A Student’s Guide to Maxwell’s Equationsassets.cambridge.org/97805218/77619/frontmatter/...A Student’s Guide to Maxwell’s Equations Maxwell’s Equations are four of the most

A Student’s Guide to Maxwell’s Equations

Maxwell’s Equations are four of the most influential equations in science: Gauss’s

law for electric fields, Gauss’s law for magnetic fields, Faraday’s law, and the

Ampere–Maxwell law. In this guide for students, each equation is the subject of

an entire chapter, with detailed, plain-language explanations of the physical

meaning of each symbol in the equation, for both the integral and differential

forms. The final chapter shows how Maxwell’s Equations may be combined to

produce the wave equation, the basis for the electromagnetic theory of light.

This book is a wonderful resource for undergraduate and graduate courses in

electromagnetism and electromagnetics. A website hosted by the author, and

available through www.cambridge.org/9780521877619, contains interactive

solutions to every problem in the text. Entire solutions can be viewed

immediately, or a series of hints can be given to guide the student to the final

answer. The website also contains audio podcasts which walk students through

each chapter, pointing out important details and explaining key concepts.

daniel fleisch is Associate Professor in the Department of Physics at

Wittenberg University, Ohio. His research interests include radar cross-section

measurement, radar system analysis, and ground-penetrating radar. He is a

member of the American Physical Society (APS), the American Association of

Physics Teachers (AAPT), and the Institute of Electrical and Electronics

Engineers (IEEE).

© Cambridge University Press www.cambridge.org

Cambridge University Press978-0-521-87761-9 - A Student’s Guide to Maxwell’s EquationsDaniel FleischFrontmatterMore information

http://www.cambridge.org/052187761X

http://www.cambridge.org


A Student’s Guide to

Maxwell’s Equations

DANIEL FLEISCH

Wittenberg University






CAMBRIDGE UNIVERS ITY PRESS

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Cambridge University Press

The Edinburgh Building, Cambridge CB2 8RU, UK

Published in the United States of America by Cambridge University Press, New York

www.cambridge.org

Information on this title: www.cambridge.org/9780521877619

ª D. Fleisch 2008

This publication is in copyright. Subject to statutory exception

and to the provisions of relevant collective licensing agreements,

no reproduction of any part may take place without

the written permission of Cambridge University Press.

First published 2008

Printed in the United Kingdom at the University Press, Cambridge

A catalogue record for this publication is available from the British Library

Library of Congress Cataloging in Publication Data

Fleisch, Daniel A.

A Student’s guide to Maxwell’s equations / Daniel Fleisch.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-521-87761-9 (hardback : alk. paper) – ISBN 978-0-521-70147-1

(pbk. : alk. paper)

1. Maxwell equations. I. Title.

QC670.F56 2007

530.1401–dc222007037901

ISBN 978-0-521-87761-9 hardback

ISBN 978-0-521-70147-1 paperback

Cambridge University Press has no responsibility for the persistence or

accuracy of URLs for external or third-party internet websites referred to

in this publication, and does not guarantee that any content on such

websites is, or will remain, accurate or appropriate.






Contents

Preface page vii

Acknowledgments ix

1 Gauss’s law for electric fields 1

1.1 The integral form of Gauss’s law 1

The electric field 3

The dot product 6

The unit normal vector 7

The component of ~E normal to a surface 8

The surface integral 9

The flux of a vector field 10

The electric flux through a closed surface 13

The enclosed charge 16

The permittivity of free space 18

Applying Gauss’s law (integral form) 20

1.2 The differential form of Gauss’s law 29

Nabla – the del operator 31

Del dot – the divergence 32

The divergence of the electric field 36

Applying Gauss’s law (differential form) 38

2 Gauss’s law for magnetic fields 43

2.1 The integral form of Gauss’s law 43

The magnetic field 45

The magnetic flux through a closed surface 48

Applying Gauss’s law (integral form) 50

2.2 The differential form of Gauss’s law 53

The divergence of the magnetic field 54

Applying Gauss’s law (differential form) 55

v






3 Faraday’s law 58

3.1 The integral form of Faraday’s law 58

The induced electric field 62

The line integral 64

The path integral of a vector field 65

The electric field circulation 68

The rate of change of flux 69

Lenz’s law 71

Applying Faraday’s law (integral form) 72

3.2 The differential form of Faraday’s law 75

Del cross – the curl 76

The curl of the electric field 79

Applying Faraday’s law (differential form) 80

4 The Ampere–Maxwell law 83

4.1 The integral form of the Ampere–Maxwell law 83

The magnetic field circulation 85

The permeability of free space 87

The enclosed electric current 89

The rate of change of flux 91

Applying the Ampere–Maxwell law (integral form) 95

4.2 The differential form of the Ampere–Maxwell law 101

The curl of the magnetic field 102

The electric current density 105

The displacement current density 107

Applying the Ampere–Maxwell law (differential form) 108

5 From Maxwell’s Equations to the wave equation 112

The divergence theorem 114

Stokes’ theorem 116

The gradient 119

Some useful identities 120

The wave equation 122

Appendix: Maxwell’s Equations in matter 125

Further reading 131

Index 132

Contentsvi






Preface

This book has one purpose: to help you understand four of the mostinfluential equations in all of science. If you need a testament to thepower of Maxwell’s Equations, look around you – radio, television,radar, wireless Internet access, and Bluetooth technology are a fewexamples of contemporary technology rooted in electromagnetic fieldtheory. Little wonder that the readers of Physics World selected Maxwell’sEquations as “the most important equations of all time.”

