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Solutions to Problems in Jackson, Classical Electrodynamics, Third Edition Homer Reid December 8, 1999

Chapter 2 Problem 2.1 A point charge q is brought to a position a distance d away from an infinite plane conductor held at zero potential. Using the method of images, find: (a) the surface-charge density induced on the plane, and plot it; (b) the force between the plane and the charge by using Coulomb’s law for the force between the charge and its image; (c) the total force acting on the plane by integrating σ 2 /20 over the whole plane; (d) the work necessary to remove the charge q from its position to infinity; (e) the potential energy between the charge q and its image (compare the answer to part d and discuss). (f ) Find the answer to part d in electron volts for an electron originally one angstrom from the surface.

(a) We’ll take d to be in the z direction, so the charge q is at (x, y, z) = (0, 0, d). The image charge is −q at (0, 0, −d). The potential at a point r is   1 1 q − Φ(r) = 4π0 |r − dk| |r + dk| The surface charge induced on the plane is found by differentiating this: 1

Homer Reid’s Solutions to Jackson Problems: Chapter 2

σ

dΦ z=0 dz   (z + d) q −(z − d) + = − 4π |r + dk|3 |r + dk|3 z=0 qd = − 2π(x2 + y 2 + d2 )3/2

2

= −0

(1)

We can check this by integrating this over the entire xy plane and verifying that the total charge is just the value −q of the image charge: Z

∞ −∞

Z

∞

σ(x, y)dxdy −∞

Z Z qd ∞ 2π rdψdr 2 + d2 )3/2 2π 0 (r 0 Z ∞ rdr −qd 2 + d2 )3/2 (r Z0 qd ∞ −3/2 − u du 2 d2 ∞ qd − −2u−1/2 2 2 d √ −q

= − = = = =

(b) The point of this problem is that, for points above the z axis, it doesn’t matter whether there is a charge −q at (0, 0, d) or an infinite grounded sheet at z = 0. Physics above the z axis is exactly the same whether we have the charge or the sheet. In particular, the force on the original charge is the same whether we have the charge or the sheet. That means that, if we assume the sheet is present instead of the charge, it will feel a reaction force equal to what the image charge would feel if it were present instead of the sheet. The force on the image charge would be just F = q 2 /16π0d2 , so this must be what the sheet feels. (c) Total force on sheet

= = = = =

Z ∞ Z 2π 1 σ 2 dA 20 0 0 Z rdr q 2 d2 ∞ 2 4π0 0 (r + d2 )3 Z q 2 d2 ∞ −3 u du 8π0 d2 ∞ q 2 d2 1 −2 − u 8π0 2 2  d  q 2 d2 1 −4 d 8π0 2

Homer Reid’s Solutions to Jackson Problems: Chapter 2

=

q2 16π0 d2

in accordance with the discussion and result of part b. (d) Work required to remove charge to infinity

= = = =

Z ∞ q2 dz 4π0 d (z + d)2 Z ∞ q2 u−2 du 4π0 2d q2 1 4π0 2d q2 8π0 d

(e) Potential energy between charge and its image =

q2 8π0 d

equal to the result in part d. (f ) q2 8π0 d

=

(1.6 · 10−19 coulombs )2 8π(8.85 · 10−12 coulombs V−1 m−1 )(10−10 m )

= 7.2 · (1.6 · 10−19 coulombs · 1 V ) = 7.2 eV .

Problem 2.2 Using the method of images, discuss the problem of a point charge q inside a hollow, grounded, conducting sphere of inner radius a. Find (a) the potential inside the sphere; (b) the induced surface-charge density; (c) the magnitude and direction of the force acting on q. (d) Is there any change in the solution if the sphere is kept at a fixed potential V ? If the sphere has a total charge Q on its inner and outer surfaces?

3

4

Homer Reid’s Solutions to Jackson Problems: Chapter 2

Problem 2.3 A straight-line charge with constant linear charge density λ is located perpendicular to the x − y plane in the first quadrant at (x0 , y0 ). The intersecting planes x = 0, y ≥ 0 and y = 0, x ≥ 0 are conducting boundary surfaces held at zero potential. Consider the potential, fields, and surface charges in the first quadrant. (a) The well-known potential for an isolated line charge at (x0 , y0 ) is Φ(x, y) = (λ/4π0 ) ln(R2 /r2 ), where r2 = (x − x0 )2 + (y − y0 )2 and R is a constant. Determine the expression for the potential of the line charge in the presence of the intersecting planes. Verify explicitly that the potential and the tangential electric field vanish on the boundary surface. (b) Determine the surface charge density σ on the plane y = 0, x ≥ 0. Plot σ/λ versus x for (x0 = 2, y0 = 1), (x0 = 1, y0 = 1), and (x0 = 1, y0 = 2). (c) Show that the total charge (per unit length in z) on the plane y = 0, x ≥ 0 is   2 x0 Qx = − λ tan−1 π y0 What is the total charge on the plane x = 0? (d) Show p that far from the origin [ρ  ρ0 , where ρ = x20 + y02 ] the leading term in the potential is Φ → Φasym =

p x2 + y 2 and ρ0 =

4λ (x0 )(y0 )(xy) . π0 ρ4

Interpret.

(a) The potential can be made to vanish on the specified boundary surfaces by pretending that we have three image line charges. Two image charges have charge density −λ and exist at the locations obtained by reflecting the original image charge across the x and y axes, respectively. The third image charge has charge density +λ and exists at the location obtained by reflecting the original charge through the origin. The resulting potential in the first quadrant is Φ(x, y)

= =

  λ R2 R2 R2 R2 ln 2 − ln 2 − ln 2 + ln 2 4π0 r1 r2 r3 r4 r2 r3 λ ln 2π0 r1 r4

where r12 = [(x − x0 )2 + (y − y0 )2 ]

r22 = [(x + x0 )2 + (y − y0 )2 ]

(2)

Homer Reid’s Solutions to Jackson Problems: Chapter 2 r32 = [(x − x0 )2 + (y + y0 )2 ]

5

r42 = [(x + x0 )2 + (y + y0 )2 ].

