PROBLEMS OF
MATHEMATICAL PHYSICS


\-ƠSELECTED RUSSIAN PUBLICATIONS IN THE MATHEMATICAL SCIENCES
Richard A. Silverman, Editor

\
,

..

ã

PRENTICE-HALL INTERNATIONAL, INC.,

London

PRF..r.;llCE-HALL 01' AUSTRALIA, PTY., LTD.,

Sydney

I'RENTICE-liALL OF CANADA, LTD., Torollto
l'RJ::NTICE-HALL OF INDIA (PRIVATE) LTD.,
PRENTICE-HAI.L OF JAPAN, INC.,

New De/hi

Tokyo

www.pdfgrip.com

•


L I 'li &t I

I

f!}!! '. /-J

Aoc. No ......... :f.

'·

·r-f

Oate ... ............... ················-·

PROBLEMS OF
MATHEMATICAL PHYSICS

N. N. LEBEDEV
I. P . SKALSKA YA
Y. S. UFLYAND
A. F. Joffe Physico-Technical Institute
Academy of Sciences, U. S. S. R.
Revised, Enlarged and Corrected English Edition
Translated and Edited by

Richard A. Silverman


With a Supplement by

Edward L. Reiss
Courant Institute of Mathematical Sciences
New York University

PRENTICE-HALL, INC.
Englewood Cliffs, N.J.

www.pdfgrip.com


© 1965 by
PRENTICE-HALL, lNC.
Englewood Cliffs, N.J.

All rights reserved . No part of this book
may be reproduced in any form, by
mimeograph or any other means, without
permission in writing from the publisher.

...

Library of Congress Catalog Card Number 65-26925
Printed in the United States of America
71767-C

www.pdfgrip.com



AUTHORS' PREFACE

The aim of the present book is to ·help the reader acquire the proficiency needed to successfully apply the
methods of mathematical physics to a variety of problems drawn from mechanics, the theory of heat conduction, and the theory of electric and magnetic phenomena.
A wide range of topics is covered, including not only
problems of the simpler sort, but also problems of a
more complicated nature involving such things as
curvilinear coordinates, integral transforms, certain
kinds of integral equations, etc. The book is intended
both for students concomitantly studying the corresponding topics in courses of mathematical physics,
and for research scientists who in their work find it
necessary to carry out calculations using the methods
described here. We also think that quite apart from its
value as a tool for acquiring technique, the book can
also serve as a handbook, especially in view of the fact
that answers to the problems are included.
A rather solid background in applied mathematics is
needed to profit from the book in its entirety. However,
most of the problems appearing in Chapters 2 to 5 will
be access~ble to those who have taken only the usual
first course in methods of mathematical physics. Chapters 6 to 8 are more specialized, and presuppose some
familiarity with special functions , integral transforms,
integral equations, and so on.
To make the book easier to use, each section begins
with a brief introduction describing its contents and
presenting a certain amount of relevant background
information. However, it is not claimed that this information is complete in any sense, and the reader
v


www.pdfgrip.com


Vi

AUTHORS'PREFACE

desiring further details must consult the literature, e.g.,
the books and monographs cited at the end of each
chapter.
The majority of problems in this collection are accompanied by hints, facilitating the choice of meaningful methods of solution. In addition, certain problems,
whose numbers are equipped with asterisks (e.g., *52,
* 148, etc.), are solved in detail in a special section at
the end of the book. The problems singled out in this
way have been selected either because they illustrate the
application of certain specific methods, or because of
their special difficulty or particular importance in the
applications. Because of the applied character of the
book, we restrict ourselves to formal solutions, whose
rigorous justification can be supplied by the interested
reader.
In compiling the collection, we have consulted not
only the classic works on mathematical physics, but
also a number of journal articles. Material accumulated
during years of teaching and research in the Department
of Mathematical Physics at the Leningrad Polytechnic
Institute, as well as work done in connection with industrial projects, plays a role in the material presented
here.
It would be impractical, and in many cases impossible,
to cite the original source where a given problem was

solved for the first time. fhus references to the literature
have been confined to cases we find particularly relevant.

