reciprocal of the number 273 per degree Centigrade This means that, as he temperature of a gas at 0° C is raised one degree, its volume increases by 1/273rd of its original value. This also means that gas shrinks in iolume 1/273rd of its value if it is cooled one degree Centigrade, and this brings up the interesting question of What would happen to a quantity of gas if its temperature should be reduced by 273 degrees from 0° C Concept of Absolute Zero of Temperature. The anstrer to this question, ie , whether or not the volume vanishes completely, cannot be found experimentally because of the impossibihty of lowering the temperature of any gas to - 273° C All known gases liquefy before this temperature is reached, and liquids do not have the same coefficients as gases There is, however, some thing unique about this temperature of - 273° C As has been ti this is significant, for the kinetic theory also requires the pressure of a gas to vanish when the temperature 18 reduced to abo. lute zero Thus, and for more technical reasons not to be considered here, an absolute zero of temperature has been postulated, a temperature so low that, on the kinetic theory, all molecular motion ceases This temperature therefore 18 – 273° C, or about - 459° F, and its existence implies the absolute inability of any lower temperature to exist This is the first place in our study where & limitation has been imposed upon our thinking of extremes of anything such as size or intensity, but it will not be the last one Absolute Temperature Scale. Thus a third, or absolute, scale of temperature is suggested, based upon this absolute zero inalt nina The Centurrade Absolute. In engineering work, however, an absolute Fahrenheit scale 19 som ·98 HEAT Purther Significance of Absolute Zero In recent se techniques have been developed by experimenters for produc very low temperatures to learn about these extreme conditic Although no one has succeeded in producing a temperature lo' than absolute zero, and of course, present-day science pred: that no one ever will, nevertheless values within a very sm fraction of a degree of absolute zero have been obtained I: dentally, it should be noted that the measurement of such te perature is by no means a simple matter The field of lo temperature research appears to hold great promise for sti tling discoveries in the near future Many of the properties matter have already been found to vary markedly from th normal values when samples are subjected to very low tempe tures Molecular Forces Involved in Thermal Expansion GC back to the question of linear expansion of solids, attention called specifically to the magnitude of the molecular forces volved when a rod, say of steel, is heated Steel increases length by approximately eleven parts in a million, per deg Centigrade This means that a steel bridge one mile long, cxample, changes in length by more than four feet under change in temperature from a twinter's minimum of 40° bel zero to a summer's maximum of around + 100°F When considers how much force is required to stretch a steel rod cleven parts in a million one can only marvel at the quiet m: ner in which a rise of temperature of only one degree Centigr produces the same effect Bimetallic Expansion-Thermostats If a strip of rol securely fastened along its length to a strip of brass of the se length, and the combination is then heated, the result will b bending of the combination, because brass expands nearly 50 cent more than iron See fig 84 This effect is utilized in Various bimetallic thermostats which are so common in numerous automatic control devices In all these devices elec contacts are simply made and broken by such a bending of metallic strips upon the application of heat Attention is called, incidentally, to the very fortunate 1 that cement and steel have practically the same coefficient expansion This makes possible the modern steel and cem ·GENERAL GAS LAW skyscraper, fhich does not buckle with changes in teroperature In fact, reinforced concrete in general, a bich plays such an important part in our highly mechanized lives, is possible only be cause of this fact tron Brass Iron Bross Brass and Iron strips welded together and at room temperature The same strips heated Broke expands some 50 per cent more the Iron Fig 84 The General Gas Law Since the volume of a gag varies with temperature, as indicated above, and since Boyle's law, previously discussed, states how the volume of a given mass of gas depends upon the pressure, it is natural to wonder if these trvo effects can be combined mio a general law This actually proves to be the case, and for the ideal gas, the relation 19 a very simple one It is usually referred to as the general gas law It states that the pressure multiplied by the volume and divided by the absolute temperature of a given wacs of gas is constant This simply means that if the volume of a given mass of gas 19 kept constant, ita pressure varies directly as the absolute temperature Of course, if the temperature 19 kept constant this is simply Boyle's law itself, ie, the pressure is inversely proportional to the volume Temperatures, in this connection, must always be expressed on the absolute scale Concept of Temperature Only Part of the Study Thug We see that one of the outstanding characteristics of matter, namely size, is affected by the emplıcation of heat, and that Variations in the size of an object may be utilized in the measa urement of temperature Of course, this is consistent with the kietie theory of matter, since an increase in the size of a body can be thought to be associated with an increased molecular activity, which requires more space for a further insight into the nature of heat we shall be required to consider the question of the quantitative measurement of heat ·102 HEAT ing temperature is raised, as in the case of the pressure cooker See fig 85 At high elevations, on the other hand, here the pres. sure is lon, the boiling temperature is also low In fact it may be too low for certain foods to be cooked by boiling, as is well known by paavengers on al liners Whereas chemistry teaches that substances can be identified by characteristic boiling points, it must be pointed out that it 19 necessary to establish some standard of pressure such as standard atmospheric pressure, 76 cm of mercury or 14 7 lbs per square inch, in this identification process Boiling under reduced pressure can also be illustrated by the so-called Franklin Fig 85 With the pulse glass This is a dumbbell-shaped glace pressure cooker the tube, ie, a slender tube with spherical bulbs boiling point of water at either end, partrally filled with colored 13 raised above 100° C or 212° F water and sealed off under reduced pressure Just the heat of the hand clasped about one bulb causes sufficient rise in temperature to make the water boil over into the other (cooler) bulb Boiling vs Evaporation-Vapor Pressure In connection with the discussion of boiling it might be well to distinguish between the phenomenon of bo ling and that of evaporation It 19 well known that water left in an uncovered dish will eventually disappear by evaporation This is the result of molecular activity at the surface, and is quite understandable from the kinetic theory The molecules simply bounce away In the case of boiling, however, the change of state more frequently than not takes place down inside the water, even at the very bottom, where heat is perhaps being applied by a flame If, however, a cover is placed over the evaporation dish in which there some water, the molecules are confined above the surface of the liquid, and build up a pressure as they accumulate At a given temperature, there 19 a certain maximum value which this pressure can reach for any particular liquid This is known as the saturated vapor pressure From this point of view the boiling point of a liquid is simply that temperature at which the saturated vapor pressure coincides with the atmospheric pressure Humidity The effects of water vapor in the air afford an ·F8 86 Relative him dty can be measured by the pling hygroro- HUMIDITY AND DET 103 interesting study Eseryone knows that atmospheric moisture ja a very important factor in connection with the weather There is obviously a limit to the amount of moisture which the air can liold at & given temperature of the maximum is reached, precipitation results This condition is referred to technically as & condition of 100 per cent relative humidity, where the term relative humidity is the ratio of the amount of water vapor in a given volume of air to the maximum amount possible at the temperature in question The actual amount of water vapor present in a gien volume of air 13 called absolute humudity The measurement of humidity is called hygrometry It can be accom eter in which wet-tral and dry bulb thermometer arc Totated plished by the use of the eling ma frame about an axle The hygrometer See fig 86 Whirling amount by which the dry bulb the device keeps the air circulatreading erceeds the wet bulb ing and enhances the evaporation reading is a measure of the tive humidity of moisture from the wet bulb Because evaporation causes cooling there is a difference in temperature readings of the two thermometers which is thus related to relative humidity Dew Since the limit to the amount of moisture possible in the air vanes with the temperature—actually it increases with temperature—we have the phenomenon of the dew point An amount of water vapor which is not sufficient to produce saturation at one te temperature moisture, or because the temperature of the pitcher is cufficiently lower than that of the surrounding air for the amount of water vapor present to produce saturation Similarly dew is formed on the graeg on dan hann 1 1 tomnarat то ܠܐ ܝܘ ܕܕܘܐ ܘ I freezes and becomes frost ·102 HEAT ing temperature is raised, as in the case of the pressure cooker See fig 85 At high elevations, on the other hand, where the pressure is low, the boiling temperature is also low In fact it may be too low for certain foods to be cooked by boil. ing, as is well known by paaeengers on 911 liners Whereas chemistry teaches that substances can be identified by characteristic boiling points, it must be pointed out that it 19 Decessary to establish some standard of pressure such as standard atmospheric pressure, 76 cm of mercury or 14 7 lbs per «quare inch, in this identification process Boiling under reduced pressure can also be illustrated by the so called Franklin Fug 8.5 Mith the pulse glass This is a dumbbell shaped glasa pressure cooker the tube, i e, a slender tube with spherical bulbs boiling point of water at either end, partially filled with colored 13 rai ed above 100° C or 212° F water and sealed off under reduced presbure Just the heat of the hand clasped about one bulb causes sufficient rise in temperature to make the water boil over into the other (cooler) bulb Boiling vs Evaporation,Vapor Pressure In connection with the discussion of boiling it might be well to distinguish between the phenomenon of boiling and that of evaporation It is well known that water left in an uncovered dish will eventually disappear by evaporation This is the result of molecular activity at the surface, and is quite understandable from the kinetic theory The molecules simply bounce away In the case of boiling horever, the change of state more frequently than not takes place down inside the water, even at the very bottom, where heat is perhaps being applied by a flame If, however, a cover is placed over the evaporation dish in which there is some water, the molecules are confined above the surface of the liquid, and build up a pressure as they accumulate At a given temperature, there is a certain maximum value which this pressure can reach for any particular liquid This is known as the saturated vapor pressure From this point of view the boiling point of a liquid 18 simply that temperature at which the saturated vapor pressure coincides with the atmospheric pressure Humidity The effects of water vapor in the air afford an ·REGELATION 100 be distinguished from gases, but today no such distinctions are attempted because it is known that all Gases can not be liquefied and even solidified Liquid alt, for example, is a very familiar cubstance in the scientific laboratory It of course, boils freely at room temperature and atmospheric pressure although its tem perature is 180° below zero Centigrade Onc has to be careful not to confine liquid air in a stoppered container becausc at room temperature its saturated vapor pressure is many, many atmoe. pheres For the same reason it is not safe to confine dry ice in a stoppered container at room temperature The vapor pressure which it will build up 19 enormous Effect of Pressure upon the Freezing Point Thus it is seen that the state in which matter exists depends among other things upon its thermal condition. At the boiling point the liquid and gaseous states exist in equilibrium with each other, and at the freezing point the liquid and solid states are in equilibrium Although the boiling temperature of water is appreciably altered by a moderate change in pres sure the freezing point is only slightly changed Enormous pres sures however, can reduce the melting point appreciably Ice skating is in reality a process of slipping along on a thin film of water immediately beneath the shate's runner a condition pro Fig 87 Regelation allustrated duced by local melting of the ice by a wire which gradually cuta" under the pressure exerted by the through a pece of ice owing to weights fastened at its ends The weight of the skater acting upon ice fuses together after the wire the relatively small area of sur passes through successive portions face supporting the skate Upon the removal of the pressure, after the skater slips along this thin film of water freezes almost instantly Regelation is the technical name for this phenomenon of melting under pressure and refreezing upon its removal See fig 87 It suggests the man ner in which the glaciers probably slipped along over large sec tions of the earth at an earlier period Summary In this chapter we have seen how heat is simply a manifestation of energy, how it is distinguished from temj ture and how the latter is measured by virtue of certain ti ·106 HEAT properties of matter, particularly expansion. We have also considered briefly hon heat is measured and how thermal considerations enter into questions involving states of matter, and change of state We shall turn next to a descriptive consideration of heat transfer and allied phenomena QUESTIONS 1. Why does the mercury thread in a glass thermometer usually fall a little when the thermometer is first placed in hot water? 2 A pyrex dish is less likely to break when heated than an ordinary glass dish. What can be said of the relative expansion coefficients of these two substances ? 3 Is there any sense to the procedure of dipping a metal can cover in hot water to make it come off more easily? 4 What is the distinction between heat and temperature? 5 List various effects produced by a temperature change which may be utilized to measure temperature 6 A Centigrade thermometer reads - 40° What would be the reading on a Fahrenheit thermometer? 7 It is often possible by gentle heating to remove the stopper from a glass bottle after it has become stuck Explain 8 le a gallon of boiling water any hotter than a pint of it? Does the one contain more or less heat than the other? 9 Why does steam at 212° F cause a more severe burn than boiling water at 212' F? 10 Why does a gas become heated on compression and cooled on expansion? 11 Explain why the air escaping from an automobile tire valve feels cold 12 Explain why it is impossible to skate on glass 13 The substance hypo is often used instead of ice on indoor skating rinks What characteristics does it have to make it suitable for this purpose? 14 Why does it become necessary to add moisture to the air of a house in the wintertime? 15 Explain why moisture collects on the outside of the icewater pitcher 16 Explain why perspiration is more bothercome on a humid day than on a day less humid even if the temperature is the same

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·108 HEAT Conduction 11 hen an iron poker is held with one end in fire, heat is observed to travel from the hot end to the cold end passing successively from point to point throughout the length of the poker This is the type of transfer called conduction In terms of the kinetic theory the molecular agitation is simply passed along from molecule to molecule in much the same manner as a Fig 91 If the end domino in a row is pusbed over kinetic energy will be passed on from domino to domino This 18 analogous to the process of heat conduction column of closely spaced, standing dominoes is toppled ver if the first one in line is pushed backward Each domino in succession is pushed over See fig 91 CONDUCTIVITY Different substances have different coefficients of conductivity, determined by the rate at which heat is conducted through unit length of the substance then a unit difference in temperature is maintained between the ends of the sample of unit croes sectional area The best known conductor of heat is silver, which, incidentally, is also the best known conductor of electricatv Insulators are simply poor conductors Whereas the metals are good thermal conductors, substances like cork and wool are good insulators Air also 13 a poor conductor, ie, a good insulator, and much of the merit of a fur lined coat lies in the poor conduc. tivity of the air spaces betteen the hairs of the fur There is the same reason for doublc walled construction of houses in which an air space is left between the walls Because powdered asbestos is al o an exceptionally good insulator, ie, a poor conductor, it 19 commonly blown into the sprce between the walls of old houses these days to improve the insulation It should be noted, of course that if heat could be presented completely from escaping through the walls, windowe roofs etc of houses they could be hcated at practically no expense at all This is why so much attention is given to the question of insulation in the house of the future' The advent of the electric refrigerator has focused considerable attention upon the matter of heat insulation For reasons of ·COVDL CTION AD COVECTION operating econom; the builders of electric reingeratora were forced to develop bores insulated to such a degree of refidcment that the manufacturers of icebores are now able to offer boxes that need to be filled with ice only once a week It is entirely Layers of Insulat on Fug 92 Insulation 19 the se cret of the successful icebox Heat 1 prevented from Bowing in from the outside to melt the ice Only hest from the food ryside the bos 19 allowed to relt tbe ice Fig 93 Convectron currents are set up by an ice cube floating It & glass of water Cold water einks and gives way to narin water which rises matter of preventing the flow of heat from the outside to the sa- Bide of the box that accomplishes this result See fig 92 Convection The second mode of heat transfer, which we shall pow consider, 13 cozy ection This is characterized by the word circulation. If an ice cube is placed in a glass of water at 19 found that currents are established in the water See fig 93 The coolest water sinks to the bottom of the glass and the warmer portions rise to the top This is due to the d fference in deneity between warm and cold water and the natural application of Archimedes principle, already described Such currents are knoty as convection currents, and the phenomenon of heat tran·fer by such means is called convectzon It is charactenzed specifically by the acquisition of heat by some material agent which moves e meg ule vy vuu registers It is to be noted that those houses which heat hest, are u tuualues Jr ·110 HEAT those in which a good circulation of air is provided by the arrange ment of the rooms The old-fashioned long, narrow house with rooms following one after another 19 hard to heat uniformly This is not the case with houses in which the rooms arc clustered Fig 94 Cooling is accomplı hed in the rad ators of certain cars by convection currents Hot water in the eng ne riscs and 19 allowed to fall through tubes in the radiator thus becoming cool r and displacing warmer uter at the bottom around some central po nt with open doors ays providing free accces from one to another, including the last and the first The principle of heat transfer by convection is still utilized in the cooling systems of some automobiles, although most of the cars today have water pumps for this purpose The hot water in the engines of these cars rises to the top of the radiator where it falls through parrow tubes of brass from which, by conduction much of the heat is removed to the out of doors At the bottom of the radiator the water becomes cool, forcing more hot water up through the motor to the top of the system, where the proces 3 repeated This is the so called thermosyphon system See fig 94 METEOROLOGICAL ASPECTS OF CONVECTION Convection cur rents of air play a very important role in the production of weather Heated air masses rising rapidly by convection on a lot day are the cause of most thundershoners The electrical phe nomena involved in such showers will be discussed in a later ·RADIATION 111 chapter, but the cloud fortsations responsible for them are pro- Lh ! IL 1 ULU ALAM currents, have advanced the study of meteorology in the last decade or co to an unprecedented degree Radiation The last of the three modes of heat transfer to be considered here is radiation It 13 a known fact that a person hand placed a few inches beneath a hot household radiator feels warmth from the radiator See fig 95 Similarly, warmth 13 felt & few inches from the floor in front of a glowing fireplace fire In neither of these cases is the transfer of heat due chiefly to conduction or convection In the first place air 19 a very poor conductor, and in the second place convection carries heat upward and not downward In these cases Fig 95 Heating underneath a m diator 19 accomplisbed largely by the heat is radiated directly out radiation ward ie, it is transferred much as light is transferred by some kind of wavelike procese Thie process of heat transfer has been studied at great length by physicists throughout the ages, because it has been one of the most difficult and most confusing of all the questions rarsed by natural philosophy It is the question of the nature of radiant energy and embraces not only the nature of heat radiation but also that of light, radio X ray, cosmic radiation, and many allied phenomena, although much of this material wall have to wait for later consideration Nature of Radiant Energy A great deal of attention has been given to the question of whether this radiation 19 corpuscu lar or wavelike, and although the present view 19 very complı cated by virtue of certain nonmaterialistic concepts associated with such subjects as quantum theory and relativity, the view has been developed that for the most part radiation is wavelike Heat radiation then, is the came as light radiation except that the length of the waves containing the bulk of the energy is dif ferent in each case Generally speaking, heat Faves are !