From this you can see that

• when x = 0, r1 = r2 and r3 = r4 • when y = 0, r1 = r3 and r2 = r4 and in both cases the argument of the logarithm in (2) is unity. (b) σ

d Φ dy   1 dr2 1 dr3 1 dr1 1 dr4 λ + − − = − y=0 2π r2 dy r3 dy r1 dy r4 dy

= −0

We have dr1 /dy = (y − y0 )/r1 and similarly for the other derivatives, so   λ y − y0 y + y0 y − y0 y + y0 σ = − + − − y=0 2π r2 r32 r12 r42  2  1 1 y0 λ − = − π (x − x0 )2 + y02 (x + x0 )2 + y02 ) (c) Total charge per unit length in z Z ∞ σdx Qx = 0 Z ∞  Z ∞ y0 λ dx dx = − − π (x − x0 )2 + y02 (x + x0 )2 + y02 0 0 For the first integral the appropriate substitution is (x − x0 ) = y0 tan u, dx = y0 sec2 udu. A similar substitution works in the second integral. # "Z Z π/2 π/2 λ du − du = − x π tan−1 − xy0 tan−1 y 0 0 0   λ π π −1 −x0 −1 x0 = − − tan − + tan π 2 y0 2 y0 2λ x 0 = − tan−1 . (3) π y0 The calculations are obviously symmetric with respect to x0 and y0 . The total charge on the plane x = 0 is (3) with x0 and y0 interchanged: Qy = −

2λ y0 tan−1 π x0

Since tan−1 x − tan−1 (1/x) = π/2 the total charge induced is Q = −λ

Homer Reid’s Solutions to Jackson Problems: Chapter 2

6

which is, of course, also the sum of the charge per unit length of the three image charges. (d) We have Φ=

λ r2 r2 ln 22 32 4π0 r1 r4

Far from the origin, r12

= [(x − x0 )2 + (y − y0 )2 ]   x0 2 y0 2 2 2 = x (1 − ) + y (1 − ) x y   y0 x 0 2 2 ≈ x (1 − 2 ) + y (1 − 2 x y  2  = x − 2x0 x + y 2 − 2y0 y)   xx0 + yy0 2 2 = (x + y ) 1 − 2 2 x + y2

Similarly, r22 r32 r42



 −xx0 + yy0 = (x + y ) 1 − 2 x2 + y 2   xx0 − yy0 2 2 = (x + y ) 1 − 2 2 x + y2   −xx0 − yy0 2 2 = (x + y ) 1 − 2 x2 + y 2 2

2

Next, r12 r42 r22 r32 so

 (xx0 + yy0 )2 = (x + y ) 1 − 4 (x2 + y 2 )2   (xx0 − yy0 )2 2 2 2 = (x + y ) 1 − 4 (x2 + y 2 )2 2

2 2



  2 0 −yy0 ) 1 − 4 (xx 2 2 2 λ (x +y ) . Φ= ln  2 0 +yy0 ) 4π0 1 − 4 (xx (x2 +y 2 )2

The (x2 + y 2 ) term in the denominator grows much more quickly than the (xx0 + yy0 ) term, so in the asymptotic limit we can use ln(1 + ) ≈  to find   (xx0 − yy0 )2 λ (xx0 + yy0 )2 −4 Φ = + 4 4π0 (x2 + y 2 )2 (x2 + y 2 )2   λ −4(x2 x20 + y 2 y02 − 2xyx0 y0 ) + 4(x2 x20 + y 2 y02 + 2xyx0 y0 ) = 4π0 (x2 + y 2 )2

Homer Reid’s Solutions to Jackson Problems: Chapter 2

= =

7

  λ 16xyx0 y0 4π0 (x2 + y 2 )2 √ 4λ (xy)(x0 y0 ) . π0 (x2 + y 2 )2

Problem 2.4 A point charge is placed a distance d > R from the center of an equally charged, isolated, conducting sphere of radius R. (a) Inside of what distance from the surface of the sphere is the point charge attracted rather than repelled by the charged sphere? (b) What is the limiting value of the force of attraction when the point charge is located a distance a(= d−R) from the surface of the sphere, if a  R? (c) What are the results for parts a and b if the charge on the sphere is twice (half) as large as the point charge, but still the same sign?

Let’s call the point charge q. The charged, isolated sphere may be replaced by two image charges. One image charge, of charge q1 = −(R/d)q at radius r1 = R2 /d, is needed to make the potential equal at all points on the sphere. The second image charge, of charge q2 = q − q1 at the center of the sphere, is necessary to recreate the effect of the additional charge on the sphere (the “additional” charge is the extra charge on the sphere left over after you subtract the surface charge density induced by the point charge q). The force on the point charge is the sum of the forces from the two image charges:

F

"

#

=

1 4π0

=

  q2 −dR d2 + dR + 4π0 [d2 − R2 ]2 d4

qq1

qq2   + d2 R2 2 d− d

(4) (5)

As d → R the denominator of the first term vanishes, so that term wins, and the overall force is attractive. As d → ∞, the denominator of both terms looks like d4 , so the dR terms in the numerator cancel and the overall force is repulsive. (a) The crossover distance is found by equating the two bracketed terms in (5):

8

Homer Reid’s Solutions to Jackson Problems: Chapter 2

[d2

dR − R 2 ]2

=

d2 + dR d4

d4 R = (d + R)[d2 − R2 ]2 0 = d5 − 2d3 R2 − 2d2 R3 + dR4 + R5

I used GnuPlot to solve this one graphically. The root is d/R=1.6178. (b) The idea here is to set d = R + a = R(1 + a/R) and find the limit of (4) as a → 0. F

= ≈

"

 a 2 R2 (1 + R ) + (1 + +   a 4 2 4 a ) R (1 + R R2 (1 + R )2 − R2   2 2 −R − aR (2R + 3a)(R − 4a) q + 4π0 4a2 R2 R4 q2 4π0

−R2 (1 +

a R)

a R)

#

The second term in brackets approaches the constant 2/R 2 as a → 0. The first term becomes −1/4a2. So we have F →−

q2 . 16π0 a2

Note that only the first image charge (the one required to make the sphere an equipotential) contributes to the force as d → a. The second image charge, the one which represents the difference between the actual charge on the sphere and the charge induced by the first image, makes no contribution in this limit. That means that the limiting value of the force will be as above regardless of the charge on the sphere. (c) If the charge on the sphere is twice the point charge, then q2 = 2q − q1 = q(2 + R/d). Then (5) becomes   dR 2d2 + dR q2 − 2 + F = 4π0 [d − R2 ]2 d4 and the relevant equation becomes 0 = 2d5 − 4d3 R2 − 2d2 R3 + 2dR4 + R5 . Again I solved graphically to find d/R = 1.43. If the charge on the sphere is half the point charge, then   dR d2 + 2dR q2 − 2 + F = 4π0 [d − R2 ]2 2d4 and the equation is 0 = d5 − 2d3 R2 − 4d2 R3 + dR4 + 2R5 . The root of this one is d/R=1.88.

Homer Reid’s Solutions to Jackson Problems: Chapter 2

9

Problem 2.5 (a) Show that the work done to remove the charge q from a distance r > a to infinity against the force, Eq. (2.6), of a grounded conducting sphere is W =

q2 a . 8π0 (r2 − a2 )

Relate this result to the electrostatic potential, Eq. (2.3), and the energy discussion of Section 1.11. (b) Repeat the calculation of the work done to remove the charge q against the force, Eq. (2.9), of an isolated charged conducting sphere. Show that the work done is   q2 a q 2 a qQ 1 . − − W = 4π0 2(r2 − a2 ) 2r2 r Relate the work to the electrostatic potential, Eq. (2.8), and the energy discussion of Section 1.11.