How is this book different from the dozens of other texts on electricityand magnetism? Most importantly, the focus is exclusively on Maxwell’sEquations, which means you won’t have to wade through hundreds ofpages of related topics to get to the essential concepts. This leaves roomfor in-depth explanations of the most relevant features, such as the dif-ference between charge-based and induced electric fields, the physicalmeaning of divergence and curl, and the usefulness of both the integraland differential forms of each equation.

You’ll also find the presentation to be very different from that of otherbooks. Each chapter begins with an “expanded view” of one of Maxwell’sEquations, in which the meaning of each term is clearly called out. Ifyou’ve already studied Maxwell’s Equations and you’re just looking for aquick review, these expanded views may be all you need. But if you’re abit unclear on any aspect of Maxwell’s Equations, you’ll find a detailedexplanation of every symbol (including the mathematical operators) inthe sections following each expanded view. So if you’re not sure of themeaning of ~E � n in Gauss’s Law or why it is only the enclosed currentsthat contribute to the circulation of the magnetic field, you’ll want to readthose sections.

As a student’s guide, this book comes with two additional resourcesdesigned to help you understand and apply Maxwell’s Equations: aninteractive website and a series of audio podcasts. On the website, you’llfind the complete solution to every problem presented in the text in

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interactive format – which means that you’ll be able to view the entiresolution at once, or ask for a series of helpful hints that will guide you tothe final answer. And if you’re the kind of learner who benefits fromhearing spoken words rather than just reading text, the audio podcasts arefor you. These MP3 files walk you through each chapter of the book,pointing out important details and providing further explanations of keyconcepts.

Is this book right for you? It is if you’re a science or engineeringstudent who has encountered Maxwell’s Equations in one of your text-books, but you’re unsure of exactly what they mean or how to use them.In that case, you should read the book, listen to the accompanyingpodcasts, and work through the examples and problems before taking astandardized test such as the Graduate Record Exam. Alternatively, ifyou’re a graduate student reviewing for your comprehensive exams, thisbook and the supplemental materials will help you prepare.

And if you’re neither an undergraduate nor a graduate science student,but a curious young person or a lifelong learner who wants to know moreabout electric and magnetic fields, this book will introduce you to thefour equations that are the basis for much of the technology you useevery day.

The explanations in this book are written in an informal style in whichmathematical rigor is maintained only insofar as it doesn’t get in the wayof understanding the physics behind Maxwell’s Equations. You’ll findplenty of physical analogies – for example, comparison of the flux ofelectric and magnetic fields to the flow of a physical fluid. James ClerkMaxwell was especially keen on this way of thinking, and he was carefulto point out that analogies are useful not because the quantities are alikebut because of the corresponding relationships between quantities. Soalthough nothing is actually flowing in a static electric field, you’re likelyto find the analogy between a faucet (as a source of fluid flow) andpositive electric charge (as the source of electric field lines) very helpful inunderstanding the nature of the electrostatic field.

One final note about the four Maxwell’s Equations presented in thisbook: it may surprise you to learn that whenMaxwell worked out his theoryof electromagnetism, he ended up with not four but twenty equations thatdescribe the behavior of electric andmagnetic fields. It was Oliver Heavisidein Great Britain and Heinrich Hertz in Germany who combined and sim-plified Maxwell’s Equations into four equations in the two decades afterMaxwell’s death. Todaywe call these four equations Gauss’s law for electricfields, Gauss’s law for magnetic fields, Faraday’s law, and the Ampere–Maxwell law. Since these four laws are now widely defined as Maxwell’sEquations, they are the ones you’ll find explained in the book.

Prefaceviii






Acknowledgments

This book is the result of a conversation with the great Ohio State radioastronomer John Kraus, who taught me the value of plain explanations.Professor Bill Dollhopf of Wittenberg University provided helpful sug-gestions on the Ampere–Maxwell law, and postdoc Casey Miller of theUniversity of Texas did the same for Gauss’s law. The entire manuscriptwas reviewed by UC Berkeley graduate student Julia Kregenow andWittenberg undergraduate Carissa Reynolds, both of whom made sig-nificant contributions to the content as well as the style of this work.Daniel Gianola of Johns Hopkins University and Wittenberg graduateMelanie Runkel helped with the artwork. The Maxwell Foundation ofEdinburgh gave me a place to work in the early stages of this project, andCambridge University made available their extensive collection of JamesClerk Maxwell’s papers. Throughout the development process, Dr. JohnFowler of Cambridge University Press has provided deft guidance andpatient support.

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A Student’s Guide to Maxwell’s Equationsassets.cambridge.org/97805218/77619/frontmatter/...A Student’s Guide to Maxwell’s Equations Maxwell’s Equations are four of the most