·112 HEAT than light waves, but the speed at which they travel in empty space is the same In fact, both kinds are liberated simultaneously and at the same speed, by a common source because they really are both part of the same thing Actually it is more accurate to say that a source of radiant energy radiates a whole spectrum ol radiation, displaying wave properties and characterized by the lengths of the waves contained therein A certain portion of this spectrum is recognized as heat, and other portions as light, X ray 9: etc Whereas the range of visible wave lengths is recognized by V s bie Short Waves Long Wowo X Rays Uitrav olet VR Infrared Heat Rado F1g 9.6 The electromagnetic spectrum extends all the way from th shortest X rays to the longest radio waves The viable portion compriscs very narrow band from violet to red the concept of color, extending from violet to red with wav Icugths from 0 00004 cm to approximately 000008 cm resper tively, heat waves extend from the infrared region out to tl radio range where the lengths of the waves are measured in mill meters, centimeters, and even meters Beyond the violet side the visible spectrum the waves range from the so-called ultri violet, responsible in large measure for sunburn and tan, to tt X rays See fig 96 Color Associated with Radiation. That color is somen hat with heat radiation may also be shown by calling a tention to the familiar kitchen stove As a stove heats up, first notices the feeling of warmth before there is any indica of color, but at a certain temperature, the stove covers glow a dull red color This color changes with temperature, becoming more yellow and perhaps finally white hot as the perature increases, at which time it is found that a very w range of wave lengths is being emitted Camera film sensi particularly for safrared is capable of detecting these radiations long before the stove becomes hot enough to glow visibly, and even in the dark Similarly it is possible to detect the variat other wave lengths present in the radiations from the white bol covers The analysis of all these radiations is usually referred as spectroscopy and will be considered more fully in the study of ·1 the poorer conductivity of the wool, rather that it railinconi of radiation and its associated phenomenon of nbrorption A NIT. stanoe has to absorb heat in order to radinte it, ani Ni Fun radiator is a good absorber On the other hand, woolen Niit la to the direct rays of the sun, even in the murier CIN HALINA / more comfortable than & cotton ono il tlio wenrrr 1% Xp to RADIATOR AND THERIOS BOTTLE 113 light, where it seems to fit more appropriately into the ecbenn of physics The study of beat 15 so closely tied in with the study of light, and even of radio, as to make the study of any one topic alone nearly impossible Law of Heat Radiation. Getting back to the study of hrat radiation, we find there are certain laws governing this phenomenon The rate of radiation depends upon the eize of the radiating surface, its absolute temperature, and its nature The dependence on temperature is not a straight proportionality but a fourth power proportionality, ie, a doubling of temperature, expressed on the absolute scale, does not double the rate of radiation, but increases it sixteenfold Thus, although all bodies with temperature above absolute zero radiate, hot bodies cool ever so much faster by radiation than cold ones Surface Characteristics of Radiators. The nature of the radiating surface is very important as regards the rate of radiation Rough surfaces cool faster than smooth ones and black eurfaces radiate faster than white ones The worst conceivable radiator, then, is a perfectly reflecting surface such as a highly polished silver mirror, and it is for this reason that modern flatırong are nickel plated all over except on the bottom, which is the only surface from which heat is allowed to escape This reasoning sur gests certain facts well known to the practical person, such as that black stoves and radiators gve off more heat than thome painted in light colors, and that polished copper hot-water tanka retain heat longer than painted ones It is not just imagination erther, that makes light-colored clothes feel cooler In tho it weather than dark ones Dark clothes actually arc tiotter but Inutes WUL IIUL tion with eonduction inny noinally HVER three The success of the therinox Lottlo, for example, la nchloved ·114 HEAT by reducing all three modes of heat transfer to a minimum Thi thermos bottle consists of a double-walled container, whose wall: are separated by a vacuum, the very worst heat conductor im aginable See fig 97 The only place wher heat can escape by conduction is through th stopper and this is usually small and mad of cork The vacuum space also eliminates th possibility of convection currents Finally the walls are usually silvered so that what ever heat might escape across the vacuum b radiation is reflected back into the bottlt Vacuum For these reasons, then, a good thermos bot tle can keep foods or liquids hot for a con siderable length of time The heat just canno escape Moreover, a thermos bottle will hee cold things cold longer than it will keep hc things hot, because hot objects radiate moi Fig 97 The ther rapidly than cold ones mos bottle is a double wallcd glass bot- Selective Transmission and Absorptio tle The reg on be of Radiation That radiant heat has muc tween the walls is the same if not exactly the same nature : highly erhausted to prevent the transfer light is also shown by the fact that it can of heat by conduction reflected by mirrors and focused by lense and convection The Of course, certain substances are more tran walls are sikered to prevent radiation parent to light waves (short) than to he: waves (long) ie, they transmit waves one length but are opaque to waves of different length Son substances also absorb pelected wave lengths The hothouse pr vides an interesting situation Light 13 readily tranamitted throug the glass windows and much radiant energy is absorbed by ti objects and ground within the hothouse Upon absorption the: bodies immediately reradiate the energy as heat waves but lengths too great to penetrate windon glass See fig 98 Thr energy enters via short war es but cannot escape because the sho waves are changed to long ones This emphasis on waves and the Icngths naturally raises the question honover as to the certunt of the view that radiant heat and light arc wavelike Furthermor it must be admitted that no discussion of radiation would b complete without some reference to the now popular quantur theory, which contradicts the have theory ·QUANTUV THEORY 111 The Quantum Theory. It was in about 1000 that Mix Pladek, the German physicist, tas making an intenske study of the distnbution of energy in the radiation spectrum In an attempt to formulate a mathematical relationship governing the amount of energy allocated to the diðerent wave lengths, 10, sesociated with different colors, he became convinced that the laws of classical physics nere inadequate to explain observable facts la other words, his scarch for the truth, which is the ulti- Fig 98 In the hot house short waves enter and are absorbed They are transformed into long hases which cannot be trasmitted by the glass mate tack of the scientist, led to the conclusion that supposedly correct laws were not altogether true, and that all natural phepomena are not necessarily explainable in terms of simple postulates In particular, he found that instead of assuming rad ant energy to be propagated like a continuous wave disturbance, it is necessary to postulate that energy 13 radiated in packets or bundlee each made up of an integral number of elementary units Timinta This seemed to many to be a very artifitrhoch a LAH the history of physics, this quauuu ju accepted view Some Philosophical Aspects of Quantum Theory Esen though this seems to give a corpuscular aspect to the nature of radiant energy, it has not eliminated thu s zve hi pothesis, becau e of other phenomena (to be discussed later in optics, pp 182-181) which cannot be explained by corpuscles unless the corpuscles themselves be thought of as bundles of waves Thus radiation is still discussed in terms of waves, but also in terms of quanta, at the same time If now this appears slightly indefinite to the stu- ·HEAT 116 dent, then he has just begun to grasp the point of view of the physicist of the last generation, who felt that the discovery of the quantum theory completely undermined all semblance of reality in physics During the last two or three decades, however, many of the philosophical aspects of this whole question have been straightened out in such a way as to show that the earlier views, although approximations to the truth, are not such good approxi mations as the later viers It 18 as if the early views explained only main features but not details, whereas the present views are more comprehensive and correspondingly more complex The "Newer" Physics Goes beyond "Classical Physics" Here it is that we begin to see how a change has come over physics since the turn of the century, and why it is that physicists refer to the New Physics as distinguished from Classical Physics The New Physics is permeated by a point of view altogether lacking in its respect for "common sense," but characterized by strictly mathematical logic and deductions from formal assumptions justified only by their observable results Although this 19 not the physics of Newton, it is nevertheless fair to point out that the major discrepancies between the two lle chiefly in the domain of submicroscopic affairs, and that most of the deductions from Newtonian physics still hold as true as ever in the ordinary mechanical world of men and machines Additional Discrepancies of "Classical Physics" If the quantum theory were the only source of worry to the classical physicist we should probably not be so concerned about it, but the facts are that many of the supposedly minor discrepancies of classical physics have become grossly magnified by the researches of modern timcs, and, strangely enough, those measures required to restore order and reason to the troubled picture are extraordi narily consistent with the newer point of view exemplified by the quantum theory Specifically, we refer to the study of relative motion, which Einstein has explained only by reference to con cepts and mathematical logic which do not seem sensible from the earlier point of view One of these, for example, is the relativity postulate that no body can have a velocity greater than that of light In brief, the modern physicists, attempting to understand nature, have discovered that instead of being faced with the task of polishing off certain rough spots of otherwise completely solved problems, they have hardly scratched the surface of much larger ·SUMMARY 117 and profoundly more significant problems There is, however, no reason to be discouraged, because the discoveries of the past have been very well consolidated and what lies ahead is merely a new era Much of the new physics deals with electrical phenomena, the elementary concepts of which we shall consider in the next chapter Summary In this chapter the three modes of heat transfer conduction, convection, and radiation have been described Insulation is seen to be the inverse of conduction Conduction refers to the transfer of molecular energy right through an object Convection involves the circulation of heat by some agent which actually carries it from one place to another Radiation is a wavelike propagation quite different from the other two modes It is electromagnetic in nature like light, radio, and X rays It is best explained by the quantum theory, which has almost completely revolutionized the point of view of physics, so that modern physics is something more than an extension of classical physics, although the latter is a first approximation of the former QUESTIONS 1 Some people boast of the length of time they can keep ice in a refrigerator by completely wrapping it in newspaper Comment 2 Why is a thick layer of air used for insulating ovens? 3 Icicles are sometimes observed to melt on the south side of a house before they do on the north side Why? 4 What is the effect of a fireplace on the ventilation of a room? 5 Why may a fireplace which smokes at first, stop smoking after a fire has burned & short while? 6 Explain how it is that a person in front of a fireplace fire may have a cold back yet feel hot in the face 7 Why does a thermos bottle keep cold things cold longer than it can keep hot things hot? 8 What re some of the features of the quantum theory?

·CHAPTER X ELECTRICAL CONSIDERATIONS STATIC ELECTRICITY, CHARGES POTENTIAL, CAPACITY In an earlier chapter reference was made to the electrical struc ture of matter It was pointed out that all matter is constructed fundamentally from charges of electricity The concept of electrical charge, although a somewhat abstract concept, is so important that it cannot go unmentioned in this description of the phy sical world in the beginning, the concept was just a device invented to describe a situation but later it was given reality by the dis covery of natural units of charge Today it is felt that such units of charge as the electron, the positron, the proton, the mesotron, etc really exist and are fundamental entities basic to the very concept of matter itself Hence they invite our study Frictional Electricity Proceeding historically and logically to the development of electrical knowledge, we start with a con sideration of frictional electricity and electrostatics As was pointed out earlier, if a rod of hard rubber or ebonite is rubbed vigorously with fur it becomes endowed with the peculiar abil. ity to pick up bits of paper and to attract small pieces of pith to itself See fig 101 Similarity between Electric and Magnetic Forces The force thus demonstrated Fig 101 An ebonte rod which has has been called an electrical beeb rubbed with fur will pick up bts force after the Greek word for of paper due to its electrical charge amber, the substance for which at was first observed A similar but quite different force has al o been observed in connection with iron filings and the ore magnetite, or lodestone, which as used as a mariner's compass because of its directional properties Sce fig 102 This force was called a 115 ·THO FLUID TIIEORY 119 VAL mlynctic forec In euch of these two cases the force was strange because unlike ordinary pushes and pulls requiring contact betreen bodies for their effects to be felt no contact was required Early physicists described such phe Domeng as action at a distance The force of gravity being the only other Esch force met Bith in physice ti e three together constitute a group which has gren rise to many problems Although they all display certain common as pects they are neverti elesy quite dif ferent and relate to entirely distinct phases of physics It is found as will be shown presently, that electrical and Fg 10.2 An elongated magnetic phenomena are related to pece of magnetste each other so that it is natural to con lodestone if suspended by a str Dg w 11 point der these together apart from the study toward the north N OT of gravity Two Fluid Theory of Electricity In an attempt to explain electrical phenomena it is supposed that electric forces are sro duced by so called electric charges which can be accumulated upon or wiped off from bodies when they are rubbed with other bodies as in the case of the ebonite rod rubbed with the fur It is found that an ebonite rod that has been thus rubbed with fur tends to repel another ebonite rod similarly treated but that it tends to attract toward itself a glass rod that has been rubbed rith - lk It is as if the charges on the ebonite and the glass were quite different-in fact opposite in character Also the fur is found to attract the ebonite and tħe silk to attract the glass after they have been thus rubbed together Such observations as these and a host of others gave rise to the early view that there were tra distinct kinds of electric charges indicated for convenience by the names positive and negative All those cubstances Thich when rubbed with other substances behaved like the giass rod when rubbed with silk were said to be charged positively and all the se bchaving like the ebonite rod when rubbed with fur Bere egid to be charged negatively The matter of which was 1 ·120 STATIC ELECTRICITY fundamental kinds of electricity were postulated and a very im portant law was discovered, namely that like charges repel and unlike charges attract each other One-Fluid Theory Benjamin Franklin, the American statesman and scientist, feeling that this so called two flud theory of electricity was unnecessarily complicated, proposed a view which in many ways resembles the modern viewpoint He proposed a so called single fluid theory, postulating that the positive charges represented an accumulation of charge (presumably positive) whereas the negative charge was merely 2 deficiency of such a positive charge Except that the modern electron theory involve charges inherently negative instead of positive, on the basis of the old classification, it explains ordinary electrostatic phenomena in a similar way, ie, by a consideration of surpluses and deficiencies of a single kind of electric charge Electrical Terminology Based upon Positive Fluid The One of the difficulties encountered in the study of electricity today is that, although the negative electron theory has superseded the positive fluid theory just mentioned, much of the terminology and many electrical concepts are actually defined in terms of the positively charged fluid of Franklin Thus even the unit of electrical charge, arbitrarily defined for purposes of a logical understanding of electrical phenomena, is not the electron, because this natural unit was not discovered until about the beginning of the present century, but the so-called unit positive charge Abstract Nature of the Study of Electricity This inportant unit is in reality only a product of the mind It is purely supposititious, but its appreciation is abeolutely necessary for the proper understanding of all electrical concepts, since the science of clectricity has been developed as an abstract subject Electricity unlike mechanics, for example, deals with concepts alone rather than visible things, and student wishes to understand a given electrical situation must, without exception, follow through a train of carefully developed logical arguments couched in precisely defined terms, because there are no revolving wheels or bouncing springs to watch here it is relatively easy to discuss academically the motion of a body under certain circumstances since bodies in motion are directly observable, and the arguments can be checked by direct observation One does not make direct oben a tions upon electric charge, honever All observations here are of ·COCLOUBS LAW 121 the indirect type ishich requires an understanding of the relationships soyolved, because te do not have direct sense perception in electricity Hence the physicist is particularly careful about his defisitions of electrical terme Coulomb's Law The unit positive charge, or the unit of electricity as it might be popularly called, 14 defined in terms of a Frtain las called Coulomb : Lau Not only do like charges repel and unlike charges attract each other, but the magnitudes of these forces 13 found to depend upon the amounts of charge and the separation between them Coulombs lan states this in mathematical language by an expression closely resembling Netston's law Vacuum Fol dyna of gravitation (which, it will be recalled, 18 co-called inverse fontana 291are law), because the force between charges diminishes as the Fig 103 11 two pointlike second power of the separation charges placed one centimeter apart an VsCUU?) repel each other The law states that the force be with a force of one dybe they tween two charges varies directly each represent the electrustatic unut of charge called the stat as the product of the charges and coulotub mersely as the square of the separation In accordance with this law, the unt of charge is defined as that amount of electricity which, placed one centimeter away from s kyke amount in vacuum repels with the force of yne dy nie See fig 103 Since the dyne 19 that force which will give a gram mass an acceleration of only one centimeter per second per second, the electrostatic unit of charge just defined is a rather grall quartit It 13 called the stat coulomb Incidentally it will be noted that all electrical units are pamed after famous physicists, but with certain prefixes attached to indicate the system to which they belong, Since there are several systems of electrical units Concept of Electric Field Thus the concept of electric charge is established as a sort of speck of electricity capable of exerting an influence upon all other such specks in its neighbor hood The regions around charges are therefore places where electrical forces are manifested, such regions are usually called elecinc fields Electric Field Intensity What 13 now more natural than to gauge the electrical intensity of a field of only in the minds eye, ·122 STATIC ELECTRICITY by the reaction of a unit positive charge? This gives rise to the important concept of electrical field intensity at a point It is defined as the force per unit positive charge at the point in ques tion It is perhaps becoming clear to the student that the study of electricity, with its abstract concepts, is developed as a study of the activity and behavior of electric charges, with the unit posi tive charge used for exploring and testing purposes, and that the procedures involved are imagined to take place rather than di rectly observed Electric Potential A common term in electrical language 13 potential One often hears references to high potential wires and the dangers associated with them Sometimes it is called high voltage What does this mean and why is voltage so important in this electrical age? What is voltage anyway? The answers to these questions are quite definite now that the concept of the electric charge has been established Since a posi tive charge, for example, exerts an attractic force on a negativa charge a force must be cxcrted to separate them If they are to be separated work must be done in the process, because work is al ways done when a body is displaced in the same direction as tha of a force applied to it Similarly work must be done to push two charges of like sign together This is equivalent to saying tha electrical potential energy is involved in the process Specifically when a positive charge is moved away from a stationary negativ charge, or a negatively charged body, work must be done upon i and it is said to acquire clectrical potential energy This energ is capable of moving it back toward the negative charge, if re leased, just as an unsupported book or other object is capable o falling to the floor, under the action of gravity In the vicinity of a positively charged body work must be done upon a positiv charge, or a positively charged body, to mote it from one poin to another point nearer the positive charge This is described by saying that points in the vicinity of positive charges are at relr trely high potential By the same argument it follows that point: near negative charges are at low potential Thus the difference u potential between two points represents the amount of work neces sary to move & unt amount of charge from the one to the other If the unit amount of charge is one stat-coulomb then the differ ence in potential