(a) The force is |F | =

q2 a 1 3 4π0 y (1 − a2 /y 2 )2

directed radially inward. The work is Z ∞ W = − F dy r Z ∞ dy q2 a = 4π0 r y 3 (1 − a2 /y 2 )2 Z ∞ ydy q2 a = 4π0 r (y 2 − a2 )2 Z ∞ q2 a du = 4π0 r2 −a2 2u2 ∞ q 2 a 1 − = 4π0 2u r2 −a2 =

q2 a 8π0 (r2 − a2 )

(6)

(7)

To relate this to earlier results, note that the image charge q 0 = −(a/r)q is located at radius r 0 = a2 /r. The potential energy between the point charge and

Homer Reid’s Solutions to Jackson Problems: Chapter 2

10

its image is PE

= = =

  1 qq 0 4π0 |r − r0 |   −q 2 a 1 4π0 r(r − a2 /r)   −q 2 a 1 4π0 r2 − a2

(8)

Result (7) is only half of (8). This would seem to violate energy conservation. It would seem that we could start with the point charge at infinity and allow it to fall in to a distance r from the sphere, liberating a quantity of energy (8), which we could store in a battery or something. Then we could expend an energy equal to (7) to remove the charge back to infinity, at which point we would be back where we started, but we would still have half of the energy saved in the battery. It would seem that we could keep doing this over and over again, storing up as much energy in the battery as we pleased. I think the problem is with equation (8). The traditional expression q1 q2 /4π0 r for the potential energy of two charges comes from calculating the work needed to bring one charge from infinity to a distance r from the other charge, and it is assumed that the other charge does not move and keeps a constant charge during the process. But in this case one of the charges is a fictitious image charge, and as the point charge q is brought in from infinity the image charge moves out from the center of the sphere, and its charge increases. So the simple expression doesn’t work to calculate the potential energy of the configuration, and we should take (7) to be the correct result. (b) In this case there are two image charges: one of the same charge and location as in part a, and another of charge Q − q 0 at the origin. The work needed to remove the point charge q to infinity is the work needed to remove the point charge from its image charge, plus the work needed to remove the point charge from the extra charge at the origin. We calculated the first contribution above. The second contribution is  Z ∞ Z ∞ q(Q − q 0 )dy 1 qQ q 2 a − dy = − + 4π0 y 2 4π0 r y2 y3 r ∞ 1 qQ q 2 a = − − − 2 4π0 y 2y r   2 1 qQ q a = − + 2 4π0 r 2r so the total work done is W =

  1 q2 a q 2 a qQ . − − 4π0 2(r2 − a2 ) 2r2 r

11

Homer Reid’s Solutions to Jackson Problems: Chapter 2

Review of Green’s Functions Some problems in this and other chapters use the Green’s function technique. It’s useful to review this technique, and also to establish my conventions since I define the Green’s function a little differently than Jackson. The whole technique is based on the divergence theorem. Suppose A(x) is a vector valued function defined at each point x within a volume V . Then Z I 0 0 (∇ · A(x )) dV = A(x0 ) · dA0 (9) V

S

where S is the (closed) surface bounding the volume V . If we take A(x) = φ(x)∇ψ(x) where φ and ψ are scalar functions, (9) becomes Z

V

  (∇φ(x0 )) · (∇ψ(x0 )) + φ(x0 )∇2 ψ(x0 ) dV 0 =

I

∂ψ φ(x ) dA0 ∂n x0 S 0

~ with the outward normal to the surface where ∂ψ/∂n is the dot product of ∇ψ area element. If we write down this equation with φ and ψ switched and subtract the two, we come up with  I  Z  2  ∂ψ ∂φ dA0 . (10) φ φ∇ ψ − ψ∇2 φ dV 0 = −ψ ∂n ∂n S V

This statement doesn’t appear to be very useful, since it seems to require that we know φ over the whole volume to compute the left side, and both φ and ∂φ/∂n on the boundary to compute the right side. However, suppose we could choose ψ(x) in a clever way such that ∇2 ψ = δ(x − x0 ) for some point x0 within the volume. (Since this ψ is a function of x which also depends on x0 as a parameter, we might write it as ψx0 (x).) Then we could use the sifting property of the delta function to find  Z I    0 0 ∂φ 0 ∂ψx0 0 2 0 dA0 . − ψx0 (x ) φ(x0 ) = φ(x ) ψx0 (x )∇ φ(x ) dV + ∂n x0 ∂n x0 V S

If φ is the scalar potential of electrostatics, we know that ∇2 ψ(x0 ) = −ρ(x0 )/0 , so we have  I  Z 1 0 0 0 0 ∂ψx0 0 ∂φ 0 φ(x0 ) = − φ(x ) ψx0 (x )ρ(x )dV + 0 − ψx0 (x ) ∂n 0 dA . 0 V ∂n S x x (11) Equation (11) allows us to find the potential at an arbitrary point x0 as long as we know ρ within the volume and both φ and ∂φ/∂n on the boundary. boundary. Usually we do know ρ within the volume, but we only know either φ or ∂φ/∂n on the boundary. This lack of knowledge can be accommodated by choosing ψ such that either its value or its normal derivative vanishes on the boundary surface, so that the term which we can’t evaluate drops out of the surface integral. More specifically,

Homer Reid’s Solutions to Jackson Problems: Chapter 2

12

• if we know φ but not ∂φ/∂n on the boundary (“Dirichlet” boundary conditions), we choose ψ such that ψ = 0 on the boundary. Then Z I 1 0 0 0 0 0 ∂ψx0 φ(x0 ) = − (12) ψx0 (x )ρ(x )dV + φ(x ) 0 dA . 0 V ∂n S x

• if we know ∂φ/∂n but not φ on the boundary (“Neumann” boundary conditions), we choose ψ such that ∂ψ/∂n = 0 on the boundary. Then I Z 1 0 0 ∂φ 0 0 0 φ(x0 ) = − (13) φx0 (x ) ψx0 (x )ρ(x )dV + 0 dA . 0 V ∂n S x

Again, in both cases the function ψx0 (x) has the property that ∇2 ψx0 (x) = δ(x − x0 ).

Solutions to Problems in Jackson, Classical Electrodynamics, Third Edition Homer Reid December 8, 1999

Chapter 2: Problems 11-20 Problem 2.11 A line charge with linear charge density τ is placed parallel to, and a distance R away from, the axis of a conducting cylinder of radius b held at fixed voltage such that the potential vanishes at infinity. Find (a) the magnitude and position of the image charge(s); (b) the potential at any point (expressed in polar coordinates with the origin at the axis of the cylinder and the direction from the origin to the line charge as the x axis), including the asymptotic form far from the cylinder; (c) the induced surface-charge density, and plot it as a function of angle for R/b=2,4 in units of τ /2πb; (d) the force on the charge.