We would like to take this opportunity to thank Prof.
G. A. Grinberg for many valuable suggestions made
in the course of writing the book.
N. N. L.
I.

P. S.

Y. S. U.

www.pdfgrip.com


TRANSLATOR'S PREFACE

The present edition differs from the Russian original
in various respects, of which three merit particular
mention:
1. The Bibliography has been expanded and updated. For example, the original sources of works
translated into Russian have been tracked down,
all references have been equipped with titles, further
references (especially, later editions and English
translations) have been added, and so on. As in
other volumes of this series, the system of references
is in "letter-number form." Thus LlO refers to the
tenth paper (or book) whose {first) author's surname
begins with the letter L, where the entire Bibliography is arranged in lexicographic order, and

chronological order as well, whenever there are
several papers by the same author.
2. Working from an extensive list of errata sent
me by the authors, I have corrected numerous
misprints and mistakes present in the Russian edition. I am particularly grateful for their help, since
the task of eliminating errors from a book of this
type (consisting primarily of problems and answers)
is both imperative and one which only the authors
themselves can perform in finite time! The authors
have also been kind enough to answer a number of
specific questions that arose in the course of the
translation.
3. It was felt that the English-language edition
would benefit greatly by the addition of material
on the approximate solution of problems of mathematical physics, since the emphasis of the Russian
vii

www.pdfgrip.com


Viii

TRANSLATOR'S PREFACE

edition is on exact solutions. This led to the writing of a Supplement on variational and related
methods by Professor Edward L. Reiss of the
Courant Institute of Mathematical Sciences of New
York University. The Supplement is independent of
the rest of the book, even to the extent of having its
own references.

Although the expression "mathematical physics" was
preserved in the title of the book as entirely fitting and
perhaps the most descriptive, one could just as well
have substituted "applied physics and engineering" or
"applied mathematics," at least in the sense in which
these terms are used in the United States.
R. A. S.

I

'
www.pdfgrip.com


CONTENTS

PART

1

1

PROBLEMS,

Page 1.

.

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS, Page 3.
1. Mechanics, 3.

2. Heat Conduction, 9.
3. Electricity and Magnetism, 11.

2

SOME SPECIAL METHODS FOR SOLVING
HYPERBOLIC AND ELLIPTIC EQUATIONS,
Page 20.
1. Hyperbolic Equations, 20.
2. E ll iptic Equations: The Green's Function Method,
27.
3. E ll iptic Equations: The Method of Conformal
Mapping, 33.

3

STEADY-STATE HARMONIC OSCILLATIONS,
Page 42.
1. Elastic Bodies: Free Oscillations, 43.
2. Elastic Bodies: Forced Oscillations, 46.
3. Electromagnetic Oscillations, 49.

4

THE FOURIER METHOD,

Page 55.

I. Mechanics : Vibrating Systems, Acoustics, 60.
2. Mechanics: Statics of Deformable Media, Fluid

Dynamics, 73.
ix

www.pdfgrip.com


X

CONTENTS

4

THE FOURIER METHOD-Continued
3. Heat Conduction: Nonstationary Problems, 77.
4. Heat Conduction: Stationary Problems, 83.
5. Electricity and Magnetism, 91.

5

THE EIGENFUNCTION METHOD FOR SOLVING INHOMOGENEOUS PROBLEMS, Page 103.
I. Mechanics: Vibrating Systems, 107.

2.
3.
4.
5.

6

Mechanics : Statics of Deformable Media, 114.

Heat Conduction: Nonstationary Problems, 119.
Heat Conduction: Stationary Problems, 124.
Electricity and Magnetism, 131.

INTEGRAL TRANSFORMS,

Page 143.

1. The Fourier Transform, 146.

2.
3.
4.
5.

7

CURVILINEAR COORDINATES,
1.
2.
3.
4.
5.
6.
7.
8.

8

The Hankel Transform, 160.

The Laplace Transform, 169.
The Mellin Transform, 189.
Integral Transforms Involving Cylinder Functions
of Imaginary Order, 194.