is one stat volt The solt, which is 1/300th of stat volt, has been adopted as the practical unit of potential It ·cotteeth, POTENTIEL DIFFERENCE 123 drechesions toltage and potential are terras often used mously, if not always correcths Mechanical Analogy of Potential Difference Regarding the question of danger acepciated with high potential or more with farge differences in potential the situation is ADOBOUS to that of a missile raised to a high eles Ation If after keng tarted it should be released energy would be liberated in 23 amount depending upon the mass of the missile as well as the petation of it so with electricity a large diference in potential represents a store of electrical potential energy If it should be released bs the introduction of a conducting path between the points esperiencing the difference in potential then energy kould be liberated in an amount depending upon the magnitude of e charge and the difference in potential This is precisely what Pnens in a flash of lightning which is nothing but a release of Electrical energy Potential difference is often referred to as eler inical pressure, with which concept there is a close analogy Electrical Conduction and Insulation. The question of how a difference in potential is maintained as an interesting one as well **an importapt one considering that opposite charges just natu Taly try to get together and resist separation Here the matter of Insulation is the key to the situation All substances can be liged up in order of their ability to conduct electrical charges Thase at the top end of the let are referred to as conductors and those at te bottom end as insulators The metals in general constitute the former group and substances like porcelain glass hard rub ber ebonste ete constitute the latter Du air is also a good in dulator as much as 30 000 volts difference in potential can be muntamed betoon conduttons ode centimeter apart in dry gir Production of Potential Differences The production of a difference in potential 19 a matter of separating charges 77. ** Ju what happens when the ebonjte rod is subbed with fir Flec trical charges are simply separated by friction the rod becon iok charged negatively and the fur nosstively But let us now bring the story up to date lest someone ask where the charges originate to be thus capable of separation by friction se let us consider the electron theory The Electron Theory As was previously mentioned the unit positive charge 19 not a natural unit even though it mat once ·124 STATIC ELECTRICITY have been so conceived by early physicists It was an inten. designed to facilitate the study of electrical phenomena During the first decade of the present century evidence was graduall accumulated which disclosed the existence of a natural unit of clectric charge This view became established hen the Americao physicist Millikan about 1910 succeeded in isolating the clectron and measuring its charge in terms of the unit positive charge It was found to be negative and very small, less than one-half a bil lionth of a stat coulomb Without going into the details of this discovery at this time we shall treat the remainder of this description of electrica physics in terms of the now accepted electron theory, which state that most electrostatic phenomena are due to the existence c free electrons associated with matter Thus it is believed toda that when a rod of ebonite 18 rubbed with fur, what really har pens is that negative electrons are removed from fur and a cumulated on the rod Similarly in the case of the glass rod an the silk, clectrons are wiped off the glass and cron ded on the sill Reference is still made, however, in all quantitative discuss101 to the old unit positive charge, but this 13 simply to be thought of as the absence of about two billion electrons Of course, the charging of a body by friction in no way involves the generation of charge aince matter itself is made of charges Charges are not created but merely separated by such processes Potential differences, on the other hand, are created as charges are separated Electric Capacity. There is still another aspect to this situation A given amount of charge does not always establish the same potential for every body any more than a given amount of water produces the same pressure at the base of its container Since hydrostatic pressure at a given place depends upon the depth of the water there, a distinction must be made between the pressure developed by the same amount of water in two different containers of different heights and correspondingly different cross sections Obviously the pressure at the base of a tall slender standripc 18 greater than that at the base of a short pipe of correspondingly larger diameter holding the same volume of water Sec fig 10 4 On the other hand, if water is released from these two pipes simultaneously, the pressure will drop more rapidly in the former case The latter might be said to have more staying poner Put ·ELECTRIC CAPACITY 123 d ferentis at 19 obvious that in the case of the second pipe more ater is required to raise the pressure a giuen amount than in ile ti e of the former because more water is required to raise the wlace level a gsen Amovat 1 A 6 F8 104 Although the two tanks conta a the same amount of mater the pressure at the base of A is greater than the pressure at the base of B Electrically tank B would be 6d to have more capac tance re the qures more patenal to increase the pressure by un t amount than does tank A Electrically & similar situation holds Conductors of large dmensions require more charge to raise the potential by A given amount than do small conductors See fig 105 Conductors thus have a characteristic known as capacitance or sometimes just called capacity by which term 18 meant not the amount of electricity that the conductor can hold but rather the amount necessary to raise the A potential bv & unit amount One stat-coulomb per stat B volt is called a stat farad of Fig 10-5 Sphere A wll develop a capacity A conductor or & greater potezual than B with the same amount of charge owng to the larger combination of conductors capac tasce of B and insulators having capaci tance or capacity is called a condenser Condensers consisting of layers of tin foil separated by layers of wated paper and packed into sealed containers are widely used in all telephones radios Automobiles and innumerable other electrical deyices and so play a very important role in modern life ·126 STATIC ELECTRICITY An Application of the Condenser To elaborate upon just one of these applications, consider the automobile The function of the spark plug is to produce sparks between two electrical ter minals at proper intervals As has been suggested above, the spark will occur when the potential difference between the electrodes 19 raiced to the proper value If there is a condencer in the spark plug circuit, the amount of charge required to raise the potential to the proper value 19 increased, with the result that when the spark does cecur a greater charge and a greater amount of energy than othermice will be released and the spark will be more intense, or hotter, as the garage man would say This may suggest the origin of the term con denser, whose function is, so to speak, to condense electricity Detection of Electric Charge-Th Fig 106 Diagram Gold-Leaf Electroscope This discussio: of a gold leaf elec would not be complete without some men troscope enclosed in & container to shield tion of the manner in which electrical charge it from air currents are detected The so-called gold-leaf electro scope is the instrument most commonly use for this purpose It consists simply of a narrow strip of gold fo: hung over a hook suspended through the insulated stopper of: glacs container in such a manner that the two ends of the foi hang down inside the glass without touching at The other en xxx Fig 107 then charged erther positively or negatively the gold leaves diserge of the hook itrmmnates in a metal ball or hnob on the oute side of the stopper See fig 106 When a charged body is touche.. to the knob of the electroscope, the gold leaves diverge because they each receive the same kind of charge and are mu- ·ELECTROSCOPE 127 ually repelled Of course the sign of the charge cannot be directly detected because the leaves will do crge upon recei 106 ther kind See fig 107 Ldirectly, honeser, it may be po sible to determine the ign of a charge, as follows if first the knob is touched with a charge of man sign, such as the po tre charge on glass which has been ribed suith silk, then the leaves till remain separated with this d'urge upon them Yon a posneli charged body brought up to tc clectroscope will cause the leases to diverge farther, but a pegatively charged body will cause the leaves to collapse more or less Thus an unknonn charge can be compared with a knon n one Charging by Induction It is possible charge an electroscore without touching it to the charged body, ie, by induction This is accomplished by first bringing the charged body near the kaob of the electroscope, thereupon the leaves will do crge This due to the fact that the charge attracts to the near end of the koob of the instrument charges of opposite sign and repels to the gold leaf end of the instrument an equal number of charges of the Eame Eign, but without adding to or subtracting from the total tái Fig 10.8 Charging an electroscope by induct on The charge left on the electroscope u of opposite Eiga to that on the charging rod amount of charge on the neutral electroscope By definition a Teutral body has an equal number of positive and negative charges If, while the charged body is thus held near the electro scope, the latter is grounded the leaves will collapse because the Eround furnishes whates er charge is needed and relieves the electroscope of the responsibility If nos, after breaking the ground connections, the charged body is removed, the leaves will #guin du crge because the extra charge supplied by the stund ·128 STATIC ELECTRICITY while the charged body was nearby has no place to go and is not needed either It constitutes an extra charge on the electroscope, which has thus been charged without coming into contact with the charged body, moreover, the sign of the induced charge is the opposite of that on the original charged body See fig 10 8 Summary The concepts of charge, potential, and capacity are so closely interrelated that no satisfactory description of electrical phenomena can be given without referring to them That is why so much attention has been devoted to them in this text Although such attention may not seem necessary for any three particular concepts in a descriptive survey, the study of electricity 18 $0 abstract that considerable emphasis has to be laid upon the basic considerations Otherwise the later study of electric currents and allied phenomena would be a mere display of words With a proper basic vocabulary, on the other hand, such material as will be considered in later chapters can be very $15 nificant even if its consideration is restricted to the descriptive level Before studying electric currents, however, it seems fitting that we should turn our attention next to a subject very closely related to electricity, namely magnetism In the next chapter mag- Detic phenomena will be considered and the subject of magnetism will be developed in much the same manner as its sister subject, static electricity 7 QUESTIONS 1 That 19 meant by the term "capacitance"? 2 that causes a small pith ball which has been attracted to a charged body to be suddenly repelled from the charged body after coming in contact with it? 3 The dangling chain formerly observed on the gasoline truck grounded the truck Explain how this minimized the risk of fire 4 Thy is it recommended that the cleaning of silk garments in gasoline be done out of doors? 5 How may the "leaking" of charge from pointed metallic objects be explained? 6 Why is it better not to say that a positively charged object hras runnu frugterne tirarges, vitu rainer to say inat it 'nas lost electrons ? 7 An uncharged pith ball 13 suspended by a string What happens to it when a positively charged rod is brought near it?

·QUESTIONS 129 8 A positusely charged rod is brought nçar a charged electroscope If the leaves collapse, what is the charge on the electroscope? 9 Is there any point in lying flat on the ground of one is caught out in the open in A etere thunderstorm? Should onc stand under an 1colated trec? 10 Hon does the mass of the nucleus of a hydrogen atom com pare with that of an electron? 11 Il tro charged ping-pong balls repel each other, what can be said about their charges?

·CHAPTER XI MAGNETISM Elementary Magnetic Phenomena Everyone probably has at least come knowledge of magnetism, if only a superficial one, socluding perhaps an appreciation of the simple fact that a com mon horseshoc magnet will attract steel pins and iron filings bu N E Fig 111 A slender bur magnet like 8 piece of lodestone will point north of suspended by a string as shown will not attract anything made of brass It is also perhaps well known that a slender bar magnet suspended transversely by a thread tied around it at the center will co orient itself as to point in a north south directron Sce fig 111 This observation was made by the ancients who suspended clongated samples of the ore magnetite, commonly called lodestone, and thus the mariner's compass was intented, supposedly by the Chinese Here, as in our study of clectrostatice, the question is again asked concerning the nature of these phenomena It is also noticed that the opposite ends of a bar magnet act as though they are magnetized oppositelt Because of the directional aspect, the terms north and south are used instead of positive and Degative to designate the two ends It is also noticed that the north ends of tho different magnets repel each other but that a 130 ·131 TERRESTRIAL VAGY ETISY end attracts & south end, just as hibe electric charges of wicity repel and unlike changes attract cach other Explanations of Magnetism Theac phenomena Euggcated to early physicists that magnetiem like clectricits, was corpuscubin Dature, insoling small units of magnetism analogous to uts of electric charge Such units were called magnetic poles, torth and south, rather than poritive and negative in vacy of the 200ciation of magnetic phenomena with geograplucal north and south poles A Coulomb law of attraction and repulsion of these magnetic poles raS also discovered just as in the clectrical ca«c, and the unit magnetic pole sas defined in a manncr similar to that employed to define the unit charge Today, hos cier, no specal significance is attached to this unit pole not only becausc no rich natural unit of magnetism has cier been discovered but also because it seems to be possible to explain magnetic phenomena in terms of moung clectric charges making the polc concept un- Decessary Aevertheless the pole roncept persists in magnetic Homenclature and is still uscd in elementary discussions of magnetic phenomena becausc explanations in terms of mouing charges fcem to be on a comes hat advanced level Terrestrial Magnetism The carth's magnetism provides a Sery interesting topic for study It appears that the earth bchaics like a large spherical magnet with a north south axis which makes a relatively small angle 12th its geometric axis The north end of a compass needle does not point truc north, 1c in the direction of the north star, but toward the magnetic pole, which is situated in the general vieiruty of Judson Bay in Canada This pole of the compres is cometimes referred to as the north-scching pole Also since the geographic and magnetic poles of the earth do not coincide it fallans that compass needles which presumably point north are likely to be in error at any on point on the earth's surface In New England this error, or angle of declination as it 13 called amounts to ncarly tnents (legiocg West the carth are the earth, but he fact that a As at a conéiderable depth the compass needle suspended 50 As to be free to move up and down as well as from right to left tends to display a downward tendency at all points except on the magnetic equator See fig 112 At +1 polcs the needle points straight up and down the, UM ·It is c 132 MAGNETISM made with the horizontal is called the angle of dip and in Ne England amounts to some seventy odd degrees Thus in New En land the downward tendency outseighs the horizontal one! three to one Technically it is said that the vertical compone of the earth's magnetic field is thi times as great as the horizon component in this vicinity As the study of electrostatics, the : tensity of a magne ic field at Dip Angle point is attributed to the exister of a neighboring magnetic pole serves to measure the polo streng Fig 112 A compass needle of the magnet, wbich to the layır. mounted so as to be free to is a measure of the strength of rotate & vertcal plane will magnet in question dip below the boruton in the porthern hemisphere Magnetic Materials tomary to associate magnetic p nomena with the substance iron This is due not only to the ea discoveries of magnetic properties among ores of iron, but a to the fact that iron is the most conspicuous of naturally occ ring magnetic substances It would be a mistake however assume that iron 13 the only magnetic substance Nichel cobalt display magnetic properties and recently alloys have b made which have far erceeded iron in magnetic properties ( of these alloys is known as permalloy and was developed in research laboratories of the American Telephone Company use in telephone equipment Another one of these alloys is kno as alnico" Magnetic Induction One of the most interesting of various magnetic phenomena is magnetic induction When a of unmagnetized iron is brought near one end of a magnetized : magnetic poles are induced n it That is to say the bar beco: magnetized by induction with the near end acquiring a pole Bign opposite to that of the inducing pole and thc far end acqı ing a pole of the same sign as that of the inducing pole It 13 a the unmagnetized bar were composed of myriads of elonga molecular magnets capable of orienting themselves at wall In unmagnetized state these molecules are to be thought of as ented at random but in the vicinity of a strong magnetic no pole let us say, all the south ends of the molecular magnets ·INDUCTIOY AND PERSIELBILITY 133 rand toward the north pole and the north ends are repelled rom it, causing the bar to become magnetized with equal and posite pole. This is the essence of one theory of magnetism Puch, although alreads out of date still senses to provide an ele muary picture It does not completely clarify the nature of the The cular wagnete, nor explain why only iron and a few other Estances display this peculiar property to any marked degree Set for elementary purposes it 13 reasonably satisfactory | Magnetic Permeability It is by induction that most magnets me made Bars of unmagnetized iron or other magnetic substances re subjected to strong magnetic fields which establish the mag etism by influence so to speah One of the features of permalloy resiously mentioned 14 its ability to become very strongly mag Luzed under the pfluence of a relatively weak magnetic field Is is due to a property technically called permeability which Words one of the most useful means of classifying inagnetic cub saces If, for example substance is strongly magnetic its per ability is very high and the cubstance 13 sud to be ferromag c Il, by its presence a substance enhances & magnetic field I not so strongly as a ſerromagnetic sub-tance it is called 'amagnetic The substance bismuth on the other hand 19 con cuous because it weakens a magnetic field by its presence It aid to be dramagnetic Paramagnetic substances are character I by permeabilities greater than unity whereas a diamagnetic tance has a permeability less than unity ummary Much of the study of magnetism deals with elec nagnetist which logically cannot be considered until electric ents have been studied Such material will be the subject for ideration in the next chapter Here we have dealt with so d static magnetic phenomens as the previous chapter dealt static electrical phenomena Such a preliminary study de ps the vocabulary and fundamental concepts upon which the * study is so dependent le shall sce presently how the two electricity and magnetism blend together in the study of ric currents QUESTIONS That is meant by the expression north secking pole ? Distinguish between angle of declination and angle oi dip that is meant by a magnetic field?