(a) Drawing an analogy to the similar problem of the point charge outside the conducting sphere, we might expect that the potential on the cylinder can be made constant by placing an image charge within the cylinder on the line conducting the line charge with the center of the cylinder, i.e. on the x axis. Suppose we put the image charge a distance R0 < b from the center of the cylinder and give it a charge density −τ . Using the expression quoted in Problem 2.3 for the potential of a line charge, the potential at a point x due to the line charge and its image is Φ(x)

=

τ R2 R2 τ ln ln − 4π0 |x − Rˆi|2 4π0 |x − R0ˆi|2 1

Homer Reid’s Solutions to Jackson Problems: Chapter 2

2

τ |x − R0ˆi|2 ln . 4π0 |x − Rˆi|2

=

We want to choose R0 such that the potential is constant when x is on the cylinder surface. This requires that the argument of the logarithm be equal to some constant γ at those points: |x − R0ˆi|2 =γ |x − Rˆi|2

or

b2 + R02 − 2R0 b cos φ = γb2 + γR2 − 2γRb cos φ. For this to be true everywhere on the cylinder, the φ term must drop out, which requires R0 = γR. We can then rearrange the remaining terms to find R0 =

b2 . R

This is also analogous to the point-charge-and-sphere problem, but there are differences: in this case the image charge has the same magnitude as the original line charge, and the potential on the cylinder is constant but not zero. (b) At a point (ρ, φ), we have Φ=

τ ρ2 + R02 − 2ρR0 cos φ ln 2 . 4π0 ρ + R2 − 2ρR cos φ

For large ρ, this becomes 0

Φ→

1 − 2 Rρ cos φ τ ln . 4π0 1 − 2R ρ cos φ

Using ln(1 − x) = −(x + x2 /2 + · · ·), we have Φ → =

τ 2(R − R0 ) cos φ 4π0 ρ τ R(1 − b2 /R2 ) cos φ 2π0 ρ

(c) σ

∂Φ = −0 ∂ρ r=b   2b − 2R cos φ τ 2b − 2R0 cos φ − = − 4π b2 + R02 − 2bR0 cos φ b2 + R2 − 2bR cos φ " # 2 b − bR cos φ b − R cos φ τ = − − 2π b2 + Rb42 − 2 bR3 cos φ b2 + R2 − 2bR cos φ

Homer Reid’s Solutions to Jackson Problems: Chapter 2

3

Multiplying the first term by R2 /b2 on top and bottom yields " # R2 τ b −b σ = − 2π R2 + b2 − 2bR cos φ   R 2 − b2 τ = − 2πb R2 + b2 − 2bR cos φ (d) To find the force on the charge, we note that the potential of the image charge is τ C2 . Φ(x) = − ln 4π0 |x − R0 ˆi|2

with C some constant. We can differentiate this to find the electric field due to the image charge: E(x)

τ ∇ ln |x − R0 ˆi|2 4π0 τ 2(x − R0 ˆi) = − . 4π0 |x − R0ˆi|2

= −∇Φ(x) = −

The original line charge is at x = R, y = 0, and the field there is E=−

1 ˆ τ R ˆ τ i=− i. 2π0 R − R0 2π0 R2 − b2

The force per unit width on the line charge is F = τE = −

τ2 R 2π0 R2 − b2

tending to pull the original charge in toward the cylinder.

Problem 2.12 Starting with the series solution (2.71) for the two-dimensional potential problem with the potential specified on the surface of a cylinder of radius b, evaluate the coefficients formally, substitute them into the series, and sum it to obtain the potential inside the cylinder in the form of Poisson’s integral: Z 2π 1 b2 − ρ 2 Φ(ρ, φ) = Φ(b, φ0 ) 2 dφ0 2π 0 b + ρ2 − 2bρ cos(φ0 − φ) What modification is necessary if the potential is desired in the region of space bounded by the cylinder and infinity?

Homer Reid’s Solutions to Jackson Problems: Chapter 2

4

Referring to equation (2.71), we know the bn are all zero, because the ln term and the negative powers of ρ are singular at the origin. We are left with Φ(ρ, φ) = a0 +

∞ X

n=1

ρn {an sin(nφ) + bn cos(nφ)} .

(1)

Multiplying both sides successively by 1, sin n0 φ, and cos n0 φ and integrating at ρ = b gives Z 2π 1 Φ(b, φ)dφ (2) a0 = 2π 0 Z 2π 1 an = Φ(b, φ) sin(nφ)dφ (3) πbn 0 Z 2π 1 bn = Φ(b, φ) cos(nφ)dφ. (4) πbn 0 Plugging back into (1), we find ) ( Z ∞ 1 2π 1 X  ρ n 0 0 0 Φ(ρ, φ) = [sin(nφ) sin(nφ ) + cos(nφ) cos(nφ )] dφ0 + Φ(b, φ ) π 0 2 n=1 b ) ( Z ∞ 1 2π 1 X  ρ n 0 0 = cos n(φ − φ ) . (5) + Φ(b, φ ) π 0 2 n=1 b The bracketed term can be expressed in closed form. For simplicity define x = (ρ/b) and α = (φ − φ0 ). Then ∞

1 X n + x cos(nα) 2 n=1

= = = = = =

∞  1 1 X  n inα + x e + xn e−inα 2 2 n=1   1 1 1 1 + + − 2 2 2 1 − xeiα 1 − xe−iα   1 1 1 − xe−iα − xeiα + 1 + − 2 2 2 1 − xeiα − xe−iα + x2   1 1 − x cos α + − 1 2 1 + x2 − 2x cos α 1 x cos α − x2 + 2 1 + x2 − 2x cos α   1 1 − x2 . 2 1 + x2 − 2x cos α

Plugging this back into (5) gives the advertised result.

Homer Reid’s Solutions to Jackson Problems: Chapter 2

5

Problem 2.13 (a) Two halves of a long hollow conducting cylinder of inner radius b are separated by small lengthwise gaps on each side, and are kept at different potentials V1 and V2 . Show that the potential inside is given by   V1 − V 2 2bρ V1 + V 2 −1 + tan cos φ Φ(ρ, φ) = 2 π b2 − ρ 2 where φ is measured from a plane perpendicular to the plane through the gap. (b) Calculate the surface-charge density on each half of the cylinder. This problem is just like the previous one. Since we are looking for an expression for the potential within the cylinder, the correct expansion is (1) with expansion coefficients given by (2), (3) and (4):

a0

= = =

an

= = = =

bn

= = =

1 2π

Z

2π

Φ(b, φ)dφ 0

Z 2π  π 1 V1 dφ + V2 dφ 2π 0 π V1 + V 2 2   Z π Z 2π 1 sin(nφ)dφ sin(nφ)dφ + V V 2 1 πbn π 0 h i 1 π 2π − V |cos nφ| + V |cos nφ| 1 2 0 π nπbn 1 − [V1 (cos nπ − 1) + V2 (1 − cos nπ)] n   nπb 0 , n even 2(V1 − V2 )/(nπbn ) , n odd   Z π Z 2π 1 cos(nφ)dφ cos(nφ)dφ + V V 2 1 πbn π 0 h i 1 π 2π V |sin nφ| + V |sin nφ| 1 2 0 π nπbn 0. 