Page 203.

Elliptic Coordinates, 204.
Parabolic Coordinates, 210.
Two-Dimensional Bipolar Coordinates, 212.
Spheroidal Coordinates, 219.
Paraboloidal Coordinates; 231.
Toroidal Coordinates, 233.
Three-Dimensional Bipolar Coordinates, 242.
Some General Problems on Separation of Variables, 247.

INTEGRAL EQUATIONS,
1. Diffraction Theory, 254.
2. Electrostatics, 259.

www.pdfgrip.com

Page 253.


CONTENTS
PART

2


SOLUTIONS,

Xi

Page 273.

MATHEMATICAL APPENDIX,

Page 381.

I. Special Functions Appearing in the Text, 381.
2. Expansions in Series of Orthogonal Functions,
384.
3. Some Definite Integrals Frequently Encountered
in the Applications, 386.
4. Expansion of Some Differential Operators in
Orthogonal Curvilinear Coordinates, 388.

Supplement. VARIATIONAL AND RELATED
METHODS, Page 391.

1. Variational Methods, 392.
1.1. Formulation of Variational Problems, 392.
1.2. The Ritz Method, 396.
1.3. Kantorovich's Method, 401.
2. Related Methods, 404.
2.1. Galerkin's Method, 404.
2.2. Collocation, 407.
2.3. Least Squares, 411.
3. References, 412.

BIBLIOGRAPHY,
NAME INDEX,

Page 415.
Page 423.

SUBJECT INDEX,

Page 427.

www.pdfgrip.com


www.pdfgrip.com


Part

1

PROBLEMS

www.pdfgrip.com


'Ii''

'

I,J


":
I

''

I

www.pdfgrip.com


1
DERIVATION OF EQUATIONS AND
FORMULATION OF PROBLEMS

Chapter 1 is devoted to problem material on the derivation of the
equations of mathematical physics and the formulation of appropriate initial
and boundary conditions. It also serves as a convenient place to list the
basic equations appearing later in the book. Throughout, we assume that the
reader is familiar with the physical Jaws underlying the mathematical
formulation of the problems which arise in various branches of physics.
The chapter consists of three sections devoted in turn to problems of
mechanics, heat conduction and the theory of electric and magnetic phenomena. Each section starts with the basic equations governjng the corresponding set of problems, with appropriate references to sources where the
derivations can be found. Special attention is dev.oted to the formulation of
problems of electrodynamics, since this subject is inadequately covered in
the available Jiterature.1

I. Mechanics
This section contains problems on the derivation of equations of motion
and formulation of initial and boundary conditions for vibrating strings,

membranes, rods and plates, as well as some examples pertaining to the
statics of deformable media. It will be assumed that the reader has already
1 Those particularly in terested in mathematical aspects of the formulation of physical
problems can find relevant material in CS, GJ, LJ, P2, Sl and SJ 3. (The reference scheme
is explained in the Translator's Preface.)

3

www.pdfgrip.com


4

DERIVATION OF EQUATIONS AKD FORMULATION OF PROBLEMS

encountered the basic equations in a f1rst course on mathematical physics. 2
Thus we shall merely list the equations concisely, at the same time explaining
the notation to be used in the book.
I. The equation of a vibrating string is

au

1

au

2

2


2

2

Ox

2

v Ot

_ q(x, t)
T

where u(x, t) is the displacement of the point of the string with abscissa
x at the timet, q(x, t) is the external load per unit length, Tis the tension,
and p is the linear density.
2. The equation for longitudinal oscillations of a rod of constant cross
section is
v,........

J£

-'

p

where u(x, t) is the displacement of the cross section of the rod with
abscissa x at the time 1, E is Young's modulus, and pis the density.