·134 MAGVETISY 1 REVIEW QUESTIONS (See pape 199 for answers > CHAPTERS VIII, IX, X, AND XI 1 That temperature on the Centigrade scale which corresponds to 68° Fahrenheit is. 1 20° C, 2 37 7° C, 3 68° C, 4 648° C,5 55 5° C 2 The amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit is called I the heat of fusion, 2 the BTU, 3 the calorie, 4 the erg, 5 the heat of vaporization 3 Franklin's pulse glass demonstrates 1 specific heat; 2 the mechanical equivalence of heat and energy; 3 boiling under reduced pressure, 4 regelation, 5 surface tension 4 As the atmospheric pressure is raised the boiling temperature of a liquid is 1 lowered, 2 raised, 3 unaltered, 4 just 100° C ,5 212° F 5 The reason for nickel-plating a flatiron except on the ironing surface is 1 to prevent radiation, 2 to encourage radiation, 3 to present conduction, 4 to encourage conduction, 5 to encourage convection 6 The reason for insulating a refrigerator is 1 to prevent outward radiation, 2 to prevent heat from coming in, 3 to prevent the cold from coming out, 4 to melt the ice, 5 to present the ice from melting 7 Cork with respect to copper is a much 1 better radiator of heat, 2 poorer insulator of heat, 3 poorer conductor of heat, 4 poorer radiator of heat, 5 better conductor of heat. 8 Heat is transferred from a steam radiator to a point in a room directly beneath it largely by: 1 conduction through the 31r, 2 conscction currents established in the air, 3 radiation through spice, 4 surface tension, 5 molecular bombardment 9 When an ebonite rod is rubbed with fur, 1 electrons are generated, 2 electrons are accumulated by the cbonite rod, 3 clectrons are accumulated by the fur, 4 protons are acquired by the fur; 5 electricity 13 generated 10 When an electroscope is charged by induction. 1 its charge is always negative, 2 its charge is always positive, 3 its charge has the same eign as that of ·135 REVIEW QL ESTIONS the source 4 its charge has the opposite sign to that of the source 5 its electron content is unaltered 1 The leaves of a charged gold leaf electroscope stand apart I because the sign of the charge is the same on each leaf, 2 because the sights are opposite 3 because charges remain on the outside of conduc tion bodies 4 because of electro tatic sttraction o because clectrons have the opposite sign to pro tong 2 never The purpose of the dangling chain on the gasoline truck 18 i to present positively charged protons from accumulating on the truck 2 to allow protons to leave the truck at will 3 to insure that sparking thich might start a fire will take place at some dis Cance from the gasoline 4 to mam the driver if the truck should become charged and thereby increase the fire hazard 5 to prevent an excess of deficiency of electrons from accumulating Lightning I never stribes the same place twice Em pire Stai 1 tall objects with lightning rods only if they are well grounded 5 13 lese hazardous to a building with lightning rods rcgardless of whether or not they are well grounded A compass needle points in a north couth direction 1 becausc it is electrically charged 2 because the earth is electrically charged 3 because the carth 13 a magnet 4 because the magneti m of the earth 13 uniformly distributed 5 because north poles repel south poles ·CHAPTER XII ELECTRICAL CONSIDERATIONS (Continuer CURRENT ELECTRICITY This is an electrical age and yet the number of people' have the ability to speak properly about electrical matter relatively maPirt or the articulty is undoubtedly due to exacting manner in which electrical and magnetic terms are y fined This has been brought out in the two preceding chaptei so that the student who has followed this presentation thus f should find himself prepared for a descriptive consideration those phenomena which make this an electrical age an age electric currents electric power, electrical devices, and all. topics The present chapter will deal with electric currents Galvani s Experiment 1 Italian scientist Galvani is cri ited with being the first to hi discovered the existence of c rents of electricity He is said have hung a pair of frogs 1 from & metal wire near which 1 a second wire of different ma rial connected with the first Fig 121 A pair of frogs legs hung from a wirr tras obeers ed by fig 121 As the frogs legs at Galvapi to twitch As contact was AC dentally touched the second cidentally made with a second tire made of a Ferent material from the they were observed to two first fire The interpretation given to phenomenon was that a difl ence in electrical potential established chemically through frogs legs and the dissimilar metals caused clectricity to f through the circuit thus established Flow of Charge It is clear from carlier considerations t if a conduc*ing path 1 provided between two points charged different potentials electrical charge should p:es from one po 136 ·FLOH OF CHARGE 137 ta the other in an attempt to cqualsze the potential This would ordinarily be juec & angle curge of charge But if some arrange. Mept could be provided to maintain a difference in potential betreen the two points shile at the same time charges were passips, 20 a so-called current of electricity would be established This benests, of course, a generation of potential difference at a rate Erat enough to offeet the dissipation allowed by the conducting P"> The situation is entirely analogous to the bow of water out of a reservoir under a certain pressure head Unless an influx 19 pipuided to maintang the pressure head, the reservoir will Peatur?ly be drained and the flow will ccace, but such an influx s made possible by the use of suitable pumps See fig 122 Tank Pump Fig 122 The pressure head of the tank 1 baintained by the pump which pepg back into the tank all the water that flows frotn it The Electric Circuit It should be noted also that in addıtran to providing the necessary pressure, such pumps must have the ability to pump the required quantity of water of the flow 18 to be maintained at a constant rate Electrical batteries are such pumps foi electric charges, and obviously the part they play in electrical circuits involves many factors See fig. 123 The study of these factors 19 the content of so called clectnc circmt conerdcrations and includes such matters as how batteries accomplish the generation of potential differences, how electric currents ·138 CURRENT ELECTRICITY Line Res stance பப்ப min key H 7of Volvo Bottery Pump Fig 12.3 Analogy between a simple series electric circuit containing , ery, a key and a resistatice element and a hydraulic circuit containing A pump, a valve and a pipe lipe detected, what laws govern their existence, how resistance 1 offered to the flow of charge, and how electrical devices operat by means of electric currents These are just a few of the man factors that might be listed Electromotive Force and Current There are many kind of electric pumps, or seats of electromo tive force (EJF), as they are calles Although the rubbing of an ebonite ro with a piece of fur may develop a ver large difference of potential between ro and fur, such a combination does not mak a good battery because it is not fa: enough, ie, it cannot maintain a lo Zinc of charge of sufficient amount to be practical importance So called chemic: Fig 124 rod of battenes, on the other hand, such as th copper and a bar of Zinc immersed in di common dry cell and 1 copper and zu lute sulphuric acid electrode combination immersed in sulph constitute an electrical ric acid (see fig 12 4), are capable of maii battery suitable for many purposes taiping & relatively large electrical flou even though they develop a potential di ference of only volt or so Chemical disintegration of th more active element of the battery (zinc) results in a separi ·pfer OIV's L45 139 tson of electnc charge An excess of electrons secumulates on this plate (electrode), Ahile a corresponding deficiency occurs at the other electrode, thus producing a difference in potential dtr een them Such cells on dead ehort circuit, 1€, with no appiable resistance in the circuit, can maintain a flor of many operes for a few minutes An ampere 18 8 flow of six and a quar- 2 billion bullion electrons per second Electric current 19 simply the rate of flow of electric charge Electric Resistance Thus it 13 seen that a battery of some kind is the prime mover in every electrie circuit There is another equally important factor, hon ever, controlling the rate at which charge flame, and that is what is called resistance As the rate d' water flow in a pipe depends upon the amount of preseure be Lund the water, it is also gos erned by the amount of resistance chered to the flow Pipes of small cross section, for example, greater resistance than do pipes of large cross section, and "kog pipes offer greater resistance than short ones Morcover, smooth-n alled pipes offer lees resistance than rough-walled ones Thece same factors also enter into electrical considerations It has already been pointed out that different substances conduct electricity deficrcntly It 13 also a fact that the electrical resistance of a wre depends upon its length and cross section Ohm's Law It would be considered a mere truism to state that the flow of furd through a pipe depends directly upon the push behind it and inversely upon the resistance offered to it the most important law of LE Ita importance lies to no ly interpreted, it explains Tactically all current phenomena and gives the physicist the leans of calculating others, ise unknown quantities associated ith circuts Specifically, it states that the current intensity in circut (or a portion of a circuit) us directly proportional to the electromotive force impressed across the circuit (or the po tentral drop across that portion of the circuit), and is inpersely proportional to the resistance of the circut (or of the portion of it under consideration) Thus it 19 seen that if the electromotive force impressed across a given circuit and the resistance of the circuit are known, then the current in the circuit can be readily calculated Moreover, the potential drop across a resetor can be calculated if the current and the resistance are knop F- 1 ·140 CURRENT ELECTRICITY sidering circuits as a whole and then in various parts taken rately, much information can often be determined in terras very little This is why Ohm's law is referred to 88 & very tant law of electric circuits Effects of Current It is appropriate at this point that & question be raised concerning the means used for detecting electre currents if they are merely charges in motion, especially if charges are not capable of direct observation The answer is that clectric currents are recognized by various effects which they have been found to produce To enumerate briefly, these effects are chemical, heating, and magnetic Practically all that is know? about electric currents is the result of information obtained directly by studying these effects Chemical Effect of Current That current is related t chemical activity is not to be doubted in view of the very exist ence of chemical batteries In theee, the chemical reaction sepe rates charges, thereby causing negative charge to accumulate o one electrode and positive charge on the other Thus all batterie have positive and negative terminals When a closod circuit : connected to these terminals, or electrodes, not only is a currer established but its direction also is specified Direction of Current It is a matter of convention that the direction of a current is taken to be from the positive terminal of the battery to the negative, through the external circuit This 18 based upon the idea that unit positive charge is repelled from the positive terminal and is attracted toward the negative terminal via the conducting wire Of course, the electron theory invalidates this view since the particles which actually move through solid conductors are the negative electrons, and they pass from the negative electrode where the surplus exists, to the positre celectrode a hich represents the deficiency of electrons Nevertheless, the conventional direction is still adhered to by most writers, on the grounds that it makes little difference which 18 actually night so long as there is common agreement, and after all it does make sense to speak of the reduction of the deficiency of electrons from the positive to the negative terminal as the clectrons flow the other way Electrolysis Metallic wires are not the only conductors of electricits Solutions of many chemical salts conduct electricity ·ODAnode Solut ortot Yooon ELECTROLYSIS AND JOULES LAW 142 the formation of particles called rons, electrically charged ender poquvely or negatively, which are attracted or repelled respectively from charged electrodes immersed in them Such powtions are called electrolytes 11 hen current 13 passed through a electrolyte in a cell the postte jobs, called cations, are drawn ward the negative terminal, and Battery Key de negative ions, called anions, are enn toward ide po'itive terminal Cathode There ons get their names from the ermis cathode and anode, the Dames 5 Tvet Nen to the negative and positive lectrodes respectively of the elec- Iver Sale puolytic cell, the cathode being the minal where clirrent leaves the ial, and the anode, the one where current enters Metallic ions, being positive, are deposited out on the Fig 125 Electroplating Silcathode of electrolytic cells to which ver 13 deposited upon the spoon hey are drawn This is the basic Tbich ng connected to the cath ode of the cell when the circut principle of electroplating Silver as closed Tay thus be electroplated on a spoon, for example, by immersing the spoon in a bath of some sil ver ealt through which an electric current is passed if the spoon s connected to the negative terminal of the cell See fig 125 Faradav, about 100 years ago, studied the phenomena oi electrolysis and formulated laws goverging it These are considered basic to electrochemical industries even today The Legal Unit of Current Electrolysis has also provided he means of specifying the unit of electric current, the inter ational legol ampere, which is defined in terms of the amount (silver derogited per second out of a standard solution of silver itrate Thus the study of the chemical effect of the electric cur ent has yielded considerable information regarding electriemy motion, and has been respotsible for the standardization of the legal unit of current Heating Effect of Current--Joule's Law Let us vext consider the heating effect of current It is a well-kpown fact that a wire carrying an electrie current becomes heated Witness the modern electme range of electric toaster for visible evidence ·112 CURRENT ELECTRICITY of this phenomenon The conversion of electrical energy int heat energy is basic to many of the electrical devices which mal this an electrical age The fundamental principle involved is give in the law named after the English physicist Joule, who discos ered that the rate at uhich heat is developed in a conductor 6 an electric current depends upon the second power of the curret and upon the resistance of the conductor Thus if the current doubled the rate of heating is quadrupled Also, coils with lari resistance heat more rapidly than coils with low resistance pr viding the current is the same in each Because of the entran of the resistance factor into this picture the conclusion 19 su gested that Joule s law must be related to Ohm's law This found to be true both theoretically and experimentally Electric Power As a result of this relationship still anot! important relationship is found to hold, namely that the pre of the potential drop, in volts, across a circuit such as that an electric heater or electric light, when multiplied by the c! rent, in amperes, through the circuit gives the power, in wat or the rate at which electrical energy is dissipated in hent T becomes a practical matter whe ono is concerned with the co of electrical energy, which vari from approximately one centi ten cents per kilowatt hour de pending upon the locality and th circumstances A hilou ott is or th thousand watts, and a holou a Fig 126 One method of meas hour represents the amount of er uring current intensity 13th ergy consumed in one hour at ti the hot wire ammeter The rate of one kilowatt Thus at wite is napred around a spindle sluch a rotated as the wire ex place where electrical energy cos prod. oming to the heat devel 10 cents per kth (kilowatt hour oped in it by the current A pointer connected to the spindle a 100 watt electric light bulb ca ulags rast a graduated scale be run for 10 hours at a cost of 1 cents or one hour for one cen A fation probably costs exeral times as much to uprctatt as th: and a toaster, about the same as a flatiron The heating effect of current is utilized in certain types ammeters, which are the instruments used for measuring curre: intensity In these instrumerts a wire is wrapped around 1 spil ·has TR 127 A compass aprile placed on top of pa konzontal wire carry- WAGNETIC EFFECT 113 de upon which is mounted 2 rointer See fig 126 As curten" is established in the wire, heat is des cloped and the circ lengthens Orring to thermal expansion This causes the epindle to be rotated through a certain angle depending upon the intensity of the current, making the pointer seep across & portion of a graduated scale placed behind it Although this principle 18 readily understood, it is not the one upon which most ammeters actually oper ate The latter utilizes the magnetic effect hich will not be described Magnetic Effect of Current The Danish phreriet Oersted was the first to observe that a compass needie is di-turbed in the vicinity of a current-carrying wire Sce fig 127 Further examination showed haz current toward the the exstence of a magnetic field con gaurth 13 deflected east- Ek Oersted discovpletely encircling the current-carrying conductor This is found to be always the case, ie, no current can be establi lied in Jamre unless at the come time a magnetic field is simultaneou Is Ret up encircling the wire See fig 128 Thus all electric currents have magnetic fields associated with them It should be noted especally, however, that the currentfarrying wire does not itself become magnetized In fact, the Current kite is more often than not made of copper, a nonmagnetic subtance, and therefore cannot become a magnet The magnetic effect is produced in the region around the wire since the field WE encircles ut rather than radiates Fig 12 & 'The magnetic field encircles the current carrying conduc- The Electromagnet. If ! tor as shown fire is wound into a long folenoldal coil, 1 e, a configuration such as would be obtained by winding & single layer of turns on a long cylindrical core and allen Withered thus effect G out from it ·146 CURRENT ELECTRICITY by the establishment of a current in it, Faraday discovered thu. the result of mechanically pushing a wire, which forms part of: closed electrical circuit, across a magnetic field is the establisha ment of current in the circuit as if by a battery In other words he discovered a way of inducing an electric current in a circuit merely by moving something In still other words he discovered a way of generating the electromotive force necessary to produce electric current, without the aid of batteries This simple d..covcry paved the way for the modern generators which, operated either by steam or water poner, are the very mainstays of today! electrical civilization Without the generators there is no elec trical power And basic to all generators is the simple matter of clectric wires being pushed across magnetic fields by the rotatior of armatures The generator principle is simply the inverse of the motor principle it represents a transformation of mechanica energy into electrical energy Lenz's Law It should be mentioned that the process de scribed above is but one way of inducing an electric current in: circuit Further study shows that the situation is far more gen eral than has just been described Any change whatever in th status quo of an electric circuit which causes a variation in th magnetic characteristics of the circuit, is sufficient to establic momentary currents in neighboring circuits This is the gist of law of induced currents known as Lenz's Lau The inductio: a sociated with a changing current is basic to the operation o the clectrical transformer and other so called alternating-curren devices The study of these devices, however, including the spar! coil, the telephone and telegraph, the radio, etc, would carry u far beyond the descriptive stage and so must be postponed for more exhaustive consideration by those students who are willin and able to des elop a more mathematical language Suffice at t say that the prisciples discussed above are basic to the operatio: of these devices and account perfectly for their behavior Summary In this chapter the various effects of electric cur rents have been described It has been ghown that the study a these effects has developed considerable information about elec tric charges in motion, although the charges themselves are no capable of being directly obeered This information has alread: been carefully organized into a body of logically developed last and praciples dunng the last one hundred years or so, and toda!

·QUESTIONS 147 we lire un an electrical age because of the many practical apsleations of these principles of these the principles of the motor the generator arc especially important QUESTIONS £ 11 hat in the nat 41 Describe the process of electroplating 6 List three major effects of the electric current 7 Distinguish between the direction of current and the direction of electron flow & Which costs more to operate a 100 watt bulb for 10 hours or & 10 watt bulb for 150 hours? What was Oersted s discovery? Explain how the clectromagnet functions ? What is the left hand rule? ! How is tbe international ampere defined?

·CHAPTER XIII ELECTRONIC CONSIDERATIONS AND ATOMIC PHENOMENA INTRODUCTION TO MODERN PHYSICS ELECTRICAL DIS- CHARGES IN GASES ELECTRONICS X RAYS RADIO ACTIVITY NUCLEAR PHYSICS The Situation about 1890 At the dawn of the twentiet century the subject of physics was in a rather unique positio with reference to the other sciences and the many fields of learn. ing This was about the end of the so-called classical period, which was characterized by the Newtonian point of view The situation was unique because the physicist, in his attempt to explain physical phenomena, was apparently almost completely successful Practically all that had been observed was under: standable in terms of simple concepts and laws such as Newtoni laws of motion, etc These were so carefully and so logically de veloped that the physicist seemed to have the situation com pletely in hand in a fashion little short of miraculous, new ob een ations had been fitted into places apparently made for then in the theories and hypotheses extending over a hundred or mor Sears On the theoretical side it was felt that physics was fas approaching the status of a dead classical subject so completel worked out in detail as to leave very little of interest, if anythin at all, for future generations to unravel All was understoot Physics was looked upon as the pride and joy of the naturi philosopher, a masterpiece of logic, successful in every exper mental test At this time famous physicists made statements 1 the cffect that all the important discoveries in physics had bee made and that most of the romance in the subject was gone, a that was left for future generations was the somewhat colorle task of extending the precision of the measurements of know physical constants and relationships That this view was all roog is the story of modern phy je 148 ·the electron DISCHARGE TCBE 149 alors that has completely resolutionized the whole outlook of pbraical science. That the physicit has been able to appreciate He change in the situation is even more significant In earlier fepten relerences have been made to the new physics, peading a deselopments since 1900 ar thereabouts The purpose of this Lpter is the consideration of some of the ideas and concepts of der physics as they have been des eloped supce the discovery Electrical Discharges in Gases About 1895 the eminent English phy szczet Sie se Thomson made torne strange observafons that rere to prose most important He was experimenting th the conduction of electricity through gases Among other Jungs he observed that dry air at atmospheric pressure 15 R rather od insulator, requiring some thirty thousand volts per centi- Tarte to break it donn, ic, to make it pasg charges of electricity of the form of the electric spark It was found, bowever, that pihen the pressure of the air was reduced, as by a vacuum pump for exataple, the necessary voltage for breakdown was apprefrably diminished, but not without a very noticeable change in lle character of the discharge These experiments were firt car ried out at a time when vacuum pumps were not only scarce but exceedingly inefficient Today however, because of technical ad- Faaces, vacuum pumps are available which make it possible to perform these experiments in a demonstration lecture right before very eyes of students Discharge Tube Demonstration If a long, slender glasa tube, say about two feet long and an inch or two in diameter, with electrodes sealed in at its closed ends, is connected to a modern vacuum pump through a tube joined to it at one side, 1 succession of interesting phenomena will be observed as the air very gradually withdrawn from the tube while a large potential difference 18 maintained across the electrodes by e spark col or Joide other source of several thousand yolts Ser fig 131 In a larkeded room the experiment is enhanced At atmospheric pres. BUFP or at the start of the experiment, no charge whatever 18 viazble between the electrodes, but very soon after the vacuum the 3 indehoren becomeg lighted by a purplish glow At this state the presture ·150 ELECTROVICS 18 probably not more than a few millimeters of mercury, or somewhat more than a few thousandths of atmospheric pressure Presently, the general glow turns somewhat pinkish and then becomes quite pipk In the meantime, a very definite blue glow appears near the negative electrode This is called the negative glow Between this glow and the main body of the pink discharge Crookes Dark Foreday Dak Space Space Cathode Anode Cathode Negative Glow Glow Strated Post ve Column (Vacuum approx one one hund edth mit meter of mercury) Vacuum Pump Induct on Col Fig 131 Discharge tube phenomena a relatively dark space develops This is called the Faraday dark pace The pink glot, si hich gradually