University Physics Electricity and Magnetism

UNIVERSITY PHYSICS BY F. C. CHAMPION, M. A., Ph. D. Cantab. Lecturer in Physics, University of London PART FIVE ELECTRICITY AND MAGNETISM BLACKIE BLACKIE SON LIMITED LONDON AND GLASGOW 1942 PREFACE This book is primarily intended for students taking a First and Second Year Course in Physics at a University. It is designed for preparation for examinations of the standard of Part I of the Natural Sciences Tripos at Cambridge, the B. Sc. General Degree of London and, by the omission of those sections marked with an asterisk, for Intermediate students who have already studied the elements of Physics at school or elsewhere. It must be remembered that at the stage covered by this book, students will not yet have become specialists in Physics. The writer has had frequent experience of students who, during a period when they are studying two or three additional subjects, feel a great need for one book on Physics which contains the basic information which they must acquire. It is not suggested that this book has made others unnecessary or, more particularly, that it has rendered lectures super fluous. It remains as important as ever for students to read widely and to acquire experience of the different methods of treatment of a subject which only a diversity of Lecturers can supply. Finally, it is becoming more and more recognized, at least as an ideal, that material usually given in formal lectures can be quite as well acquired from good text-books and that lectures will gradually develop into a tuitional tutorial system under which the time and energy of the lecturer can be devoted to the detailed elucidation of difficult points, apt illustrations and demonstrations, the discussion of essaysand exercises done by the student, and the exercise of per sonality to engender an enthusiasm without which a subject remains dry bones . F. C. CHAMPION. CONTENTS CHAP., Page I. INTRODUCTORY 1 It. ELEMENTARY ELECTROSTATICS 3 HI. ELECTROSTATICS 18 IV. LECT SJAXIClJlEASUREMENTS 28 V. ELECTROSTATIC MACHINES - 34 VI. ELEMENTARY MAGNETISM 39 VII. MAGNETIC MEASUREMENTS - 45 VIII. ELEMENTARY PROPERTIES OF THE ELECTRIC CURRENT 53 IX. GALVANOMETERS, AMMETERS, AND VOLTMETER - t - 65 X. MAGNETIC INTERACTIONS OF THE ELECTRIC CuRRESPT 7 - 73 XI. MEASUREMENT or RESISTANCE AND POTENTI T - 82 XII. ELECTROLYSIS 98 XIII. ELECTROMAGNETISM 114 XIV, MAGNETIC PROPERTIES OP MATERIALS - - - -142 XV. ELECTRICAL OSCILLATIONS 153 Xyir. ELECTROMAGNETIC WAVES -. . . . . 161 XVH. THERMOELECTRICITY, - - -J-168 XVIII. ELECTRICAL DISCHARGE THROUGH GASES - - - 178 XIX. ELECTRONICS - 186 XX. ATOMIC AND NUCLEAR PHYSICS 200 EXAMPLES 217 ANSWERS AND HlNTS FOR SOLUTION .... 229 INDEX 241 CHAPTER I Introductory Unlike many physical phenomena, electric and magnetic forces are not directly detectable by the senses. Further, according to modern ideas the branches of physics classified under the title of Electricity and Magnetism are on a different footing from those branches already discussed in Vols. I-IV. In problems involving General Physics, Heat, Sound, and to a lesser extent Light, the majority of the experimental results are adequately explained in terms of mechanics and the mole cular theory of matter, together with the mathematical aid afforded by thermodynamics. With electric and magnetic phenomena, however, , the molecular theory is of little assistance, the simplest experiment re-i quiring interpretation on themodern view in terms of particles smaller than atoms. Of these sub-atomic particles, the electron plays at present the dominant role. It is true that electricity and magnetism achieved tremendous advances during the hundred years prior to the discovery of the electron, yet to attempt to interpret those advances to-day without constant reference to the electron theory would be analogous to an attempt to account for chemical phenomena in terms of phlogiston theory...