Z

With these coefficients, the potential expansion becomes Φ(ρ, φ) =

V1 + V 2 2(V1 − V2 ) X 1  ρ n sin nφ. + 2 π n b n odd

(6)

6

Homer Reid’s Solutions to Jackson Problems: Chapter 2

Here we need an auxiliary result: X 1 xn sin nφ = n n odd = =

1 X 1 (x = iy) (iy)n [einπ − e−inφ ] 2i n n odd ∞  1 X (−1)n  iφ 2n+1 (ye ) − (ye−iφ )2n+1 2 n=0 2n + 1  1  −1 iφ tan (ye ) − tan−1 (ye−iφ ) 2

(7)

where in the last line we just identified the Taylor series for the inverse tangent function. Next we need an identity:   γ1 − γ 2 −1 −1 −1 tan γ1 − tan γ2 = tan . 1 + γ 1 γ2 (I derived this one by drawing some triangles and doing some algebra.) With this, (7) becomes   X 1 1 2iy sin φ xn sin nφ = tan−1 n 2 1 + y2 n odd   2x sin φ 1 −1 tan . = 2 1 − x2 Using this in (6) with x = ρ/b gives V1 − V 2 V1 + V 2 + tan−1 Φ(ρ, b) = π π



2ρb sin φ b2 − ρ 2

(Evidently, Jackson and I defined the angle φ differently).



.

Homer Reid’s Solutions to Jackson Problems: Chapter 2

7

Problem 2.15 (a) Show that the Green function G(x, y; x0 , y 0 ) appropriate for Dirichlet boundary conditions for a square two-dimensional region, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, has an expansion G(x, y; x0 , y 0 ) = 2

∞ X

gn (y, y 0 ) sin(nπx) sin(nπx0 )

n=1

where gn (y, y 0 ) satisfies   2 ∂ 2 2 − n π gn (y, y 0 ) = δ(y 0 − y) ∂y 2

and gn (y, 0) = gn (y, 1) = 0.

(b) Taking for gn (y, y 0 ) appropriate linear combinations of sinh(nπy 0 ) and cosh(nπy 0 ) in the two regions y 0 < y and y 0 > y, in accord with the boundary conditions and the discontinuity in slope required by the source delta function, show that the explicit form of G is G(x, y; x0 , y 0 ) = ∞ X 1 −2 sin(nπx) sin(nπx0 ) sinh(nπy< ) sinh[nπ(1 − y> )] nπ sinh(nπ) n=1 where y< (y> ) is the smaller (larger) of y and y 0 . (I have taken out a factor −4π from the expressions for gn and G, in accordance with my convention for Green’s functions; see the Green’s functions review above.) (a) To use as a Green’s function in a Dirichlet boundary value problem G must satisfy two conditions. The first is that G vanish on the boundary of the region of interest. The suggested expansion of G clearly satisfies this. First, sin(nπx0 ) is 0 when x0 is 0 or 1. Second, g(y, y 0 ) vanishes when y 0 is 0 or 1. So G(x, y; x0 , y 0 ) vanishes for points (x0 , y 0 ) on the boundary. The second condition on G is  2  ∂ ∂2 ∇2 G = G = δ(x − x0 ) δ(y − y 0 ). (8) + ∂x02 ∂y 02 With the suggested expansion, we have ∞

X   ∂2 G = 2 gn (y, y 0 ) sin(nπx) −n2 π 2 sin(nπx0 ) 02 ∂x n=1 ∞

X ∂ 02 ∂2 G = 2 g (y, y 0 ) sin(nπx) sin(nπx0 ) 02 2 n ∂y ∂y n=1

8

Homer Reid’s Solutions to Jackson Problems: Chapter 2

We can add these together and use the differential equation satisfied by gn to find ∇2 G = δ(y − y 0 ) · 2 0

∞ X

sin(nπx) sin(nπx0 )

n=1

= δ(y − y ) · δ(x − x0 ) since the infinite sum is just a well-known representation of the δ function. (b) The suggestion is to take  An1 sinh(nπy 0 ) + Bn1 cosh(nπy 0 ), gn (y, y 0 ) = An2 sinh(nπy 0 ) + Bn2 cosh(nπy 0 ),

y 0 < y; y 0 > y.

(9)

The idea to use hyperbolic sines and cosines comes from the fact that sinh(nπy) and cosh(nπy) satisfy a homogeneous version of the differential equation for g n (i.e. satisfy that differential equation with the δ function replaced by zero). Thus gn as defined in (9) satisfies its differential equation (at all points except y = y 0 ) for any choice of the As and Bs. This leaves us free to choose these coefficients as required to satisfy the boundary conditions and the differential equation at y = y 0 . First let’s consider the boundary conditions. Since y is somewhere between 0 and 1, the condition that gn vanish for y 0 = 0 is only relevant to the top line of (9), where it requires taking Bn1 = 0 but leaves An1 undetermined for now. The condition that gn vanish for y 0 = 1 only affects the lower line of (9), where it requires that 0 = An2 sinh(nπ) + Bn2 cosh(nπ) = (An2 + Bn2 )enπ + (−An2 + Bn2 )e−nπ

(10)

One way to make this work is to take An2 + Bn2 = −e−nπ

and

Bn2 = enπ + An2

→

− An2 + Bn2 = enπ .

Then

so An2 = − cosh(nπ)

and

2An2 = −enπ − e−nπ

Bn2 = sinh(nπ).

With this choice of coefficients, the lower line in (9) becomes gn (y, y 0 ) = − cosh(nπ) sinh(nπy 0 )+sinh(nπ) cosh(nπy 0 ) = sinh[nπ(1−y 0 )] (11) for (y 0 > y). Actually, we haven’t completely determined An2 and Bn2 ; we could multiply (11) by an arbitrary constant γn and (10) would still be satisfied. Next we need to make sure that the two halves of (9) match up at y 0 = y: An1 sinh(nπy) = γn sinh[nπ(1 − y)].

(12)

9

Homer Reid’s Solutions to Jackson Problems: Chapter 2 70000

60000

g(yprime)

50000

40000

30000

20000

10000

0 0

0.2

0.4

0.6

0.8

1

yprime

Figure 1: gn (y, y 0 ) from Problem 2.15 with n=5, y=.41

This obviously happens when An1 = βn sinh[nπ(1 − y)] and γn = βn sinh(nπy) where βn is any constant. In other words, we have  βn sinh[nπ(1 − y)] sinh(nπy 0 ), gn (y, y 0 ) = βn sinh[nπ(1 − y 0 )] sinh(nπy), = βn sinh[nπ(1 − y> )] sinh(nπy< )

y 0 < y; y 0 > y. (13)

with y< and y> defined as in the problem. Figure 1 shows a graph of this function n = 5, y = .41. The final step is to choose the normalization constant βn such that gn satisfies its differential equation:   2 ∂ 2 2 gn (y, y 0 ) = δ(y − y 0 ). (14) − n π ∂ 2 y 02 To say that the left-hand side “equals” the delta function requires two things: • that the left-hand side vanish at all points y 0 6= y, and • that its integral over any interval (y1 , y2 ) equal 1 if the interval contains the point y 0 = y, and vanish otherwise. The first condition is clearly satisfied regardless of the choice of βn . The second condition may be satisfied by making gn continuous, which we have already done, but giving its first derivative a finite jump of unit magnitude at y 0 = y:

Homer Reid’s Solutions to Jackson Problems: Chapter 2

10

y0 =y+ ∂ 0 g (y, y ) n 0 − = 1. ∂y 0 y =y

Differentiating (13), we find this condition to require nπβn [− cosh[nπ(1 − y)] sinh(nπy) − sinh[nπ(1 − y)] cosh(nπy)] = −nπβn sinh(nπ) = 1 so (14) is satisfied if βn = −

1 . nπ sinh(nπ)

Then (13) is gn (y, y 0 ) = −

sinh[nπ(1 − y> )] sinh(nπy< ) nπ sinh(nπ)

and the composite Green’s function is G(x, y; x0 , y 0 )

= 2

∞ X

gn (y, y 0 ) sin(nπx) sin(nπx0 )

n=1 ∞ X

= −2

sinh[nπ(1 − y> )] sinh(nπy< ) sin(nπx) sin(nπx0 ) (15) . nπ sinh(nπ) n=1

Problem 2.16 A two-dimensional potential exists on a unit square area (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) bounded by “surfaces” held at zero potential. Over the entire square there is a uniform charge density of unit strength (per unit length in z). Using the Green function of Problem 2.15, show that the solution can be written as   ∞ 4 X sin[(2m + 1)πx] cosh[(2m + 1)π(y − (1/2))] Φ(x, y) = 3 . 1− π 0 m=0 (2m + 1)3 cosh[(2m + 1)π/2] Referring to my Green’s functions review above, the potential at a point x0 within the square is given by  Z I  1 ∂G 0 0 ∂Φ Φ(x0 ) = − − G(x ; x ) G(x0 ; x0 )ρ(x0 )dV 0 + Φ(x0 ) 0 0 dA . 0 V ∂n ∂n 0 S x x (16) In this case the surface integral vanishes, because we’re given that Φ vanishes on the boundary, and G vanishes there by construction. We’re also given that

11

Homer Reid’s Solutions to Jackson Problems: Chapter 2

ρ(x0 )dV 0 = dx0 dy 0 throughout the entire volume. Then we can plug in (15) to find Z 1Z 1 ∞ 2 X 1 Φ(x0 ) = sinh[nπ(1−y> )] sinh(nπy< ) sin(nπx0 ) sin(nπx0 )dx0 dy 0 . π0 n=1 n sinh(nπ) 0 0 (17) The integrals can be done separately. The x integral is sin(nπx0 )

Z

1

sin(nπx0 )dx0 0

sin(nπx0 ) = − [cos(nπ) − 1] nπ  (2 sin(nπx0 ))/nπ , n odd = 0 , n even

(18)

The y integral is sinh[nπ(1 − y0 )] = = =

Z

y0 0

0

sinh(nπy )dy + sinh(nπy0 ) 0

Z

1 y0

sinh[nπ(1 − y 0 )]dy 0

y0 1 o 1 n sinh[nπ(1 − y0 )] · cosh(nπy 0 ) 0 − sinh[nπy0 ] · cosh[nπ(1 − y 0 )] y0 nπ 1 {sinh[nπ(1 − y0 )] cosh(nπy0 ) + sinh(nπy0 ) cosh[nπ(1 − y0 )] − sinh(nπy0 ) − sinh[nπ(1 − y0 )]} nπ 1 {sinh[nπ] − sinh[nπ(1 − y0 )] − sinh(nπy0 )}. (19) nπ

Inserting (18) and (19) in (17), we have  X sin(nπx0 )  4 sinh[nπ(1 − y0 )] + sinh(nπy0 ) Φ(x0 ) = 3 1− . π 0 n3 sinh(nπ) n odd The thing in brackets is equal to what Jackson has, but this is tedious to show so I’ll skip the proof.

12

Homer Reid’s Solutions to Jackson Problems: Chapter 2

Problem 2.17 (a) Construct the free-space Green function G(x, y; x0 , y 0 ) for twodimensional electrostatics by integrating 1/R with respect to z 0 − z between the limits ±Z, where Z is taken to be very large. Show that apart from an inessential constant, the Green function can be written alternately as G(x, y; x0 , y 0 ) = − ln[(x − x0 )2 + (y − y 0 )2 ] = − ln[ρ2 + ρ02 − 2ρρ0 cos(φ − φ0 )]. (b) Show explicitly by separation of variables in polar coordinates that the Green function can be expressed as a Fourier series in the azimuthal coordinate, ∞ 1 X im(φ−φ0 ) e gm (ρ, ρ0 ) G= 2π −∞ where the radial Green functions satisfy   m2 δ(ρ − ρ0 ) 1 ∂ 0 ∂gm ρ − 02 gm = . 0 0 0 ρ ∂ρ ∂ρ ρ ρ

Note that gm (ρ, ρ0 ) for fixed ρ is a different linear combination of the solutions of the homogeneous radial equation (2.68) for ρ0 < ρ and for ρ0 > ρ, with a discontinuity of slope at ρ0 = ρ determined by the source delta function. (c) Complete the solution and show that the free-space Green function has the expansion G(ρ, φ; ρ0 , φ0 ) =

 m ∞ 1 X 1 ρ< 1 ln(ρ2> ) − · cos[m(φ − φ0 )] 4π 2π m=1 m ρ>

where ρ< (ρ> ) is the smaller (larger) of ρ and ρ0 . (As in Problem 2.15, I modified the text of the problem to match with my convention for Green’s functions.) (a) R

= [(x − x0 )2 + (y − y 0 )2 + (z − z 0 )2 ]1/2 ≡ [a2 + u2 ]1/2 , a = [(x − x0 )2 + (y − y 0 )2 ]1/2

, u = (z − z 0 ).

Integrating, Z

Z −Z

du 2 [a + u2 ]1/2

h i +Z = ln (a2 + u2 )1/2 + u

−Z

Homer Reid’s Solutions to Jackson Problems: Chapter 2

= ln = ln ≈ ln

13

(Z 2 + a2 )1/2 + Z (Z 2 + a2 )1/2 − Z

(1 + (a2 /Z 2 ))1/2 + 1 (1 + (a2 /Z 2 ))1/2 − 1

2+

a2 2Z 2

a2 2Z 2 2

4Z + a2 a2 2 = ln[4Z + a2 ] − ln a2 . = ln

Since Z is much bigger than a, the first term is essentially independent of a and is the ’nonessential constant’ Jackson is talking about. The remaining term is the 2D Green’s function: G = − ln a2

= − ln[(x − x0 )2 + (y − y 0 )2 ] in rectangular coordinates

= − ln[ρ2 + ρ02 − 2ρρ0 cos(φ − φ0 )] in cylindrical coordinates.