shortens to make room or the Faraday dark space, 19 known as the positive glow or the posituie column Soon thie positive column presents a very unique appearance by breaking up into innumcrable striations while at the same time the pink color fades into a pale orange color, even turning white The shortening continucs as the Faraday dark sprcc and the negative glos secm to break away from the cathode and to more nearer the positie electrode, leaving a second dark snace between the negative glow and the cathode utseli This durk space has become known as the Crookes dark space The cathode itself alco glows with a violet light known as the cathode glou By this time the preasure has been reduced to probably a matter of tenths or even hundredths of a millimeter of mercury As ·the wave variety like light and is characters artim CATHODE AND X RAYS 151 the exhaustion continues thc etrations of the positive column present a more and more fluted appearance, becoming coarser and coarcer The positive column, which is now definitely gaco ciated with the positive end of the tube, becomes shorter and shorter The Faraday dark space and negative glow move farther and farther anay from the cathode, with the result that tlie Crookes dark space gets larger and larger, crowding all else right out of the positive end of the tube By the time the pressure is reduced to about a thousandth of a millimeter of mercury the whole positie column has been thus completely crowded out and the Crookes dark space completely fills the whole tube That is to say, the whole tube appears very dark except that the glass walls glow with a peculiar green light called fluorescence Fluorescence is the name given to the process by which radiation of a given mave length is transformed into a radiation of a different wave length (usually longer) In this case fluorescence is produced by cathode ray bombardment from the negative electrode Cathode Rays At this stage of exhaustion it is noted that any object inside the tube between the electrodes casts a shadow on the positive end of the tube This indicates the existence of poine sort of radiation from the cathode toward the anode In addition to producing fluorescence, the radiations travel in straight liner, forming sharp shadows of objects in their paths It 19 nko found that the radiation can be focused by using a spherically concayo cathode when it is thus focused upon a target, hent is generatrid at the spot on which the rays fall Because the radiation inste T 11118 len uw trical properties It was the discovery of these electrome buy one siderations of electrical discharges in gases that wird in tfn new era in phyics When cathode rays are allowed to fall unom * X Rays w UVA > ·152 ELECTROVICS its ability to penetrate relatively opaque substances It not only makes possible the modern \ ray shadow photographs but 18 used extensively today in hospitals for therapeutic work The nature of the X radiation depends upon several factors, includ- X Rays Cathode Rays Target Fia 132 X rays may be produced by cathode rsys bombarding & target ing the potential drop across the tube, the degree of vacuum, and the substance of which the target is made Applications of Discharge Tubes Partially exhausted tubes vrough which an electrical discharge is produted, as in the experiment just described, are called Geussler tubes The color of the discharge depends upon the gas used The modern neon advertising bigas are nothing but such discharge tubes, using the gas neon, however, instead of air because of the characteristic red color of neon when activated in this manner The blue violet light of the photographer's shop 19 a similar light, but one whose tube contains the substante mercury In this tube, however, a mercury Arc accounts for the radiation At the present time it appears certain that ordinary household electric lights of the future will be just such discharge tubes in which the phenomenon of fluorescence is utilized by coating the walls of the tube with various chemicals capable of producing different colors Such so called fluorescent lamps are becoming quite common Incidentally, they are considerably more economical to operate than the present day filament type lamps Thermionic Emission With the discovery of the electron came the understanding of another phenomenon which is fun damental to modern communication, ie, the Edison effect, or the thermionic cmrsalon of electrons Heated filaments in vacuum ·EDISON AND PHOTOELECTRIC EFFECTS 153 tubes are found to emit electrons a bich can be drawn acrosan ezopty region by electrostatic attraction toward positively charged plates See fig 133 Such electronic behavior is the basic principle of all radio tubes and of the host of so called control tubes for chaumierable electronic appliances See fig 134 The modern Flament, - Plata Fig 133 When fila ment 14 heated in 2013 um electrons are emit led by it These elec trons are readily attracted to a positively charged plate producing a current between filament and plate Thug explains the Edison effect of a cur rent between plate and filament ben the former is charged porttively Fig 134 The rectifying action of the gnd in a three electrode vacuum tube The and if alternately charged positively and negatively acts so as to control the low of electrons from filament to plate It allows the flow to take place only when it is positively charged Thus al ternating currents can be rectified into direct eur rents X ray tube utilizes thermionic ero184100 for the production of electrons Oace obtained by emission from a hot filament, the electrons are directed toward a target from which X rays are emitted Photoelectric Effect Another interesting electronic phenomenon is that of the emission of electrons from the surfaces of certain substances by the action of light of tadiatios generally including rays This is called the photoelectric effect It 13 basic to the operation of the photoelectric cell or electric eye as it 18 more popularly known tyhen light shines upon these cells electre currents are established, but when the light 19 shut off the ·154 ELECTRONICS current is interrupted See fig 135 Thus an electric current can be controlled just by regulating the amount of light falling upon the cell Because of this phenomenon the photoelectne cell is 3 very useful device for controlling electric currents It play e a very important part in the operation of sound movies and in televi Lght sion in the case of the former, 3 beam of light is varied in in tensity or direction by certain markings on the side of the film, Whereupon an electric current is varied accordingly to control the sound mechanism In the case of Frg 13.5 The action of a photoelectric cell Light falung upon television, light reflected from the inside of a vacuum tube the subject is picked up by a coated with certain actite sub scanning device in which an elecstance causes electrons to be emitted These electrons can be tric current is vaned according attracted to a po tively charged to the variations in intensity of rc rode in the tube to produce the reflected light in the so en electric current capable of be 18 controlled by vanations in called television receiving set the Ight intensity operations are essentially reversed and a varging current 13 translated into a visual image which is cast upon a screen Electronics Applications of principles euch as those just discussed constitute the important field commonly referred to as clectronics Since many of these applications lie in the field of communications, many people have acquired the erroneous idea that electromanu means simply radio Of course, many var application and developments such as radar (chort-wave-directed radio) were directly concerned with radio, but probably the most toportant peacetime applications will be in the field of industrial controls of machinery, not to overlook the all important medical field Since the term "electronics" covers all clectronic pbenomena, it 13 actually imposeible to define the field sharply in view of the fact that the electron is a basic constituent of all matter Atordic Structure The discovery of the electron marked the beginning of an extraordinanly active era of research pertaining to the structure of the atom Before this the atom was considered to be an indivisible entity Today, however, this 13 ·ATOMIC STRUCTURE 155 not the case Along with the discovery of cathode, or negative, rays streaming from cathode to anode, a positive radiation was also observed This positive radiation was discovered by the use of perforated cathodes which allowed particles originating at the anode to pass through the perforations in the cathode and be detected at the glass wall opposite to the anode See fig 136 Post ve Rays Cathode Anode Cathode Cathode Rays Fig 136 Positive rays are emitted by the anode as well as negative rays by the cathode These are positive Jons Both sets of rays are capable of detection by elec tric and also magnetic fields Positive Rays Whereas the cathode rays consist of negatively charged particles, all exactly alike as shown by the action of a magnetic field upon them, the positive rays are found to consist of positively charged ions of various masses and charges The smallest of these ever observed has a mass approximately equal to that of a hydrogen atom This positive unit is called the proton Its charge 13 numerically equal to that of the electron, but its mass is some 1845 times the electronic mass It is the nucleus of the hydrogen atong The other positive ions seem to be combinations of protons and electrons, and have masses coughly corresponding to whole numbers of protons Bohr Theory These elementary facts regarding electrical particles led to the development of a succession of theories peraining to atomic structure Probably the most famous of these .s the Bohr theory of 1913 previously mentioned in this text On :his theory the hydrogen atom consists of a single proton about hich a single electron revolves like the moon around the earth See fig 137 The atom of helium has a positively charged nucleus about which two electrons tevolve In this way all the elementary Atoms were thought to be made up of combinations of positive and Pegative charges of electricity forming dvnamical systems of ·156 ELECTRONICS Varying complexity The original success of this theory was its explanation of optical spectra, about which more will follow in succeeding chapters In many details, however, this theory has been found inadequate and is no longer accepted as final Yet the views which have supplanted this theory are not altogether suitable for pictorial representation, and so the Bohr theory 13 Atill referred to in many popular discussions Hydrogen Heltum Fig 137 Bohrs concept of the hydrogen and the helium atom with negative electricity circulating around positively charged nucle: The helium nucleus 29 the alpha particle of radioactivity For many years after the discovery of the electron and the proton, these two particles were considered the sole constituents of matter It was not until the fourth decade of the present century that this number was increased by the discovery of the neutron and then within a few months by the discovery of the positive electron The neutron is a particle carrying no net charge and having a mass approximately equal to that of the proton The positive electron (or positron) carries a positive charge but otherwise has all the characteristics of the negative electron It now appears that the nuclei of all atoms are composed fundamentally of protons and neutrons It is also felt today that there are probably other subatomic particles which have pot yet been discovered, and, whereas only a few years ago the electron and the proton held the stage, there are now at least four recognized fundamental particles Much of today's research work is des oted to the possible discovery of additional fundamental particles and to the study of atomic nuclei Only yester day the atom was found to have a structure, and today the attention of physicists 19 focused upon the question of the structure of one of its component parts, namely the nucleue Tomorrow may bring yet more surprising discoveries ·#rays Brays RADIOACTIVITY 157 Radioactivity. This question of the atomic nucleus has become of additional importance today because of the discovery of artificial radioactivity Radioactivity is the name applied to the spontaneous disintegration of certain substances such as radium, the radiations from which were discovered by Becquerel, and the isolation of which was yrays achieved by the researches of Me. dame and Pierre Curie These substances actually emit electrically charged particles along with radiation similar to X rays The rad12- tions were named alpha, beta, and gamma before their natures were understood The alpha radiation consists of particles now known to be the Fig 13.8 Radioactive subnuclei of helium atoms Since they stance in a clot in a lead block The action of a magnetic field carry two extra positive charges they transverse to the diagram upon are called doubly sonized helium at a (alpha) rays B (beta) rays, oms The beta radiations are streams and y (gamma) rays respectively Beta rays are sharply of negative electrons, and the gamma curved one way, alpba rays radiation is wavehke rather than cor are less sharply curved the other way, gamma rays are puscular, being a very short-wave unaffected type of Xray See fig 138 These radiations, like X rays themselves, have been put to use in hospitals, where their therapeutic value has been recog- Aized, but radium is very rare and very expensive Today it has been found possible to make substances artificially radioactive by bombardment with atomic particles accelerated to high velocities by super-high-voltage devices Of these the Van de Graaf high voltage generator and the Lawrence cyclotron have been widely publicized Both of these devices are capable of endowing particles such as electrons, protons, and other lons with energies equivalent to millions of electron volt: The electron volt 19 the amount of energy acquired by an electron accelerated through a difference of potential of one volt These devices have really been developed for the purpose of smashing atoms and atomic nuclei to bits in order that the products of such disintegrations may be studied. It is the hope of today that the disintegration of the nucleus may open up new fields of knowledge as did the breaking up of atoms a couple of generations or so ago ·158 ELECTRONICS The Atomic Bomb Early in 1939 the remarkable discovery was made that the atom of a certain form of uranium, hnoma as the 235 isotope, could be split by bombardment with slow neutrons into two lightcr atoms, with the liberation of considerable atomic energy This process is now referred to as uranium fission The military possibilities of atomic energy were immediately recognized This led to the development of uranium fission as the wars greatest secret enterprise at a cost reputed to be around two billion dollars When a critical amount of U235 is accumulated a self sustaining nuclear reaction is a necessary consequence This takes place extremely rapidly and with the evolution of a tremendous amount of energy As a neutron splits the uranium atom, additional neutrons are evolved which split additional atoms and so on indefinitely until all the U235 19 uscd up Another important feature of the process is the creation of a new element, plutonium, which also exhibits fission By methods which at the time of this writing are still carefully guarded military secrets, devastating atomic bombs utilizing nuclear fission were created in the summer of 1945 This marked the first occasion on which man had released sizable amounts of atomic energy, and no one can predict the social and economic consequences of this important event It took World War II to provide the stimulus for crowding into just a few years an amount of research in nuclear physics that would normally have taken probably scores of years Summary The subject of modern physics has been des eloped in this chapter from the early experiments in electrical conduc tion through gases to the modern super high voltage machines used in nuclear disintegration experiments Although many topics have been mentioned here it must not be asumed by the student that the discussion is anything like complete Modern physics 1 far too large & subject to be treated cxhaustivel in a single chapter but for purposes of this descriptive survey of physics the treatment should be adequate to give the beginning student a proper orientation ſuch of modern phycics also comes under the heading of optics and as such will be considered in succeeding chapters Te turn next to a description of optical phenomena and the question of the nature of light ·159 QUESTIONS QUESTIONS 1 Descnbe briefly the appearance of the electric discharge in an elongated glass tube as the pressure of the air within is gradually reduced 2 To that, in general, does the term 'electronics" refer? 3 Distinguish between cathode rays and I rays 4 How docs a neutron differ from an electron or a proton? 5 What is meant by uranium fisjon?

·CHAPTER XIV OPTICAL CONSIDERATIONS PHOTOMETRY LAWS OF GEOMETRIC OPTICS, OPTICAL INSTRUMENTS Scope of Optics Of all physical phenomena there are probably none more fascinating and of more immediate interest to the average person than those related to human vision The worlds of color, sunshine, geometric design, and fine instruments also contribute to the complex realm of optics This field also includes such studies as illumination, photography, microscopy, spectroscopy, optometry, and polarization, as well as the study of certain philosophical questions such as the nature of light itself Thus it is seen that optıcs is indeed a very broad subject, yet at the same time, it is & part of that larger subject, physics Complex Nature of This Study From a strictly logical point of view, this subdivision of physics 19 less well adapted to the methods employed in the foregoing parts of this text, 1e, the logical development from one topic to another, than those which have preceded it This is not only because of the complexity of the subject but also because of the lack of any logical starting point on the historical side, many complex and apparently unrelated facts were known about optics before other simpler ones were discovered on the philosophical side, too, there has existed the perpetual controversy between the proponents of the corpuscular theory and those favoring the wave theory of the nature of light Moreover, the points of contact between thiº and other parts of physics are numerous, yet at the same time they are as diversified as the studies of geometry and the colore of the rainbow It 19 this very same difference, however, that proudes us with a key to a systematic study of optical phenomena Geometric vs Physical Optics The study naturally divides itself into two major divisions thich are designated as geometno ophics and physical optics The former deals with the light rays 18+ ·RECTILINEAR, PROPAGATION 161 traveling in straight lines and the images which are produced by reflection and refraction, whereas the latter deals with the physical nature of light and related phenomena We shall consider each of these in turn, starting with geometric optirs partly because of the historical de clopment of the subject The more recent researches of the last decade or so have had a particularly important bearing upon physical optics Early Theories It was held by the ancients that light and Vision were practically the same They thought that all light originated in the eye, for which contact was made in some mygterious way with observed objects It seems strange that they were not disturbed by the fact that objects invisible to blind people were visible to others. A later view credited so-called luminous objects with the ability to emit particles of light capable of traveling in straight lines and endowed with the property of exciting the eye of the observer That the luminous body does the emitting and the eye does the receiving as the modern view, but whether the radiation itself 18 corpuscular or wavelike us a matter about which physicists have argued throughout the entire history oi physics Straight-Line Propagation of Light One fact, however, has become very certain, pamely that light travels in straight lines This is technically called the rectilinear propagation of Fig 141 The rectilinear propagation of light light, and 19 proved, is a superficial way at least, by the ability of light to cast well-defined shadows See fig 141 The pinhole camera also demonstrates this fact See fig 142 The concept of light rays is thus suggested Such rays either are emitted by selfluminous 'bodies or ate reiecita Tua Wawuted Water towe appreciated by the eye Objectively speaking, of course, the eve Aeed not be a party to optical phenomena in general ·102 LIGHT Fig 14.2 The piphole camera demonstrates the rectilinear propagat on of light Photometry The concept of rays leads to the notion of spherical symmetry of the radiation about a point source of light and suggests a way of measuring amounts of light This is a field of optics usually referred to as photometry A source of light is compared with a candle and sources are rated for intensity in units called candle power Thus & 100 candle power source is approximately 100 times as intense as a specified type of candle A standard candle is one which emits a specified number of lumens of light where the lumen is a unit of light quantity If a source of light is so small that it can be thought of as a mathe matical point then rules of solid geometry enable one to calculate the amounts of light emitted by sources of different candle power intensities II a luminous source is not a point but an extended surface its Lumens of Lght brightness rather than its intensity is designated Brightness refers to the cmission of so many lumens per unit of area Illumination 11 hen light falls upon & surface the Eurface is said to F8 113 The illumina be illuminated Sce fig 143 The num tion of a surface is measured ber of lumens per square foot incident by the number of lumens upon 1 surface is a measure of the incident upon & unit of area illumination and this quantity is usu One lumen per square foot 14 a foot-candle and is the ally expressed in foot candles The amount of all mination re foot candle is the amount of light that cenic I on A surface one foot falls upon a square foot of surfacc one away from a standard candle foot en ny from a roint fource of one candle power intensity Foot candle meters have become rather common instruments today with the growth of popular intercat in photography It ja a foot candle meter which the photographer }olls up to the light to determine the proper exposure time for 1 fixen enapshot ·LAW OF ILLUMINATION 163 Practical Importance of Photometry These and other pholometric matters are of great roprern today to the increas ingly important profesgion of illuminating engineering Industrialists have become aware of the increase in efficiency of work ens when proper conditions of illumination are maintained in offices and factories Instead of the one to three foot candles of illumination which prevailed in many 1actories in the past, proper values of five to ten foot-candles are maintained for general illumination in the modern factory For close work from ten to trenty or more foot Candles are desirable The illumination of surface exposed to full summer sunlight may run as bigh as 10 000 foot Candles, while under full moonlight this figure muy be reduced to a