3. The equation for transverse oscillations of a rod (beam) is


where u(x, t) is the displacement of the points along the midline of the
rod, q(x, t) is the external load per unit length, E is Young's modulus,
J is the moment of inertia of a transverse cross section, pis the density,
and Sis the cross-sectional area.
4. The equation of a vibrating membrane is
2

0u

ox'

+ o'u - ! o'u ~ oy 2 v' ot'

q(x,

y, t)

T

v~Jf;.
p

where u(x, y, 1) is the displacement of the point (x, y) of the membrane
at the timet, q(x, y, t) is the external load per unit area, Tis the tension
per unit length of the boundary of the membrane, and p is the surface
density.
5. The equation for transverse oscillations of a thin elastic plate is

!l.'u


+

_1_
b'

o'u
ot'

=

q(x,

y, t)

D

'

b'~

JD

ph,

2
See 56 (Vol. II), S14, Tl and T2. Concerning the derivation of the equations of
vibrating plates, see T4.

www.pdfgrip.com



I' ll! 111.

I

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

5

where u(x, t) is the displacement of the point (x, y) of the midplane of
the plate at the timet, q(x,y, t) is the density of the external load, D
is the flexural rigidity, h is the thickness, p is the density, and

02

t::. = -

02

ox2

+- 2
oy

is the two-dimensional Laplacian operator.
The above equations lead to corresponding equations for static
deflections, if we regard the external load q and the unknown displacement u as independent of the time t. For example, the equilibrium
equation for the membrane is


6.
02U

+0

2
U

ox 2 o/

= _ q(x, y)
T

the static deflection of the plate satisfies the equation

7.
and so on.
Among the other equations governing the statics of elastic bodies
which will figure in this book, we cite the familiar equation
8.

for twisting of a prismatic rod, where u(x, y) is the torsion function.
We now give some problems on the formulation of initial and boundary
conditions for these equations, and also some problems on the derivation of
o ther differential equations.
1. Describe the initial and boundary conditions for a vibrating string with
fixed ends (0 < x < /), which is stretched at the point x = c and time t = 0
to a heig\lt h, and then released without initial velocity.

Ans.


hx

uJ,_o = f (x)

=

c '
( h(l - x)
l - c '

0

< X< C,

C< X< l,

uJ:t=O= uJx=l = 0.

www.pdfgrip.com

auot

I - o·
t- o -

'


6


2

PROB.

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

2. A concentrated load of mass m 0 is fastened at the point x = c of a
string 0 < x < I of length I. Find the equations describing vibrations of the
string with arbitrary initial conditions, assuming that the ends of the string
are fastened.

Ans.
0 <;X<; C,
C

<

~

(i

1, 2),

X< [,

with initial conditions

au I


ul,~o ~ f(x),

-

ot

~~·

~

g(x),

and boundary conditions

3. Formulate initial and boundary conditions for the problem of longitudinal oscillations of a rod in the foiiov-.ring special cases:
a) A rod oflength I is clamped at the end x ~ 0 and stretched by a force F
applied to the other end; at the timet ~ 0 the force is suddenly discontinued;
b) A tensile force F(t) is applied at the timet- 0 to the end x
cantilever in equilibrium;

~I

of a

c) A cantilever clamped at the point x = 0, with a load of mass M 0 at the
free end x = !, undergoes longitudinal oscillations subject to arbitrary initial
conditions.

Ans.


ou / ~ o·

Fx
a) ul,~o ~ - ,

Ox

ES

b) ul,~o ~ 0,

au

Ot

c) ul,~o ~ f(x),

-au

I ~o
t=O

at

I

t~o

~


x=l

~

au/

ax

'

X=l

'

F(t) ;
ES

ivr, a'u2 I

g(x),

ES

ot

x~t

0

4. Derive the differential equation for longitudinal oscillations of a thin

rod of variable cross section S ~ S(x). As an example, derive the equation
for oscillations of a conical rod.

Ans.

_1_1._[s(x)
S(x)

ax

au] - _!_ a'u ~ 0

OX

v2

at'

,

www.pdfgrip.com


l'llO B.