(b) The 2d Green’s function is defined by Z ∇2 G(ρ, φ; ρ0 , φ0 )ρ0 dρ0 dφ0 = 1 but ∇2 G = 0 at points other than (ρ, φ). These conditions are met if ∇2 G(ρ, φ; ρ0 , φ0 ) =

1 δ(ρ − ρ0 )δ(φ − φ0 ). ρ0

(20)

You need the ρ0 on the bottom there to cancel out the ρ0 in the area element in the integral. The Laplacian in two-dimensional cylindrical coordinates is   1 ∂ 1 ∂ 2 0 ∂ ∇ = 0 0 ρ − 02 02 . ρ ∂ρ ∂ρ0 ρ ∂φ Applying this to the suggested expansion for G gives    ∞  0 m2 1 X 1 ∂ 0 ∂gm ρ − 02 gm eim(φ−φ ) . ∇ G(ρ, φ; ρ , φ ) = 0 0 0 2π −∞ ρ ∂ρ ∂ρ ρ 2

0

0

If gm satisfies its differential equation as specified in the problem, the term in brackets equals δ(ρ − ρ0 )/ρ0 for all m and may be removed from the sum, leaving ∇2 G(ρ, φ; ρ0 , φ0 )

∞ 1 X im(φ−φ0 ) e 2π −∞

=



δ(ρ − ρ0 ) ρ0



·

=



δ(ρ − ρ0 ) ρ0



δ(φ − φ0 ).

Homer Reid’s Solutions to Jackson Problems: Chapter 2

14

(c) As in Problem 2.15, we’ll construct the functions gm by finding solutions of the homogenous radial differential equation in the two regions and piecing them together at ρ = ρ0 such that the function is continuous but its derivative has a finite jump of magnitude 1/ρ. For m ≥ 1, the solution to the homogenous equation     m2 1 ∂ 0 ∂ ρ − 02 f (ρ0 ) = 0 ρ0 ∂ρ0 ∂ρ0 ρ is f (ρ0 ) = Am ρ0m + Bm ρ0−m . Thus we take gm =



A1m ρ0m + B1m ρ0−m A2m ρ0m + B2m ρ0−m

, ρ0 < ρ , ρ0 > ρ.

In order that the first solution be finite at the origin, and the second solution be finite at infinity, we have to take B1m = A2m = 0. Then the condition that the two solutions match at ρ = ρ0 is A1m ρm = B2m ρ−m which requires A1m = γm ρ−m

B2m = ρm γm

for some constant γm . Now we have   m  γ m ρ0  ρ m gm =  γm ρ0 ρ

,

ρ0 < ρ

,

ρ0 > ρ

The finite-derivative step condition is dgm 1 dgm − = dρ0 ρ0 =ρ+ dρ0 ρ0 =ρ− ρ or

−mγm so



1 1 + ρ ρ

γm = −



=

1 ρ

1 . 2m

Then gm

  0 m  − 1 ρ , ρ0 < ρ 2m  ρ m =  − 1 ρ0 , ρ0 > ρ 2m ρ  m 1 ρ< . = − 2m ρ>

Homer Reid’s Solutions to Jackson Problems: Chapter 2

15

Plugging this back into the expansion gives  m ∞ 0 1 X 1 ρ< eim(φ−φ ) G = − 4π −∞ m ρ>  m ∞ 1 X 1 ρ< = − cos[m(φ − φ0 )]. 2π 1 m ρ>

Jackson seems to be adding a ln term to this, which comes from the m = 0 solution of the radial equation, but I have left it out because it doesn’t vanish as ρ0 → ∞.

Problem 2.18

(a) By finding appropriate solutions of the radial equation in part b of Problem 2.17, find the Green function for the interior Dirichlet problem of a cylinder of radius b [gm (ρ, ρ0 = b) = 0. See (1.40)]. First find the series expansion akin to the free-space Green function of Problem 2.17. Then show that it can be written in closed form as   2 02 ρ ρ + b4 − 2ρρ0 b2 cos(φ − φ0 ) G = ln 2 2 b (ρ + ρ02 − 2ρρ0 cos(φ − φ0 )) or

 (b2 − ρ2 )(b2 − ρ02 ) + b2 |ρ − ρ0 |2 . G = ln b2 |ρ − ρ0 |2 

(b) Show that the solution of the Laplace equation with the potential given as Φ(b, φ) on the cylinder can be expressed as Poisson’s integral of Problem 2.12. (c) What changes are necessary for the Green function for the exterior problem (b < ρ < ∞), for both the Fourier expansion and the closed form? [Note that the exterior Green function is not rigorously correct because it does not vanish for ρ or ρ0 → ∞. For situations in which the potential falls of fast enough as ρ → ∞, no mistake is made in its use.]

(a) As before, we write the general solution of the radial equation for gm in the two distinct regions:  A1m ρ0m + B1m ρ0−m , ρ0 < ρ 0 gm (ρ, ρ ) = (21) A2m ρ0m + B2m ρ0−m , ρ0 > ρ. The first boundary conditions are that gm remain finite at the origin and vanish on the cylinder boundary. This requires that B1m = 0

16

Homer Reid’s Solutions to Jackson Problems: Chapter 2

and A2m bm + B2m b−m = 0 so B2m = −γm bm

A2m = γm b−m

for some constant γm . Next, gm must be continuous at ρ = ρ0 :    m  ρ m b m A1m ρ = γm − b ρ   m  b γ m  ρ m . − A1m = ρm b ρ With this we have  m   0 m ρ b gm (ρ, ρ ) = γm − b ρ ρ  0 m  m  b ρ − = γm , b ρ0   ρ m

0

,

ρ0 < ρ

ρ0 > ρ.

Finally, dgm /dρ0 must have a finite jump of magnitude 1/ρ at ρ0 = ρ. dgm dgm 1 = − ρ dρ0 ρ0 =ρ+ dρ0 ρ0 =ρ−  m−1     m  bm ρ ρ m 1 b = mγm + m+1 − mγm − bm ρ b ρ ρ  m 1 b = 2mγm ρ ρ so γm = and 0

gm (ρ, ρ ) = =

1  ρ m 2m b

 0 m  0 m  1 ρρ ρ − 2 2m b ρ  0 m  m  ρ ρρ 1 − 2 2m b ρ0

or gm (ρ, ρ0 ) =

1 2m



ρρ0 b2

m

−



ρ< ρ>

,

ρ0 < ρ

,

ρ0 > ρ.

m 

.