very small fraction of a foot candle Law of Illumination There is a very important law of photometry known as the inverse square law This law states that the illumination of a surface by a given point source of light Varies invercely as the second power of the distance between source and surface This means, for example, that if the distanre between the tho 19 doubled, the allumination is decrea®ed to one fourth of its original valup Another form of this law states that for equal illuminations due to two point sources of different intensity, the ratio of intensities 19 proportional to the second Translucent Screen Feeble Source Intense Source Fig 144 To produre the same illumination on a screen a feeble source must be much closer than an intense source The illumination produced by a point source 13 inversely proportional to the second power of the distance from ource to screen Power of the separation meaning that for a light twice as intense as another the former must be about one and one-half times as far away as the latter to produce the same illumination at a given pice See fig 14 4 Reflection In the study of geometric optics two phenomena are of the utmost importance These are reffection and refraction, ·164 LIGHT each of which is characterized by a basic relationship or natural law In the case of reflection the so called law of regular reflection states that the angle made by a reflected beam of light and a perpendicular line drawn to the reflecting surface at the point of reflection 18 always equal to the angle made by the incident beam and the same perpendicular lide It states, moreover, that No mal to Su face Inc dent Beam Reflected Beam Reflect ng Su foce Fig 145 The law of regular reflection states that r=1 the reflected beam, the incident beam, and the perpendicular line just mentioned all lie in the same plane See fig 145 Refraction Refraction is the phenomenon by which a ray of light appears to be bent as it passes from one medium into an other of different optical character because of a change in the speed of light from the one medium to the other The basic law here 19 Snell s Law This law is usually expressed in mathematica language and refers to the relative bending of light rays in two adjacent media in terms of angles and speed factors hence the reference to geometric optics agun Although incapable of beina precisely expressed without the use of mathematical language Snells law states essentially that with reference to a line draw perpendicularly to a boundary surface between two media at the point where a ray of light passes from the one to the other the ray 13 bent toward this perpendicular if the second medium has 1 greater refractive inder than the first and away from it if thi second medium has a smaller index By refractive index of: medium 19 meant the ratio of the velocity of light in vacuum to that in the medium Thug as a ray of light passes from air int glass or water substances in which it is elowed up, it is so ben that the angle made with the perpendicular in the glass, or th ·REFRACTION 165 water, 13 emaller than the angle made by the incident beam and the same perpendicular in air See fig 146 This is to say that the angle of refraction in glass is smaller than the angle of incidence in air Moreover, the angle of refraction, as thus defined, is Glass Glass Fig 146 Two cases of refraction showing the bending of light as It passes from one medium to another maller for glass than for water because the refractive index of dass is greater than that for water In Snells law, it is specifically he trigonometric quantity known as the sine of each angle which nters into the formulation, although this mathematical detail leed not be emphasized in this discussion The Phenomenon of Total Internal Reflection An intersting consequence of this law is the phenomenon of total internal A Water Water Fig 147 Az Ight passes from water toward air a certain limiting angle incidence is encountered such that when it is prceeded all the hght is tally internally reflected back into the water Thus aggle is indicated by 1, the second diagram 1 fle fs ·LIGHT AS 166 incidence exceeds this critical angle, as it is called, the light instead of being refracted is all reflected back into the initial medium as if by a mirror, and obeys the law of reflection See fig 14 7 It is obvious that as the ray is bent away from the normal under these circumstances it cannot be bent so as to make an angle greater than 90°, hence the limit which is imposed upon the size of the initial angle of incidence For glass in air this angle is approximately 42°, which is less than the 45° of an isosceles right triangle Fug 148 The critical angle Thus light attempting to pass for glass air surfaces is about 42° or less than the 45º in through a right triangular piece of cident angle in the diagram glass parallel to one side will, upor Therefore the light is totally incidence at the oblique side, be re internally reflected as shown flected out of the glass in a directio parallel to the other side See fig 148 In other words it will b bent through 90° as by a mirror held at 45° to the incident ray but without the ud of any silvered material whatever The bend ing is said to be the result of total internal reflection Images Formed by Plane Mirrors Considerations of th laws of reflection and refraction lead to numerous conclusions re garding the behavior of rays of light in optical media, accountin Plene Mrror Object Image Object D stance = Ima je D stance Fig 14.9 The image formed by a plane mirror is crect and crtica! It is formed just as far behind the mirror as the object 13 in front of it for such things as the formation of images by the use of mirror: prieme and lenses Since all optical instruments depend for the operation upon these conºderations, their importance 19 not to 1 underestimated Consider the plane mirror for cxample It follos ·PLANE AND SPHERICAL VIRRORS 107 frora the law of reflection that a night-side-up image, formed apparently just as far behind the mirror as the object is in front of it, 13 accounted for See fig 149 This simple phenomenon 39, 3 2 4 90° A 5 Object Object Three images of the object are formed Five images are formed by an y two plane mirrors making an apgle object placed as shown beof 90° if the object 15 placed on the line tween two piane mirrors mal- Lecting the angle between them ing an angle of 60° with each other Fig 14 10 Examples of multiple images formed by reflection for plade mirrors course, an every day occurrence, but nevertheless an important ne from the viewpoint of physics Other examples may easily be nonstrated See fig 14 10 Reflection from Spherical Surfaces As rays of light are flected from curved mirrors the results are even more interest- 8 A spherical convex mirror always makes objects appear re iced in size, but nght side up whereas a spherical concave irror can sometimes produce inverted images atanding right it in space, and at other times produce right-side up images rated apparently behind the mirror See fig 14 11 Such inverted lages are known technically as real images because they appear really exist where the rays of light are focused They can be alized upon a screen Many optical illusions are produced by al images The right-side-up images are called virtual images ese, however can never be captured on a screen It is obvious that the formation of images by cursed mirrors, lindrical as well as spherical, can be of considerable practical portance in optical instruments ·168 LIGHT Image Object Ob ect Image by a conser spher Image (one case) by a ical mirror Concave spherical mirror Fig 1411 Images by spherical mirrors Focal Points The largest astronomical telescopes in the world such as the one on Mt Palomar in California, operate on the principle of the formation of real images of heavenly bodies by large concave mirrors One of the characteristics of these in struments is the fact that all rays of light from distant objects are brought to focus at a single point known as the focal point See fig 14 12 The distance from this point to the mirror is called the focal distance It can be shown that for spherical mirrors the Ray Parallel to Axs Center Obect Focus Object FOCUS Centet Fig 14 12 hlustrating the focal point of the concave spherical mirror and the virtual focal point of the convex spherical mirror The focal distance is one-half the radius of curvature focal distance is just half the radius of curvature Whereas th convex spherical mutor always produces virtual images reducer in size with respect to the object, and the focal point is virtual the type and size of the image produced by the concave spherica mirror depends upon the distance between object and mirror a compared with the focal length and radius of curvature of th mirror The concave mirror 19 an example of a converging systen because it actually converges light The convex mirror, on thi other band, always makes light appear to diverge and hence i called a diverging mirror ·SIMPLE LENSES 169 Simple Lenses Another example of a converging system 19 the double convex lens which, by refraction, causes light to be bent tnore at the edges than at the center and so to be converged F18 14 13 The converging lens hus a real focal point toward which Taj s originally parallel to the axis pies after refraction Fig 1414 The diverging lens has a virtual focal point from which rays originally parallel to the are appear to come after refraction The double concave lens, on the other hand being thicker at the dge than at the center produces the opposite effect and so is an xample of a diverging system See figs 14 13 and 14 14 The convergence of a system can be shown by passing a beam of sunlight restricted by an aperture, through the system if it 18 a lens, or by reflecting it from it if a mirror, in a smoke-filled room It will be noted that originally parallel rays (parallel because the sun is about 93 000 000 miles away and a narrow beam does not have much opportunity to spread in the short space of an ordinary room) either converge to a focus or diverge from an apparent focus as indicated See again figs 14 13 and 14 14 It Fig 14 15 Point source at focus of converging lens The bean wb ch is translaitted by the lens is a parallel beam will also be noted that the rays from a small pointlıke source situated on the principal axis at the focal distance away from a converging lens will be parallel upon emergence from the lens ·170 LIGHT This provides a means of producing parallel light in the laboratory without resort to sunshine See fig 14 15 Ilith a converging system, ie, either a double convex lens or a concave mirror it is found that images of objects lying farther away than the focal length are always inverted and real For objects located within the focal distance, however, images arc virtual, ercct, and usually enlarged The size of the image is to the size of the object as the image distance is to the object dis tance Ray Diagrams From a consideration of the laws of reflection and refraction and a knowledge of the characteristics of the focal points of a simple lens or sphercal mirror it is possible to locate the image of an object by a so-called ray diagram Ray Diagram for Concave Mirror Consider an object in the form of an arrow whose base reets on the axis of a concave spherical mirror See fig 14 16 Treating the arrow as a luminous source of light, consider first the rays of light emitted from the Image FA 0 A Ob ect Image Real Ima, Vetus! Imago Fug 14 16 Two examples of images located by the ray diagrm method These are formed by conserging mirrors (concase) head end of it or all the rays cmitted from this point there are at least two shoce directione after reflection from the mirror are determinable The ray which travels paralel to the axis must be reflected through the focal point, and the ray which pq«ces through the center of the curvature must be reflected back upon itself Except when the base of the arrow is at a distance equal to the focal length anay from the mirror, these two rays will intersect in front of the mirror, or thcir extensions will appear to intersect behind the mirror, and such an intersection will reprefent an image of the point from slach the rays originally emanated If the intersection takes place in front of the mirror the ·RAY DIAGRAMS 171 image 13 real, if behind the mirror, then the image 18 virtual Now it is found that if the object lieg outside the focal point the image is always real and inverted If, however, the object lies inside the focal point, ie, between the mirror and the focal point, the image is virtual and erect It will be recalled that when we look into a shaving mirror at very close range the observed image is nght side up, behind the mirror and usually magnified If, honever, the mirror is held at quite some dis tance, the image of the face is tyrong side up, or real This is because the focal distance has been exceeded Ray Diagram for Convex Mirror The same ray tracing procedure can be followed with the convex mirror only to dis cover that in this case the image is alsays virtual and reduced in size See fig 14 17 An interesting check on both these cases results when a third ray is considered Obviouely the ray from the head end of the Otect Image Fig 14 17 Virtual image by a diverging tirror (con- 1ex) located by ray dragram method object which pasees directly through the focus must be reflected parallel to the axis and it should also pass through the point of intersection of the other two rays Of course it would necessary to consider such rays as here called first second and third for every single point of the object to reproduce every single point of the image, but this is hardly necessary aince the head alone locates the position of the Image ahen the base is on the axis Ray Diagrams for Thin Lenses The foregoing considerations of rays is applicable also to thin lenses except that instead of being refceted the rays are transmitted through the lens and ·172 LIGHT instead of considering the ray through the center of curvature, there being no such thing for a two-sided lens, the ray passing through the actual center of the lens is used See fig 14 18 It 2 F Image Ob ect F Ob ect Image Fig 14 18 Images found by ray diagram method for converging and diverging lens These show bow a third ray through the focal point emergey parallel to the axis and checks the result given by rays 1 and 2 can be shown that this ray is undeviated after passing through a thin lens By such an analysis it is found that diverging lenses alsays produce virtual and diminished images whereas converging lenses produce real images of the objects lie outside the focal point and virtual images of the objects lie within this distance The Magnifying Glass A common example of the latter situation is that allustrated by the ordinary reading glass or Vrtual Image F Object Fig 14 19 The simple microscope or ordinary magnifying glae represents a special case of image formation by a converging lens when the object 19 placed between the focal point and the lens The image is virtual and on the same gide of the lens 89 the object Bimple microscope as it is technically called In ure, the glass 13 held at a distance from the printed page less than the focal length of the lens and the resulting image is right side up (virtual) and ·which also take part in the refraction phenomena The aperture Object outside objects These images falling on the retina, which is arrangement serve as an optical system to produce mages of really the inner lining of the rear part of the eyeball, stimulate the nerve cells contained in it to send appropriate Messages to the brain See fig 14 21 The whole eye is filled with water-like funda THE EYE 173 on the same gide of the lens as the object itself See fig 14 19 If the glass is held too far away from the printed page the image of the print 18 Inverted (real) and is located out in front of the lens, 1e, on the opposite side from the object. Almost all optical instruments are combinations of mirrors and lenses, and in all cases it is possible to trace rays of light through the instruments to understand how they function Such instru. ments include telescopes, microscopes, opera glasses, cameras, and the all-important human eye See fig 14,20 For an optical Fim (Long Rest Image Fig 14.20 The action of the photographic camera is illustrated A real image of the object is formed on the film placed at the rear of the camera system consisting of more than one mirror or lens it is proper, in ray diagram considerations, to first locate the image formed by the first unit of the system and then treat it as the object for the second unit, and so on in succession The Eye. Possibly the most important optical instrument 19 the human eye, although the mistake must not be made of confusing the phenomenon of 'seeing with that of the image formation by the optical system of the eye Seeing involves the interpretation by the brain of the image that is formed Here we peld a lens ·174 LIGHT or pupıl, is controlled in size automatically by the action of the ris diaphragm The smaller the opening, the sharper the image that results, but too small an opening allows insufficient light to enter to stimulate the retina, and so the adjustment of the size of this opening, although performed automatically, is indeed very critical One of the important features of the eye is the focusing arrangement Muscles are so attached to the lens that it can be flattened or bulged, with the focal length correspondingly altered Ret na I Daphragm Cornea Image Object Opt ca Nerve Pup 1 Lens and Muscles Fig 1421 The optical system of the human eye Berves to form real images on the retina the serve tissue lining the back portion of the eyeball It is by this arrangement that the eye is said to be accommodated for objects at different distances Otherwise it would not be possible to focus the eyes first upon a distant scene and immediately thereafter upon a newspaper held at ordinary reading distance It is this power of accommodation which 18 gradually lost with old age, and this loss necessitates the use of so-called 'double- 114100 glasses Indeed the entire question of spectacles is an ap plication of physxs in every day life which will be considered presently Although the whole retina is very sensitre to light, there is one epot in particular that is extremely sensitive In fact it 19 80 sensitive that in all close frork such as reading, for example, the eyeballs are continually moved so as to focus the work on these spots of the two ez es Indeed, theee spots are so small that it 19 necesary to actually move the eyeballs to shift the attention from the one to the other of the trvo dots which make up the semi colon mark of punctuation Thus it is that reading makes it neces sary for the eyes to be moved jerhily along the lines of the printe! page, focusing upon one spot after another in rapid succession Is it any wonder that these eyes of ours sometimes get tirer and require the assistance of spectacles, especially in middle life or later, for many people?

·EYE DEFECTS 175 Eye Defects and Spectacles -Nearsightedness It bas already been suggested that the adaptation of spectacles to 454st, human vision is a practical application of physics Some people are born with eyeballs too long for distinct vision Images formed by the lens system fall short of the retina, and no matter how In myopi (neareighted In hypermetropia (lars gbtness) the image is formed edness) the image is fonned before the retina With behind the reting With a diverging lens in front of converging lips in front of the eye the image is formed this eye the image 13 forned on the retida Thus faulty on the retina VISION 18 corrected Fig 14.22 Eye defects hard the eye attempts to accommodate distinct vision is not achieved except for very close objects This condition is popularly called nearsightedness Technically it is referred to as myopa The obvious remedy is the use of a diverging lens in front of the eye to increase the focal length of the whole system This is, ther, why nearsighted persons wear glasses which are thicker at the edges than in the centers These are diverging glasses See fig 14 22 Farsightedness The opposite condition is of course farsightedness, or hypermctropa, as it is called It represents a condition in which the eyeball is too short and, as a result, images are always formed behind the retını This condition 19 relieved by the use of supplementary convergiog lenses to shorten the overall focal length of the syetemi As people lose the power of accommodation with increasing age they generally become more and more farsighted The near point, so to speak, recedes with advancing age Thus it is that many a neareighted person finds his vision improved as time goes on Astigmatism Many eyes suffer from a defect known as astigmatism It is probably safe to say that core spectacles are sorn for this reason than for any other Astigmatism is an aberration involving the shape of the front surface or the u ·176 LIGHT the eye This defect shows up when a person attempts to see with equsl distinctness all the lines of a series drana radially from a point in a plane The astigmatic eye sees the lines in certain directions more distinctly than in others Headaches constitute a common symptom of this difficulty as a result of strain on the eye muscles when they attempt to compensate for the defect to correct a condition of astigmatism cylindrical spec tacles are used with their cylindrical axes properly adjusted for the direction of the astigmatism Of course several defects can be compensated for at the same time by a pair of spectacles 80 that spectacle lenses are often combinations of spherical and cylindrical surfaces, representing something of an achievement on the part of the optician Indeed, it is no exaggeration to repeat that applications of physics in the aid of human vision are of the utmost importance Summary In this chapter we have scen that geometric optics plays a very important role in the world of human experience From the ordinary spectacle lens to the largest astronomical telescope, light rays form images of luminous objects Reflection and refraction, mirrors and lenses, allumination and darknessall are within the realm of optics, all a part of the study of physics Let us consider next the action of a prism on a beam of light, the wonders of the rainbow, and other optical phenomena related to physical optics as contrasted with geometric optics. QUESTIONS 1 What is meant by photometry? 2 Does the intensity of illumination at a place depend upon the distance it is away from the source? 3 Define the foot-candle 4 Why can it be assumed that light passes through the center of a thin lens almost undeviated in direction? 5 Indicate the path of two rays from & point on an object which may be drawn to locate the image formed by (a) a spherical mirror, (b) a lens 6 Show by a diegram that a person can sce his entire image 10 & plane mirror one half as tall as he 19 7 How can farsightedness and dearaightedness be corrected with spectacles? 8 What type of lens must be used to produce an enlarged image on a screen?

·QUESTIONS
177
9 Show by e diagram how light passes (a) through a lens
which is thicker at the center than at the edges, (b) through
a lens whose edges are thicker than the center
10 Can a double convex lens be used to start a fire? A convex
mirror? A concave mirror?
11 What type of image is formed on the film of a camera by
the lens?