8

DERI VATION OF EQUATIONS AND FORMULATION OF PROBLEMS

7


5. Derive the equation for torsional oscillations of a shaft of circular
cross section.

Ans.

ae
ax

ae
at

2
1 2
-2- -2 -2 - 0

v

-

,

where G(x, t) is the angular displacement of the cross section x relative to the
equilibrium position, v = .JG/p, pis the density, and G is the shear modulus.
Hint. The torque at the cross section x is given by the expression
M

=

GJ


ae
ax,

where J is the polar moment of inertia of a cross section of the shaft.
6. Formulate initial and boundary cond itions for the problem of torsional
oscillations of a shaft of circular cross section and length /, where the end
x = 0 is clamped and a disk-shaped mass with moment of inertia J 0 is attached
to the other end. At the time t = 0, the disk is rotated through a given angle
IX and then released without initial velocity.

Ans.

7. A cantilever of length lis clamped at one end x = 0 and loaded by a
force Fat the other end. At the timet = 0, the action of the force is suddenly
discontinued. Formulate initial and boundary conditions for the corresponding oscillations.

Ans. Initial conditions
u

I

I= O

= -F- ( 3lx2 - x 3),
6EJ

auat

I


- 0

1=0 -

'

and boundary conditions
ulx=O

=

au I =
ax x=O

0,

a2~ I = a3~ I
ax x=! ax

= 0.

X=!

8. Describe initial and boundary conditions for the problem of free
oscillations of a disk-shaped plate with clamped edge, whose initial deformation is due to a concentrated force F applied at the center of the disk.

Ans.

au

at I

- o·

1=0-

www.pdfgrip.com

'


8

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

PROB.9

Hint. To determine the static deflection due to the concentrated force,
consider the force as the limiting case of a load of density F/m:2 uniformly
distributed over a small disk of radius e:.
9. Show that the problem of the deflection of a plate with a simply
supported polygonal boundary reduces to the solution of Poisson' s equation

6-w = f(x , y),
with boundary condition wlr = 0 (Jis a known function).
Hint. Note that in the present case, the boundary conditions on the
supported edge can be written in the form ulr = 0, 6-ulr = 0.
10. Show that the velocity potential for the three-dimensional flow of an
ideal incompressible fluid containing no sources is described by Laplace's
equation

6-u = 0.

Hint . Use the condition

fs v • ndS = 0
(v is the vector describing the velocity of fluid particles at a given point, Sis
an arbitrary closed surface inside the flow, and n is the exterior normal to the
surface S) and the condition
v = - grad u
for potential flow.
11. Formulate mathematically the problem of the flow of an ideal fluid
past an object bounded by a surface S, where fluid emanates from a point
source of strength m located at a point M 0 in the region exterior to S.
Ans. The problem reduces to finding a solution of the equation

6-u = 0
which is regular (i.e., has no singularities) in the region exterior to S, except at
the point M 0 • In a neighborhood of M 0 ,
u =

m

47tp

IMMol

+ a regular function

where M is a point near M 0 and p is the density of the fluid (IMM0 1 denotes
the distance between M and M 0 ). The desired function u must satisfy 'tho

boundary condition

au
on

I=o
s

www.pdfgrip.com


PROB.

12

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

9

and the condition
u

= O(R- 1),

R -+ oo

at infinity.

2. Heat Conduction
As proved in courses on mathematical physics (see Sl , Tl), the flow of heat

in a body of thermal conductivity k, specific heat c and density p is governed
by Fourier's equation

where T(M, t) is t he temperature at the point M , and Q is the density of heat
sources within the body. 3 T he boundary conditi ons to be satisfied on the
surface of the body (or its parts) depend on the particular problem under
consideration. Most often it is assumed that the s urface of the body has a
given temperature Tis = f(P, t), where Pis a point of the surface S, or that
the body radiates heat into the surrounding m edium according to Newton's
law, which states that the am ount of heat radiated by a unit area of the
surface per unit time is proportional to the difference between the temperature
of the surface and that of the surrounding medium. In the latter case, the
boundary condition takes t he form

(~: + hT) Is= hTmcd,
where of on indicates differentiation with respect to the exterior normal to S,
is the temperature of the surrounding mediu m, a nd h is the heat
exchange coefficient or emissivity. Without loss of generality, we can assume
lftat Tmcd = 0 ; th is assumptio n is made in all t he problems involving heat
~.:ond uction except Prob. 155. 4
We now give a few problems on the formulation of initial and bou ndary
l'Onditio ns fo r t he equation of heat conduction (and for the related diffusion
rq uation).
'fmed

12. Let the temperature of a conductor in the form of an infinite cylinder
nf radius a be initially th e same as that of the su rrounding medium. Suppose
I hat starting from the time t = 0, the conductor is heated by a constant
3


The density of heat current (i.e., the heat flux) is descri bed by the vector
q

1
'

=

-k grad T.