·CHAPTER XV OPTICAL CONSIDERATIONS (Continued) PAYSICAL OPTICS, DISPERSION, SPECTROSCOPY, INTERFERENCE, DIFFRACTION, POLARIZATION In the preceding chapter, questions dealing with the directional aspects of light rays were considered It was observed that rays were reflected, bent, made to intersect one another to form images, and affected by the presence of matter The geometry of each situation was of more immediate importance than the nature of light itself In this chapter we are to be more concerned with the latter question, namely the nature of light. We shall first consider another phenomenon in which direction also plass an important part The Visible Spectrum When a narrow beam of sunlight 18 passed through a triangularly shaped glass prism a very interesting and perhaps strange result is produced by refraction Sco fig 151 Not only is the ray bent, as the study of the preceding chapter would require, but it is spread out into a whole spectrum of visible colors ranging from red to violet This indicates that there is something about a beam of light which makes pog- Bible its resolution into component parts The phenomenon 19 referred to as dispersion. If the source of light is an illuminated narrow alit and an image of it is formed on a screen by a lens held near the priem, there will be reproduced on the screen succesave overlapping images of the slit in every single color present in the source Thus the spectrum formed by & prism is character. ized by the light source used, and many different types of spectra have been classified Spectroscopy All incandescent light sources are found to emit continuous spectra, ie, continuous bands of light containing all the colors of the rainbow Indeed, the rainbow itself is a spectroscopic phenomenon produced by sunlight passing through droplets of water in the air when the sun makes a certain angle 178 ·SPECTROSCOPY 179 with the horizon after a shower The solar spectrum, however, 18 not sincthy & continuous spectrum, as will be pointed out presently On the other hand, electric ught filaments and glowing furnaces do give spectra which are continuous Hot vapors, hke those in the modern fluorescent lamps, give tise to spectra in which only selected colors are present Again, ROY G BV Screen Glass Prism SI Fig 151 A beam of sunlight is spread out into a colored spectrum by a triangularly shaped glass przemn of the source is a slit illuminated by such a light, the spectrum will be a set of discrete lines separated by dark gaps Such a spectrum is ordinarily referred to as a brght line spectrum No trochemical elements yield the same bright-line spectrum, whenre identification of chemical substances is possible by spectroscopic methods Indeed, new chemical elements have been discovered after spectroscopic studies have indicated their prescnce It was in this way that helium was discovered The bright lines of the spectra of all chemical substances follow patterns of varying degrees of complexity which have bech core related with atomic structure Much of a hat 19 hnown tony about atomic structure has been determined from spectroscopic studies, and the subject of spectroscopy has been one of the most fruitful subjects of research in physics to the last two lerndes for this reason ·180 LIGHT The third type of spectrum in this classification of spectra 15 the dark-line or absorption spectrum This type is produced where an incandescent source is placed in such a way with respect to a hot vapor that the light from the former is intercepted by the latter before it passes through the spectroscope Under these conditions the colors which would ordinarily constitute the spectrum from the hot vapor are absorbed from the continuous spectrum that would have been produced by the incandescent source alone This results, in the case of the slit-type spectroscope mentioned twice before, in an almost continuous spectrum, but one crossed by dark lines Fraunhofer Lines The solar spectrum is a dark-line spectrum and indicates that the outer portions of the sun contain elements whose characteristic wave lengths have been absorbed from the otherwise continuous spectrum of the inner and hotter portions of the sun The dark lines in the solar spectrum, having first been observed by the German physicist Fraunhofer, have been called Fraunhofer lines The more conspicuous of these are labeled with alphabetical letters See fig 15.2 Thus the Fraunhofer D line RO Y G B V (Colors) I C D Eь F G (Fraunhofer Lines) Fig 152 The appearance of the solar spectrum crossed by the Fraunhofer lines The colors in this spectrum are unequally spaced when formed by a prism The D lide is due to sodium, the Iwe to hydrogen etc refers to the codium doublet, or the pair of very close lines in the yellow-orange region of the spectrum It occurs at precisely the same position as that occupied by a similar pair of yellow-orange lines in the spectrum of a sodium vapor lamp The so-called optical dispersion of a spectroscope is often measured by the degree to which the instrument can separate the lines in this doublet Thus the whole question of the nature and structure of light 19 rared by the study of spectroscopy It is evident that so called white light, or simply sunshine, can be broken down into constituent colors by the prison of the spectroscope Moreover, it 19 pos- ·COLOR MIXTURES 181 able to recombine colors so as to reproduce white light These are simple experimental procedures which can be performed quite readily in the classroom today just as Newton demonstrated them some three hundred years ago Color Mixtures This inatter of the superposition of colors or the mixtures of colors is an interesting study in itself It 18 referred to by some as the physics of art It is found that only three colors, the so called primary colors, are necessary to pro duce a fairly accurate reproduction of white light, and that com. binations of these three colors will reproduce practically any Color Color printing utilizes three primary colors in this manner In a less accurate way, two colors sometimes produce a fairly good reproduction of white light Such pairs of colors are called complementary colors Blue and yellow, red and green, are examples of such complementary pairs Color Addition vs Color Subtraction An important distinction must be made between the addition of two colors and the addition of two pigments, which is really the subtraction of two colors Yellow and blue hght when added together produce white light, as just noted, but yellow paint applied to a blue surface pro duces a green surface Since the color of yellow paint results from the fact that the colors but yellow, probably bordered by some green, have been absorbed, and since the color of blue paint results from the fact that all the colors except blue, probably bordered by some green are absorbed, the only color reflected by the combination is that which 19 reflected in common by each, napely greea Thus the addition of yellow pigment to blue pigment is the subtraction of all colors except green This distinction between addition and subtraction can be shown in another way If two spotlights, one with a yellow filter before it and the other with a blue filter, are caused to shine their light upon the same screen, the resulting spot of light on the screen will be practically white Jf, bowever, a single spotlight is used The only color, then, that will probably get through both Alters ·182 L.GHT is green Of course, a complete filtering job by the filters would produce black, or the complete absence of all color It might be noted parenthetically that no object has any color in the dark Color is the result of selective absorption of light Similarly, 1 red apple appears black under strictly green light because there 19 po red light to be reflected if the light is truly green Interference The superposition of light not only involves the matter of additive colors but also suggests another phenomenon of considerable interest Under very special circumstances hght ap parently coming from two different sources can be made to pro duce darkness that is to say, two beams of light can be made to nullify each other Thomas Young (1773-1829) observed that when a candle is placed behind a double slt, ie, two parallel slits rather close together, the light coming from the two clits forms a peculiar pattern on a screen held in front of the slits This pattern consists of a set of alternate bright and dark bands oriented parallel to the shts When white light such as a candle source is used, the bright bands are colored, but when light of a single color is used, eg, light from a yellow sodium source, the bright bards, or fringes as they are called, are all of one color, separated of course by dark bands The explanation of this phenomenon involves the wave theory of light Whereas water waves are known to be able to interfere with one another when they are superimposed of one 19 a half wave leagth out of phase with the other, so it follons that light saves can interfere destructively under proper circumstances The dark bands occur at those places where the distance to one slot exceeds the distance to the other slit by an integral number of half wave lengths Of course the distances between the fringes are very small since the wave length of light is very small, being of the arder of several hundred thousandths of a centurneter The whole phe. norocnon 13 called interference of light The dark bands represent destructive interference and the bright bands represent construc. trie interference Diffraction Although the priem spectroscope provides o means of breaking light down into its component parts, and ir. particular of separating white light into colors by refraction, there 1$ still another nay of producing the same effect This is by dif. fraction, which is a phenomenon of selective bending of light ray! According to travelength as light passes around the edges of ob- ·DIFFRACTION 183 structions, the longer waves being bent more than the shorter Odes A grating produced by ruling several thousand lines per inch on a transparent glass plate by a diamond point constitutes a device by means of which white light can be thus resolved into its colors As the light passes through this grating some of it is bent en direction, with red light bent more than blue light Copce quently, spectra are produced by a fine ruled grating See fig 153 Got ng R Spectrum Red to Volet Wh te Light Undeviated Wh te Beam V Spectrum Vclet to Red R) Fig 153 A got ng formed by ruling several thousand lines per inch on a glass plate will produce spectra by diffraction as indicated This is in realits an interference phenomenon and requires for its iterpretation the assumption that light 13 a wave phenomenon r corpuscular light would not be so affected by a grating As has rep noted the bending of light is selective arcording to wave ngth Grating Spectroscope Furthermore this co called grating pe of spectroscope makes possible the very simple matter of escunng the wave lengths of light The bending of waves around Iges is very familiar to all who have watched water waves play ound docks and piers 1l bat is not so familiar is the manner in hich the bending depends upon the length of the waves con mned Very short light waves (00001 cm to 00007 cm) are 1 LJ at this phenomenon of diffraction might seem incapable of monstration except under very favorable circumstances Such Wever, is not the case because inexpensive replicas of costly led gratings can be made very easily out of collodion that work actically as well as the originals themselves ·184 LIGHT Colors of Thin Films Due to Interference Another interesting phenomenon in this connection is the colonog produced by very thin films such as thin films of oil on water, or those films which constitute the so-called mother-of-pearl in oyster shells This also is an interference effect and one which can be produced only when the films are exceedingly thin The light reflected from one surface interferes with the light reflected from the other surface, re-enforcement or nullfication results depending upon the number of half wave lengths included between the two surfaces Indeed, the thickness of the film can sometimes be measured by the color effects produced This 19 & way of utilizing light waves for measurement purposes Such interferometric methods are used in precision measurements to such extent and sefinement today that it is possible to measure to A millionth of an inch with certainty Thus is made possible the interchange of working parts in the modern automobile, which have to fitted with as little clearance as a few thousandths of an inch Invisible Glass A very novel and interesting application of the laws of interferometry has recently been developed to make glass invisible This is accomplished by coating the glass with a film of some convenient substance The thickness of this film 15 & very critical matter, it is just so thick that light reflected from one side of it will interfere destructively with light reflected from the other siden (not over & quarter of a wave length of light at the most), thereby eliminating the beam that would pormally be reflected from the front surface of the glass This, of course, makes the glass invisible since the line of demarcation between it and the surroundings is eliminated It is to be noted that the actual surface of A front-surfaced mirror is really invisible since the reflection 9 total rather than selective Camera lenses coated so as to maahe their surfaces invisible are found to be very effective in producing glareless photographs Similarly, invisible-glass vindoms in front of instruments such as clochs and electrical meters cut down the glare to observers, who can thereby see the faces of the instruments more clearly Polarized Light. As a final topic to be considered in this descriptive survey of optics, and of physics generally, we turn to polarized light, a subject which has gained coneiderable popu larity in recent years through the advent of polaroid, the material used in certain sunglases and in three-dimencional movies The ·POLARIZATION 185 concepts of interferenco and diffraction just considered have defi nitely established the wave aspects of light and yet it should be recalled from the quantum theory viewpoint that the corpuscular aspects are not to be ignored Nevertheless it must be obvious that, for the most part the wavelike features of light seem to be invoked more often to explain common phenomena than the cor puscular features With this in mind the next question we en counter 18 whether light waves are longitudinal like sound waves or trangverse like elastic waves This question is best answered by the study of the polarization of light When a beam of light is passed through a crystal of the sub ܘܐܗܘ T4 exactly as the first one If the second disk of polarord for exam, ple 19 rotated through an angle of 90° with respect to the first one, no light is passed through it The phenomenon 18 called polarnatron. The light is said to be polarized by the first disk which is called the polarzer, and in this condition its angle of polarization can be determined by the second disk which is called the analyzer Nature of Polarized Light To help understand the matter better it is useful to consider an analogy If a horizontal rope fixed at one end 18 wayed up and down by the other end a transveree mine 1. the ph 11 wave vibrating in a vertical plane is established Furthermore the situation would not be altered if the rope were to pass through a vertical alot or gate If however such a clot were turned so as to be horizontal it would effectively top the wave motion beyond the point on the rope where it was placed See fig 154 Moreover, ·186 LIGHT if two slots were used, one at sone distance beyond the other, then a random sidewise motion of the free end of the rope would be restricted by the first slot to a motion parallel to itself and, unless the second slot were oriented in the same direction as the first, no wave motion would pass through the second slot There is something about the molecular or atomic arrangement in tourmaline and polaroid which restricts waves of light to a particular planc of vibration just as the slots in the above examples restrict the motion of the rope Then the mode of vibration of light waves is so restricted the light is said to be plane-polarized Thus it is secn further that the polarizability of light requires that light be ? transverse wave motion if it is a nave motion at all, because longitudinal waves cannot be polarized Polarization by Reflection In addition to the phenomenon of polarization by transmission through tourmaline or polaroid, light is polarized to a large extent just by reflection alone Indeed, all reflected light is polarized somewhat, even the glare from a ccment highway on a bright day Thus polaroid sunglasses are clicctive in reducing glare because they eliminate the polarized component of the reflected light Sky light 18 also polarized because all the fine particles of water vapor, dust, etc in the air reflect light Such reflection, or scattering, of short wave-length light is much more pronounced than that of long wave-length light and so the light reflected from the sky is blue Although the midday shy 19 blue, the sunset is reddish This is because the light from the «uneet is transmitted through the earth s atmosphere and appears red because the blue has been removed by scattering The sky at noon is seen by reflected light, whereas the sunset is seen by transmitted light Both the scattered and the transmitted components are polarized Photoelasticity. Polarized itght has been found to be of considerable value to the machine designer Thus 19 because many Eubatances which are transparent to ordinary light have quite & different appearance in polarized light Furthermore, many substancee, ordinarily transparent, under polarized light becoine opaque alien struned Glies and celluloid, for example, display regular-colored interference patterns then strained and viewed under polarized light Celluloid models of gears and other parts of machinery will show the regions of strees then sicted in operation under polarized light, and many industrial concerns cant) ·HOTUELASTICITY 187 out such photoelastic tests on new designs before going into the production of machine parts and manufactured products In the aviation industry, in particular, where close tolerances in weight are very important, such photoelastic analyses are utilized to prevent & sacrifice in strength along with eight Indeed, the cub ject of polarized light is a branch of physics which illustrates quite aptly the manner in which physics has been consciously put to work by practical men ako no longer are content with just engineenng experience but who are quich to take advantage of every new development in the scientific laboratory In Conclusion No wonder this is a scientific age, and no wunder laymen are interested in the intricacies of phyoics even af they do not, and in some cases cannot, dele deeply into the mathematics of the situation It is for students, and people generally, who are thus sincerely interested in a sober treatment of physics on the descriptive level that this text has been written It will not, and cannot, provide the necessary comprehension for the person who wishes to utilize the material in a professional way, but it is the hope of the writer that it will arouse interest and develop a sense of appreciation for physics in those who wish to make its acquaintance in a manner which one might call serious et unprofessional QUESTIONS 1 Sketch the passage of a ray of light through a triangular premn 2 What kind of spectrum is obtained from moonlight? from starlight? 3 Describe the appearance of a red card as it is moved through A continuous spectrum 4 Why does a mixture of ali pigments produce black thereas 5 6 7 What is meant by destructive and constructive interference of light waves? 8 to 9 cul methods of measurement to the phy ciert?

·188 REVIEW QUESTIONS REVIEW QUESTIONS (See page 190 for answers ) CHAPTERS XIV AND XV 1 Light is thought to be a wave motion principally because of 1 its interference effect, 2 its enormous Velocity 3 its action on the human eye 4. Ein stein a theories 5 its propagation in straight lines 2 Diffraction is 1 the same as refraction, 2 the same as fluorescence, 3 governed by the law of reflection, 4 the cause of red sunsets, 5 the bending of light around cdges 3 The age old conflict between the corpuscular and the wave theories of light has been settled by present day physicists 1 in favor of the wave view, 2 in favor of the corpuscular theory 3 by considering light to be neither wavelike nor corpuscular, 4 by A corpuscular view to explain diffraction and a wave view to explain the enormous velocity, 5 by coosidering light to be both wavelike and corpuscular 4 The velocity of light was determined very precisely by 1 Newton 2 Galileo, 3 Michelson, 4 Einstein 5 Huygens 5 The shortest plane mirror in which a man can sce his entire erect image 1 19 just as long as himself, 2 is dependent upon his distance away from the mirror 3 18 just half bis height 4 is necessarily 6 brightness of a ource of light, 3 solid angle 4 glare 5 illumination of a surface 7 The illumination produced by a source of light on & surface varies 1 directly as the distance between them, 2 inversely as the distance between them, 3 inversely as the square of the distance between them, 4 directly as the square of the distance between them, 5 independently of the distance between therr 8 Light 13 CO pal's by i 4 eclective absorption 5 interference 9 When a beam of light falls obliquely on a glass gurface after passing through air it is 1 bent toward ·RETTEW QUESTIONS 189 the homar C) V Uut 4 006 cm, 10 pailia11) immersed in water appears 'broken" because of i refraction, 2 diffraction, 3 polarization, 4 interference, 5 absorption 11 The spectrum of the sun 1 is an almost continuous band of colored light covering the whole visible range, 2 is a bright-line spectrum, 3 18 produced only by means of a prism 4 may be produced simply by a concave mirror 5 is exactly like the spectrum of a mercury arc light 12 Real images differ from virtual images that 1 real images are always inverted, 2 real images are always erect, 3 real images are sometimes inverted but sometimes erect, 4 virtual images can be impressed on a screen, 5 only converging lenses can produce virtual images 13 The wave length of yellow light is approximately 1 06 cm , 2 0006 cm, 3 00006 cm 5 0000006 cm 14 Yellow light and blue light when added together produce 1 green, 2 yellow, 3 blue, 4 white, 5 black 15 A double convex glass lens is 1 A diverging system, 2 capable of producing virtual images 3 unable to produce real images, 4 always free from chromatic aberration, 5 always free from spherical aberra tion 16 The human eye is 1 essentially & spectroscope capable of analyzing the component colors in a couroe, 2. esentially a radio receiver permanently "tuned' to a Darrow band of electromagnetic frequencies, 3 ja no respect whatever & radio receiver 4 absolutely unaffected by infrared light, 5 essentially a compound microscope 17 Myopia (nearlightedness) is corrected by the use of 1 concave lenses, 2 convex lenses, 3 concave mirrors 4 convex mirrors, 6 astigmatic lenses 18 The largest telescopes in the world are mirrors because 1 inverted images are desired, 2 they are cheaper and more practical than lensea of the same size 3 they will gather more light than lences of the () ·190 REVIEW QUESTIONS same size, 4 they can be made with longer focal lengths, 5 greater magnification is possible than with lenses of the same size, 19 Polarized light 1 was discovered by the use of polaroid, 2 indicates the