Examples of other boundary conditions encountered in the applications are given in

l'mbs. 365, 367 and 370.

www.pdfgrip.com


I0

DERIVAnON OF EQUATIONS AND FORMULATION OF PROBLEMS

PROB.

13

electric current rc]easing an amount of heat Q per unit volume of the conductor. Give a mathematical formulation of the corresponding problem of
heat conduction, assuming that the heat exchange at the surface of the conductor obeys Newton's law.'

Ans. The temperature T(r, t) satisfies the equation


oT) oT

1iJ ( r -

kt
cp

Q

~---

ror or

a.

't"=-~

k'

with initial condition

Tj,_ 0 = 0
and boundary condition

(arar + hr)/

=

o.


r=a

13. A homogeneous sphere of radius a is heated for a long time by heat
sources uniformly distributed throughout its volume with density Q. Write
the equations which describe the cooling of the sphere after the sources are
turned off, assuming that the heat exchange between the surface of the sphere
and the surrounding medium, during both the heating and cooling, obeys
Newton's Jaw.
Ans.

!_2 ~(r' iJT)

or

r iJr

(arar + hr)/

iJT
OT '

Tl ,_.
_ = 6k
Q (a 2

-

r 2)

=


o,

r=a

+ 3kh
Qa .

14. Two slabs of thicknesses a 1 and a 2 , made from different materials and
heated to temperatures T~ and Tg, are put into contact with each other at the
time t = 0. Write the equations governing the resulting process of temperature equalization, assuming that the free surfaces are thermally insulated from
the surrounding medium.
Ans.

a'T,

ox

2

iJT,
k, iJt

= c, p,

(0 <

X /

.


~

2

a )
1

'

iJ T2
iJx 2

=

c2 p2 iJT2
k, iJt

(a,

< x < a1 + a 2 ),

with initial conditions

5
It is recommended that the problem be solved directly from underlying physical
principles, without regarding Fourier's equation as known.

www.pdfgrip.com



PROB.

16

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

JJ

and boundary conditions
oTl

I

= 0,

OX a:=O

TlLo~a, = T2la:~a1'

15. A non uniformly heated body in the form of a circular ring of radius a
with a small cross section cools by giving off heat from its lateral surface.
Write the equations describing the corresponding process of temperature
equalization, assuming that the temperature drop inside the ring can be
neglected and that the surface cooling obeys Newton's law.

Ans.
kt

'"t" = - ,


cp

where p is the perimeter, S the cross-sectional area and h the heat exchange
coefficient. The temperature, which must be a periodic function of the angular
coordinate cp, satisfies the: initial condition
where f is a given function.

Tl-r=O= f(cp),

16. Show that the concentration C(x, y , z, t) of a substance diffusing in a
gas or liquid obeys the differential equation
/).C

= 1. ac
Dot

_Q
D'

where Q is the source density of the diffusing substance and D is the diffusion
coefficient.
Hint. Starting from Nernst's law q = - grad C (where the vector q is the
density of flow of the diffusing substance), write a conservation equation for
an arbitrary volume element.

3. Electricity and Magnetism
An important class of problems of mathematical physics involves integralion of the differential equations arising in various branches of electromagnetic
theory. Assuming that the reader has previously encountered this subject
(see G5, 16, PI), we shall regard the following basic equations as known:

1. The equations of electrostatics
flu = -

4

7tp,

E = -grad u,

<:

where u is the potential of the electrostatic field E, p = p(M) is the
volume density of charge at the point M, s: is the dielectric constant of
the medium, and !l is the Laplacian operator.

www.pdfgrip.com


12

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

2. The equations
/',.u

--~

~

--


,

j -- -crgradu,

(1)

for the distribution of d-e current density inside a homogeneous
conductor, where u is the potential of the current field, j js the current
density vector, Q ~ Q(M) is the volume density of current sources (in
particular, Q may vanish), and cr is the conductivity.

I

3. The equations
'A = - 4nf'
. l•·J,
-J

L..l.

H

c

I

~-curl

A


1-'

for the magnetic field due to d-e currents, where A is the vector potential of the magnetic Oeld H, the vector jl•l is the density of the (external)
currents producing the magnetic field, 1-' is the magnetic permeability
of the medium, c is the velocity of light in vacuum, and /',. is the
Laplacian operator. 6

4. Maxwell's equations
curl H ~ ~

aE

at

c

I 47tcr E

+ 4n i"\

c

c

~-'au

curiE~---,

c


at

divE~ 4"P
divH

~

0

•

for the electromagnetic fteld in a homogeneous isotropic medium,
where E and H arc the electric and magnetic field vectors, z, 1-' and "
arc the dielectric constant, the magnetic permeability and the conductivity of the medium, cis the velocity of light, and p and i''' are the I
charge and current densities producing the field.'
'
6 The components of the vector ~~A in a Cartesian coordinate system are flA,.., L\A ,
1
and flAz. To calculate the components of the vector flA in other coordinate systems, one
should use the relation
llA -----' grad divA
curl curl A.

I

Expressions for the components of llA in cylindrical and spherical coordinates are given
on P- 389 3907
It should be noted that if j!'- 1 is given, then p cannot be chosen arbitrarily, but mU'st
satisfy the difTerential equation


ap

4rrcr

--'-- o
Ot
e: '

=

-div)"(e)

implied by the first and third Maxwell equations.

www.pdfgrip.com


I'ROB.

18

DERIVATION OF EQUATIONS AND FORMULATION OF PROBLEMS

13

If we use the relations

1
1 aA

H = ..:.curl A,
E = - grad u - - c
fl.
8
to introduce the vector and scala r potentials A and u, the problem of determining the electromagnetic field reduces to integrating the system of equations

at

2

f).A - Efl. a A - 47tcrfL aA = - 47tfL j<e>,

at
Efl. au _
at

c
/).u _

2

2

2

C

2

2


at

c2

47tfl.CJ au

at

2
C

c

= _

4np
E

We now con sider the mathe matica l fo rmulation of various problems
invo lving electric and magnetic fields (both static and variable), as well as
some problems on transformation s of the d ifferential equations of electrodynamics which are useful in special cases.
17. Formulate mathematically the problem of finding the three-dimensional electrostatic field between N conducto rs of arbitrary shape at given
potentials Vi (i = I, .. . , N) .

Ans. In the region D bounded by the surfaces Si (i
conductors, the potential u satisfies Laplace's equation

= 1, . .. , N)


o f the

0.

/).u =

The boundary conditions have the form
i

= J, . .. , N,

where, in the case where the point at infinity belongs to D, these conditions
'"'!.ust be supplemented by the requirement that a t infinity the potential u
approach zero uniformly in all directions.

Comment. If none of the s urfaces S; extends to infinity, then the products
Ru and R 2 grad u (where R 2 = x 2
y 2 z 2) remain uniformly bounded as
R-->- oo. However, these conditions need not be included in the fo rmu lation
of the problem, since the uniqueness of the solution is guaranteed by the
above requirement th at the potential u a pproach zero uniformly as R-->- oo.

+ +

18. A charge Q is placed at the point M 0 = (x 0 , y 0 , z0) near a conductor at
potential V, bounded by a surface S. Formula te the corresponding pro blem
of electrostatics.
8

The quantities A and u a re no t independent, but are connected by the relation

.
dJ V

A

ou + 41ti.L
+ E:!J.
II = 0.
c ot
c

www.pdfgrip.com