corpuscular aspect of light, 3 may be used to detect strain in transparent substances; 4 is blue, 5 is of no commercial value as yet U) 1 (1) 2 (1) 3 (4) 4 (1) Answers to Rerrew Questions pp 60-53 7 (1) 13 (1) 19 (4) 25 (1) 8 (5) 14 (3) 20 (5) 26 (3) 9 (2) 15 (3) 21 (3) 27 (3) 10 (3) 16 (3) 22 (1) 28 (1) 11 (4) 17 (4) 23 (2) 201 (4) 12 (1) 18 (3) 21 (4) 30 (3) 31 (1) 32 (2) 33 (2) 31 (3) 35 (1) 36 (1) 37 (1) 6 (5) 1 (3) 2 (2) 3 (2) 4 (3) Answers to Reviero Questions pp 89-93 10 (3) 15 (2) 20 (1) 6 (5) 11 (5) 16 (5) 21 (3) 7 (3) 12 (1) 17 (1) 22 (3) 8 (2) 13 (4) 18 (2) 23 (2) 9 (4) 14 (1) 19 (1) 24 (3) 25 (2) 26 (5) 27 (2) 28 (5) 1 (1) 2 (2) Answers to Retres que trons pp 134-135 3 (3) 5 (1) 8 (3) 11 (1) 4 (2) 6 (2) 9 (2) 12 (5) 7 (3) 13 (4) 1 (1) 2 (5) Answers to Romero Questions pp 188--190 7 (3) 10 (1) 13 (3) 5 (3) 8 (1) 11 (1) 14 (4) 9 (1) 15 (2) 16 (2) 17 (1) 18 (2) 19 (3) ·APPENDIX () () SUPPLEMENTARY REVIEW QUESTIONS 1 The Scientific method is based on deductive rea soning 2 is based on inductive reasoning 3 13 illustrated by the writings of Anistotle 4 has al ways been followed in physics 5 disregards facts 2 A hypothesis is 1 the same thing as a scientific theory 2 not allowed in physics 3 never success ful 4 a necessary step in inductive reasoning 5 al ways successful 3 Force is the same as 1 mass 2 push 3 power 4 moinentum 5 acceleration The resultant of two forces of 10 lbs each acting in opposte directions is 1 20 lbs 2 10 lbs 3 14 lbs 4 5 lbs 5 zero 5 A force is always necessary 1 to heep a body mov ing steadily in a fixed direction 2 to change the speed of a body 3 to account for the velocits of a body 4 to keep a body at rest 5 to determine the position of body 6 Displacement is 1 the change of position 2 the same as distance traveled 3 the time rate of change of velocity 4 the same as momentum 5 synony mous with magnitude 7 Velocity may change 1 m magnitude only 2 in magnitude and/or direction 3 in direction only 4 only by Newton s law 5 only horizontally 8 The acceleration of gravity 18 I zero 2 32 cm per sec 3 32 ft per sec 4 32 cm per sec per sec 5 32 ft per sec per sec 9 Acceleration is 1 the same as velocity 2 the same as displacement 3 the time rate of change of dis placement 4 the time rate of change of velocity 5 always zero 30 A body traveling in a circle with constant speed 1 has constant velocity 2 is accelerated 3 is not affected by gravity has zero displacement 5 is weightless 191 4 ·() 192 APPENDIX 11 Newton second law of motion proposes that 1 force is mass times acceleration, 2 weight is force, 3 a body at rest continues at rest unless a force acts, 4 to all actions there are equal and opposite reactions, 5 all bodies are attracted to the center of the earth 12 VVork is 1 mass times acceleration 2 force times displacement when these two quantities have the same direction, 3 force times displacement only when the two are mutually perpendicular, 4 force times velocity, 5 mass times velocity 13 The work done when a 10 lb force acts horizontally on a 50 lb body on a frictionless surface is 1 10 ft-lbs, 2 10 lbs , 3 500 ft-lbs , 4 500 lbs, 5 zero 14 The capacity to do work is called 1 mechanical advantage, 2 energy, 3 power, 4 efficiency, 5 momentum 15 The ratio of the force overcome by a machine to the force applied to it is called 1 mechanical advantage 2 energy, 3 power, 4 efficiency, 5 work 16 The maximum displacement of a vibration from equilibrium is called i frequency, 2 speed, 3 amplitude, 4 period 5 inertia 17 Simple harmonic motion 1 takes place in a straight line, 2 18 rotary 3 is torsional, 4 is always audible, 5 always involveg nodes and loops 18 Sound is a wave phenomenon of the following type 1 transverse, 2 circular, 3 longitudinal, 4 electro magnetic, 5 elliptical 19 The velocity of a wave is 1 the product of the frehe fretimes to the next crest, 5 always 186 000 miles per second 20 In a tube open at each end sound waves 1 display & node at either end, 2 display an antinode at the center, 3 resonate with a fundamental wave length of four times the tube length, 4 have a fundamental wate length of twice the tube length, 5 can never re-opate 21 The number of overtoncs in & EQupd I determibes the pitch, 2 determines the quality, 3 gov- ·SUPPLEMENTARY REVIEY CUESTIONS 193 () emns the loudness, 4 accounts for all beat phenomena, 5 is always zero When a 256-cycle tuning fork and a 260 cycle tuning fork are sounded together, the number of beats one hears s 1 zero, 2 one, 3 two 4 three, 5 four (CAUTION! Do not confuse the answer with the number attached to the answer ) 23 Whenever two waves of the same frequency, speed, and amplitude traveling in opposite directions are superimposed 1 destructive interference always results 2 constructive interference always results, 3 refraction 18 demonstrated, 4 the phase difference is always zero, 5 standing waves are produced 24 Atoms are 1 rigid spheres, 2 composed of molecules, 3 always stationary, 4 composed of electrons, protons, and neutrons, 5 nonexistent 25 If the pressure of a gas is increased threefold at constant temperature the volume 1 must be increased threefold, 2 must be decreased threefold, 3 must be decreased niefold, 4 remains the same, 5 decreases to zero 26 If the temperature of a gas 19 raised its molecules 1 are speeded up 2 are slowed down 3 undergo no change in speed, 4 are made smaller, 5 are made larger 27 The temperature on the Centigrade scale which corresponds to -40° Fahrenheit 18 1 0°C, 2 - 222° C 3 - 72° C 4 10° C,5 - 40° C 28 Boyle's law has to do with i buds seeking their own level, 2 the diffusion of gases through porous substances, 3 the law of multiple proportions, 4 Magdeburg hemispheres, 5 the pressure-volume relations of a gas 29 Heat 1s 1 a fluid, 2 energy 3 power, 4 momentum, 5 the same as temperature 30 Pressure may be measured in 1 pounds, 2 inchez, 3 pounds per inch, 4 pounds per square inch, 31 32 The reason for insulating a refrigeralul ls i wple- ·() 194 APPENDIX vent outward radiation, 2 to prevent heat from coming in 3 to prevent the cold from going out, 4 to melt the ice, 5 to prevent the ice from melting 33 When an ebonite rod is rubbed with fur 1 electricity is generated, 2 positive and negative electricity are sepirated 3 the fur is said to be negatively charged, 4 the rod is said to be positively charged, 5 nothing happens 34 The proton compared with the negative electron has 1 the same sign of charge, 2 the same size, 3 the opposite sign of charge, 4 twice the charge, 5 1/1800 as much charge 35 Two Irke electric charges 1 attract each other, 2 repel each other 3 neutralize each other 4 have no effect on each other 5 must be neutrons 36 When glass is rubbed with silk 1 electricity is gencrated 2 the glass is positively charged 3 the glass is negatively charged 4 the bulk is uncharged, 5 the silk acquires twice as much charge as the glass 37 The leaves of a charged gold leaf electroscope move apart because I like charges attract, 2 they are el nharrond nns + 2 thn he T 38 ate rod is rubbed with fur 1 negative electrons are then away from the fur 2 positive electrons are accumulated on the fur, 3 protons accumulate on the rod, 4 protons are triped off the rod, 5 the rod becomes a good conductor of electricity 39 The function of a condenser is 1 to produce a spark 2 to increase potential 3 to diminish the charge, 4 to increase the intensity of a spark at a giten electrical pressure, 5 to condense & given amount of charge into a smaller volume 49 In a metal conductor the two ends of which are kept at a constant difference in potential 1 current of electricity 19 established 2 electrons flon from posttise to negatre 3 protons flow from plus to minus 4 charge piles up at the negative end, 5 magnetic cffects are never produced 41 Electrolu 45 13 an effect usually clasayfied as 1 mag. netic 2 I eating 3 chemical 4 acoustical, 5 static () ·SUPPLEMENTARY REVIEW QUESTIONS 195 () 42 In this locality the north end of a compass needle points 1 east of north 2 north, 3 weet of north, 4 toward the north magnetic pole of the earth, 5 south 43 A unit positive charge at rest and a unit north magnetic pole 1 attract each other 2 repel each other, 3 exert no effect on each other 4 produce other magnetic poles, 5 produce other electric charges 41 When an electroscope is charged by induction 1 the sign of the charge induced is the same as that on the charging body, 2 the sign of the induced charge is opposite to that on the charging body 3 charges are transferred directly from the charging body to the electroscope 4 the induced charge is always positive since the leaves diverge 5 the charged electroscope has the same number of electrons before and after 45 When a current is passed through a wire which crosses a magnetic field 1 the wire is attracted toward the north side of the field 2 the wire is attracted toward the south side of the field 3 the wire is not moved in any way 4 the wire always becomes magnetized 5 the wire is caused to move at right angles to the field and at right angles to its own direction 46 Electromagnetic induction wall discovered by Marwell, 2 Edison, 3 Einstein, 4 Oersted, a) () 47 48 The electric toaster depends for its operation upon which of the following effects of a current 1 the heating effect, 2 the magnetic effect 3 the chemical effect, 4 the thermoelectric effect, 5 the electrolytic ect 49 Which of the following names is asociated with cathode rays 1 Joule 2 Franklin, 3 Thales, 4 Oersted, 5 J J Thomson 50 The following are all modern physicists except 1 Michelson 2 Compton, 3 Millskan, 4 Maxwell, 5 Heisenberg 51 All of the following terms suggest modern physics wa ·196 APPENDIX except 1 quantum theory 2 relativity 3 posi tron 4 proton 5 resistivity 52 Light is thought to be a wave motion principally because of 1 its enormous velocity 2 its ability to be polarized 3 its propagation in straight lines 4 its action on a photographic plate 5 its ability to be focused by a lens 53 A foot-candle meter is used to measure 1 illumina tion 2 glare 3 brightness of a source 4 candle power 5 distance 54 The bending of light around edges is due to i re fraction 2 reflection 3 polarization 4 diffraction 5 absorption 55 The illumination on the flat top of a table 2 feet directly beneath a 40 candle power lamp is 1 10 foot-candles 2 10 candle power 3 20 foot candles 4 zero 5 80 foot-candles 56 The visible spectrum of the sun is 1 A continuous spread of color from red to violet 2 a spread of color cro sed by dark lines 3 a bright-line spec trum 4 exactly like the visible spectrum of sodium light 5 completely lacking in green light 1 (2) 2 (4) 3 (2) 4 (5) 5 (2) 6 (1) 7 (2) 8 (5) Answers to Supplementary Perrew Quest ons 10 (2) 19 (1) 29 (2) 39 (4) 11 (1) 20 (4) 30 (4) 40 (1) 12 (2) 21 (2) 31 (3) 41 (3) 13 (5) 22 (5) 32 (2) 42 (3) 14 (2) 23 (5) 33 (2) 43 (3) 15 (1) 24 (4) 34 (3) 44 (2) 16 (3) 22 (2) 35 (2) 45 (5) 17 (1) 96 (1) 36 (2) 46 (5) 18 (3) 27 (5) 37 (5) 47 (1) 23 (5) 38 (1) 48 (1) 49 (5) 50 (4) 51 (5) 52 (2) 53 (1) 51 (4) 55 (1) 56 (2) ·INDEX Aren References to names of physicists are not included For such pames, see the chapter on 'Hi torical Considerations pages 12-25 Absolute temperature 84 97 Candle power 162 Abeolute zero 86 97 98 Capac tance 125 Absorption of beat 113 Capauty electr cal 125 Absorpt on spectrum 180 Cep llary attraction 87 Acceleration 35 36 of gra ty 36 37 Cathode 151 Accommodation 174 Cathode glow 150 Adhesion 87 Cathode rays 151 Advantage mechanical 46 Cation 141 Aur liqwd 104 105 Cell photoelectrie 153 154 Alpha particle 157 Centigrade scale 96 Altitude 63 Centimeter 8 Arapete 20 139 141 142 Centrifugal force 89 Amplitude of vibration 56 of wave 09 Centr petal force 39 Analyzer 185 CGS unta 89 Angle of declination 131 of dp 131 132 Charge electric 80 1204 Anion 141 Charles s law 83 Anode 141 Chemn cal effect of current 140 Arabic gyetem of numbers 15 Chladni figures 72 Archimedes Principle 61 109 Clock 8 Clockwise moment 33 torque 33 Armature 145 Closed organ P pes 78 Astigmatisin 175 Coeffic ent of frict on 48 49 of heat con Atmospherio pressure 63 duct vity 109 of thermal expans on 96 Atom 79-81 154 155 179 Cohesion 87 Atomic bomb 158 Color 112 181 addition, 181 Bubtract on Atomic energy 21 159 281 Attract on capillary 87 Colors of thin films 184 Average velocity 35 Compass magnetic [80 Avogadro s law and number 84 Component of force 28 Compression 57 08 67 Balance beam 9 spring ? Concave mirror 107 Balloob 82 Concept 5 Barometer 63 Condenser 125 Daec concepte 2 Conduction of electricity 123 of beat 107 Bento 69 Conservat on of energy 14 Bets particle 157 Constructive interference 63 Bimetall e thermostat 08 Cont nuous Spectrs 178 179 Bohrs atom theory 80 155 Convect on of heat 107 108 109 Bojling 101 102 Conve ging optical systern 103 Boiling temperature 101 Convex mirror 187 Bomb atomic 158 Cordes 174 175 Bottle thermos 113 114 Coulomb a law 121 131 Boyle & law 63 Counterclockwise moment 33 torque 83 Brakes hydrone 84 Cztical angle of incidez Bright line spectrura 179 Crookes dark space 150 151 Br ghtaegs 103 Current electricity 1368 direction of 140 British Engineering Unts 8 Cyclotron 24 157 Brit s6 Thermal Cats 99 100 Browman motion 81 82 Dark Lape spectrum 180 Buoyancy e1 Day mean solat 8 Declmation angle of 131 Calculus 18 Detetive reasoning 2 Csuper micrometer 10 Verdier 10 Defects eye 173 Calorie 100 Dens ty 19 Calorinetry 100 101 Destruct ve zoterference 88 Camera photographie 173 pinhole 102 Dew point 103 197 ·198 INDEX Diamagnetist 133 Fluorescence 131 Diffraction 192 Focal point 163 Diffusion 83 Foot Dp angle of 131 132 Foot candle 162 Drection of electric current 140 of force Foot pound 13 28 Force 626ff 80 electrical 80 molecular, Dispersion 178 87 99 Displacement 34 Franklin pule glass 102 Distortion 57 Fraunhofer Lunes 139 Derging optical system 169 Freely faling bodies 36 toppler & principle 73 74 Frequency 07 of vibration 58 Dry ice 104 105 Friction 43 48 49 Duration Frictional electricity 113 Dynastrs 28 Frost 103 Dune 7 39 121 Fusion heat of 10% Dy ne centimeter 43 Gamms rays 157 Id con effert 152 Gas 53 82 83 84 99 ideal 85 89 Eficiency of machine 47 Gas expansion 97 Elastie *#*es 65 Gas pressure 8% Elasticity 51 Gaseous discharge 119 Electric capacity 125 Gessler tube 152 Electric charge 80 1201 General gas law 83 81 99 Tlectric conduction 123 Generator 118 C'ectrc current 138 direction of 140 Geometr c opties 160 effects ol 140 Glans visible 131 Flectric discharges an gases 119 Glow cathode 150 Itectric field 121 Gold-leaf electroscope 126 lectric Induction 197 Gram 8 121 Tlectric motor 145 Grating spectroscope 183 Electric potencial 722 Gravitational puit 40 Electric power 142 Gravity 38 37 40 Flertric resistance 139 Flectricity 80 1181 1368 fretional 113 Clectrolys s 140 141 Heat 03F ot tosion 107 of vaporization Electrolyte 141 101 104 Electromagnet 143 Heat insulation 103 Flctromagnetic induction 145 Heat transfer 107 Flectromagnetic #pretrum 112 TIeating effect of current 140 T'lectromantive force 139 Ilelrum 179 Electron 21 23 79 81 123 124 151 Hooko, law 65 Flectron volt 157 Iforsepower <3 Flectronics 21 154 flothouse 115 I lectrostatics 119 House beating 104 rimator hsdraulic of Hundity 102 103 Fmugon therminnie 152 Hydraulic brakes of Energy 43 44 94 159 Hydraulic elevator 04 Inginering Hydrogen atom 235 158 Faglih sten of unit 89 Hydrometer 02 TAIhrum of forces 31 Hydrostatics 63 Fte <3 Ilygrometry 103 fiber 31 llyfermetropia 173 Evaporation 102 Ilypothesis 3 1 xparuinn coefficient of 96 therma) 95 Fpmentation 2 Ideal gas 85 93 Fxplanation 3 Illumination 102 103 Lye 173 Ise defects 175 Image optical 180 187 real, 167, virtual, 107 Inch Fahrrnhest scale 06 Fara 131 dark once 150 Toel nal plane 47 Isclex of refruction 161 Famihelore 175 Induction electrical 127 Feromareils 133 electromagnet 113 mametle 13! Diel pletnical 121 Inductive reax dire ? Il Held tensity 111 12 Inertu o 39 Fim laps on thin 14 Fwnon 133 Infrarel Ight 112 Bal points of thermometer 99 Instantaneniu veiority 23 Ft 39 Instrument of measure Flyt pitsure Insulation heat 109 (soul.stor 123 ·IDEXX 199 Intens ty of entric Reld 121 122 of the Molecular force 87 99 pet e Geld 132 Molecules 79 Isterlerepce 192 of rased 3 63 Moment of force 33 Laterrat obal me er Momentum 33 Isyus ble glass 134 Motion 6 26 Brown 81 82 Neoton Ion 141 laps of 27-40 Isotope of ursnitt 158 Votor elecinc 145 Vyopis 175 Joule B taw 141 Mears ghtedness 175 Klogram 7 8 legst ve electnc charge 119 huutowatt 142 Megat ve flow 150 Kilowatt hour 142 Neutron 81 156 Kinemat ca 26 Mewtou s law of gravitation 10 w of Ketc energy 43 mot on 37-40 Kinet e theory of matter 82 83 Todes 70 Kundes apparat:29 72 North magnet e pole 131 Morth seek ng pole 131 Law of Boyle 83 of Coulomb 121 of Nuclear phys 21 gravitst on (hewton) 4po! Hoche 55 of illum nation 163 of Joule 112 of Ob ervat on 3 11 Lena 148 of mot ou (Newton) 37-40 of Octave 75 Ohm 139 of reffect OD 163 164 of re Ohms Law 139 frac on (Spell) 164 Open organ P pes 76 Left hand rule 145 Opt cal image 168 107 Length 25 of wave 67 Opto 1000 Lena 169 Osmosi 88 Lenz slar 146 Osthote sure 89 Lever 47 Overtones 7176 Leve arm 32 Lght 1608 polarwed 184 185 Parallel Light 169 Lght raye 161 Paramagnetism 133 Lines Fraunhofer 180 Paseale naple 64 Liquid 59 83 Pendu um unple ST Luud air 104 105 Per od of vibrat on $6 Lodestone 119 136 Permaflov 132 Longitudinal wave 68 Permeabli v magnet c 133 Loops 70 Phase 57 68 192 Loudness of sound 75 Photnelaat sty 180 Lumen, 182 Photoelectnc cell 153 154 Photoelectr c effect 22 153 Mach he 45 Photograph c camera 173 Magnet c compass 130 Pbogometry 162 163 Blagnet cetect of current 340 Ph 1 cal opt o 180 Aragtet e field intensity 132 Puhole camers 182 Machet c induction 132 Ptch of screw 48 of sound 75 Neguet • rement1 ty 133 Plumb ne 39 Vognet e pole 131 Par uma 158 Magnetern 13017 terres ria! 131 Polariza on by reflect on 186 Vagopt te 119 13 Polarized Ight 184 185 Magnifying gla89 172 Polarizer 135 Man 2 SO 1 Polaro 184 185 Matter T9X Pue magnet c 131 Mean solar das 8 Porto 34 Mean solat second 8 Pos ve columns 150 Measurement 3 10 Post ve electre charge 30 it Measuring instrudents 9 Pont ve glow 150 Mechanical adutage 45 Port. 153 Mechanical equivalence of best 84 Poa tron 81 158 Velde apparatus 73 Potent al electrie 122 Mercury 5 Po ent al dfference 193 Ale cury thermomete 95 Polen w enerb 46 Meteorolor 110 Pound 7 Vete ne natione Power $5 electre 161 Me ne system of uruts 3 Preure tmpber 01 11 sat V chlson Vo ley experiment 23 85 Ostof 8 Yapor 102 Mometer cal per 10 Pressure cooker 102 A roncope Euple 1 2 Prunery colon ist Mirrors 167 Prem 173 M.K.S unuta Proton 73 81 135 ·SV PU4 173 Pusb € 20 Striktione 150 Sublimatzon 104 Submarine 2 SupersonLCS Sutiara (ETS2093 87 Bystems of unsts BS Quality of sound 75 78 Quata 133 Quantum theory 22 113 Radar t3t Radont enero 121 Radisson of heat $07 135 Radio 152 Radio wave 132 Rad oactiv 31 337 Rainbow 178 Naretacsson 68 Ray alpha 157 beta 137 cathode 133 Rama 15 heht 585 Dostne 155 Ray diagram 170 Rate 167 Nemi near propagation 163 cheetan 20 303 285 total internal 105 Retraction 70585 Refractie wes 184 Refngrration 104 Regelscson 05 relative humid ts 103 Rent vsey 2 118 les alonce electrie 139 JXraODC 56 ) ) Troulant foror 27 28 Retina 573 Rotary motion Television 21 04 Temperature 93 absolute 97 Tension sustace 87 Terrestrial roagnetism 131 Theoretical meconteal advantage 47 Thermal expansion 83 Thermionac eraikon 252 Thermometry 85 Thermos bottle 113 114 Thermostat bitmetallo 93 5 Syys 185 Threshold of bearing 75 Time 258 Torque 33 Total internal reflection 185 Transfer of hest 107 Translatory motion 32 Transverse ware * Twist 37 Ultraviolet light 212 Unt magnetic pole 137 Vart positive electric cbarge 123 Unta S8 Uranium 338 Uranurn fision 158 Uranum otope 359 Vacunma 03 Vapor pressure 703 I aportation beat of 103 Sector 28 29 Vector uddy1200 29 Seloc ty 34 33 al sound 7 of wave 67 Terbser cal 10 Ibration 38 Isrtual image 167 Toft $22 oltage 122 Sasing into the wind 30 60 Amerme brontatic law 3 39 Scleatic butlost * $ Screw 45 cond asean solar Selective transmission 114 Slear 67 Aide thrust 344 145 Ample barnan motto3 $4 Sunnle microscope 172 staple pendulum 57 SKBK Lokometer 103 Anellian 01 Folenoid 368 int 53 13 Found 75 charactertias al 73 reloa ity of 75 Forind movies 244 mtb menetic pole 133 Specific martty Epecifio beat 100 Spectroscope pat 131 Freelrorory 122 178 170 Spectrum boplon 150 bright line 119 fontziz 179 dark-sse 383 electro tre $12 yindle 173 Free 23 Contit tart > Fram ng reret 70 la tra! 127 134 srat farv at volt, 123 PRAIA na 3 watt 113 Wave 048 elastic 65 Songitudinal 181 standing 70 trannverne 78 40 thamurth t More frequency 47 are interference 68 Wpre length 87 Wsve motron 67 Ware 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D gest of (EH) FOLK DANCES For All (EH) PSYCHOLOGY HUNTING Sportsman s D gest of (EH) PSYCHOLOGISTS Teach ags of Great (EH) MANNERS FOR MILLIONS (EH) PSYCHOLOGY CHILD (CO) PARTY GAMES For All EH} PSYCHOLOGY EDUCATIONAL (CO) PSYCHOLOGY GENERAL (CO) STUDY AIDS PSYCHOLOGY READINGS (CO) EXAMINATIONS COLLEGE (CO) HANDWRITING BETTER (EH) SCIENCE RESEARCH and REPORT WRITING (CO) ANATOMY and PHYSIOLOGY (CO) STUDY BEST METHODS of (CO) BACTERIOLOGY (CO) WRITING TERM PAPERS BIOLOGY (CO) and REPORTS (CO) *This ist beg ns at the front of the book ·a SUVAI 3 1 Quality of sound 75, 78 Quanta 115 Qz aktu theoy 2 1 Stristioss 199 Sublimas 100 104 Subocerine 82 Bupersonnes 71 Surface tension 37 83 steres af unt & 9 Rastar 254 Radiant energy. 111 Had atron of heat 107, 111 Radio 452 Radio waves 153 Rad bactsvaty 21 147 Rainbow 178 Rarelaetsors 83 Ray xlshs 137 bete 137 catbode 151 Kama 257 light 182 Domni ve $55 Ray dumRTATAR 1706 Neal unake 107 Rrets nens propagation 161 Befecsson 73 33 189 total internal 165 Refracciou 70 104 Retract ve tee 164 Tereptat on 104 Regolatron 105 Pelative hy duty 103 Relat vity 32 113 Res CAtce electric 137 RAONARKT 50 70 71 Revuttant force 27 28 Ret 173 Rotary motion 32 Televisios 21 134 Tersperature $3 Absolute 97 Tetus on surface 87 Terrestrial magart 131 Theoret, cal mechanical advantage 37 Thermal expans on $5 Theronic emission 152 Thermotsetry 85 Thermos bottle 113 114 Thermostat bimetaldc 99 Thin Biss 134 Threshold of bearing 75 Tome ? $ & Torque 23 Total anternal reflection 185 Transfer of brat 107 Translatory motion 22 Transvese wave 63 Twist 37 Ultraviolet Isght 118 Ust egnetsc pole 131 Vast positive electric charge 120 Units X 8D Lerum 159 Uranium fianch 16S Uranium isotope $58 Vacun 03 apor pressure 304 Vsportistsod best of 100 Veetor 24 Tector add tro 29 Velocity 34 35 of sound 75 of wave 67 Seniet caliper 10 Vibratson $6 Istust smage 383 Saitago 122 Rare into the wind 30 Serene 8 fcienti5c Bw 3 39 Scient fe tsvethod 2 3 Krew 45 Recond tear solar Releetav tromisoa 114 Shesr $T Nute thrust 144 143 Sumple hactcnus motion 35 ) Cumple meroncone 172 Symple pendulum $7 51 DR bychometer 103 Enell Law 161 Solenoid 16 Sold 88 83 Syurd 788 characterutice of 75 relacity of 73 Round mye 156 Fouth magnetic pole 33 Specie frusty * pecube host 100 Spectroscose mutung 183 Fremtroncosy 113 178 179 Rrentum leorptios 100 bright-One 177 tonttu 179 dark line 39 Sectro martie 1u 1 dle {75 Sport 3 Syrine halance ? sandid IvM 70 Seat erilah 1 $24 Brut-turut t25 Wat volt 19 RM 35 Euro 35 *att 143 tave elastie 63 longitudinal 682 Mandung 70 travete 03 Wave charactertates 67 Wave frequency 07 Wave sterference 68 are lengte Ware motion 87 Ware repremutattat 43 Ware velorsty 67 Wrather #3 110 edge 43 me she 7 Height denx ly 39 Work, (2 Yray 22 112 131 132 und Aspriear # English 3 Zero absolute &