Plugging into the expansion for G gives G(ρ, φ, ρ0 , φ0 ) =

 0 m  m  ∞ 1 X 1 ρρ ρ< cos m(φ − φ0 ). − 2π n=1 m b2 ρ>

(22)

Homer Reid’s Solutions to Jackson Problems: Chapter 2

17

Here we need to work out an auxiliary result:  ∞ ∞ Z x X X 1 n n−1 0 u du cos m(φ − φ0 ) x cos n(φ − φ ) = n 0 n=1 n=1 ) Z x( X ∞ 1 n 0 = u cos n(φ − φ ) du u n=1 0  Z x cos(φ − φ0 ) − u du = 1 + u2 − 2u cos(φ − φ0 ) 0 x 1 = − ln(1 − 2u cos(φ − φ0 ) + u2 ) 0 2 1 = − ln[1 − 2x cos(φ − φ0 ) + x2 ]. 2 (I summed the infinite series here back in Problem 2.12. The integral in the second-to-last step can be done by partial fraction decomposition, although I cheated and looked it up on www.integrals.com). We can apply this result individually to the two terms in (22):   1 1 + (ρρ0 /b2 )2 − 2(ρρ0 /b2 ) cos(φ − φ0 ) 0 0 G(ρ, φ; ρ , φ ) = − ln 4π 1 + (ρ< /ρ> )2 − 2(ρ< /ρ> ) cos(φ − φ0 )   2  4 ρ> b + ρ2 ρ02 − 2ρρ0 b2 cos(φ − φ0 ) 1 = − ln 4π b4 ρ2> + ρ2< − 2ρ< ρ> cos(φ − φ0 )   2  4 ρ> b + ρ2 ρ02 − 2ρρ0 b2 cos(φ − φ0 ) 1 = − ln 4π b4 ρ2> + ρ2< − 2ρ< ρ> cos(φ − φ0 )  2 ρ> 1 = − ln 4π b2   4 b + ρ2 ρ02 − 2ρρ0 b2 cos(φ − φ0 ) 1 (23) ln 2 2 − 4π b (ρ + ρ02 − 2ρρ0 cos(φ − φ0 )) This is Jackson’s result, with an additional ln term thrown in for good measure. I’m not sure why Jackson didn’t quote this term as part of his answer; he did include it in his answer to problem 2.17 (c). Did I do something wrong? (b) Now we want to plug the expression for G above into (16) to compute the potential within the cylinder. If there is no charge inside the cylinder, the volume integral vanishes, and we are left with the surface integral: Z ∂G dA0 . (24) Φ(ρ, φ) = Φ(b, φ0 ) ∂ρ0 ρ0 =b where the integral is over the surface of the cylinder. For this we need the normal derivative of (23) on the cylinder: ∂G 1 =− 0 ∂ρ 4π



2ρ0 − 2ρ cos(φ − φ0 ) 2ρ2 ρ0 − 2ρb2 cos(φ − φ0 ) − 2 4 2 02 0 2 0 b + ρ ρ − 2ρρ b cos(φ − φ ) ρ + ρ02 − 2ρρ0 cos(φ − φ0 )



.

Homer Reid’s Solutions to Jackson Problems: Chapter 2

18

Evaluated at ρ0 = b this is   ρ2 − b 2 ∂G 1 . =− ∂ρ0 ρ0 =b 2π b(ρ2 + b2 − 2ρb cos(φ − φ0 ))

In the surface integral, the extra factor of b on the bottom is cancelled by the factor of b in the area element dA0 , and (24) becomes just the result of Problem 2.12. (c) For the exterior problem we again start with the solution (21). Now the boundary conditions are different; the condition at ∞ gives A2m = 0, while the condition at b gives B1m = −γm bm .

A1m = γm b−m

From the continuity condition at ρ0 = ρ we find  m    b ρ m − A2m = γm ρm . b ρ The finite derivative jump condition gives  m   m      1 b b 1 ρ m 1 ρ m − mγm = − + −mγm b ρ ρ b ρ ρ ρ

or

γm = −

1 2m

 m b . ρ

Putting it all together we have for the exterior problem  2 m  m  1 b ρ< gm = . − 2m ρρ0 ρ> This is the same gm we came up with before, but with b2 and ρρ0 terms flipped in first term. But the closed-form expression was symmetrical in those two expressions (except for the mysterious ln term) so the closed-form expression for the exterior Green’s function should be the same as the interior Green’s function.

Solutions to Problems in Jackson, Classical Electrodynamics, Third Edition Homer Reid June 15, 2000

Chapter 3: Problems 1-10

Problem 3.1 Two concentric spheres have radii a, b(b > a) and each is divided into two hemispheres by the same horizontal plane. The upper hemisphere of the inner sphere and the lower hemisphere of the outer sphere are maintained at potential V . The other hemispheres are at zero potential. Detemine the potential in the region a ≤ r ≤ b as a series in Legendre polynomials. Include terms at least up to l = 4. Check your solution against known results in the limiting cases b → ∞ and a → 0. The expansion of the electrostatic potential in spherical coordinates for problems with azimuthal symmetry is Φ(r, θ) =

∞ h X l=0

i Al rl + Bl r−(l+1) Pl (cos θ).

(1)

We find the coefficients Al and Bl by applying the boundary conditions. Multiplying both sides by Pl0 (cos θ) and integrating from -1 to 1 gives Z 1 i 2 h Al rl + Bl r−(l+1) . Φ(r, θ)Pl (cos θ)d(cos θ) = 2l + 1 −1 At r = a this yields V

Z

1

Pl (x)dx = 0

i 2 h Al al + Bl a−(l+1) , 2l + 1 1

2

Homer Reid’s Solutions to Jackson Problems: Chapter 3

and at r = b, V

Z

0

Pl (x)dx = −1

i 2 h l Al b + Bl b−(l+1) . 2l + 1

The integral from 0 to 1 vanishes for l even, and is given in the text for l odd: Z 1 (l − 2)!! 1 Pl (x)dx = (− )(l−1)/2 l+1
. 2 2 2 ! 0

The integral from -1 to 0 also vanishes for l even, and is just the above result inverted for l odd. This gives i 2 h 1 (l − 2)!! Al al + Bl a−(l+1) V (− )(l−1)/2 l+1
= 2 2l + 1 2 2 ! i (l − 2)!! 2 h l 1 −V (− )(l−1)/2 l+1
= Al b + Bl b−(l+1) . 2 2l + 1 2 2 ! or

αl −αl with

1 (2l + 1)(l − 2)!!
αl = V (− )a(l−1)/2 . 2 4 l+1 2 !

The solution is Al = α l

= Al al + Bl a−(l+1) = Al bl + Bl b−(l+1)



bl+1 + al+1 a2l+1 − b2l+1



Bl = −αl



al+1 bl+1 (bl + al ) a2l+1 − b2l+1



The first few terms of (1) are  2    (a + b2 )r a2 b2 (a + b) a4 b4 (a3 + b3 ) 3 7 (a4 + b4 )r3 − 2 3 − 4 7 Φ(r, θ) = V P1 (cos θ)− P3 (cos θ)+· · · 4 a3 − b 3 r (a − b3 ) 16 a7 − b 7 r (a − b7 ) In the limit as b → ∞, the problem reduces to the exterior problem treated in Section 2.7 of the text. In that limit, the above expression becomes 7  a 4 3  a 2 P1 (cos θ) − V P3 (cos θ) + · · · Φ(r, θ) → V 4 r 16 r

in agreement with (2.27) with half the potential spacing. When a → 0, the problem goes over to the interior version of the same problem, as treated in section 3.3 of the text. In that limit the above expression goes to 3 r  7  r 3 Φ(r, θ) → − V P1 (cos θ) + V P3 (cos θ) + · · · 4 b 16 b This agrees with equation (3.36) in the text, with the sign of V flipped, because here the more positive potential is on the lower hemisphere.

Homer Reid’s Solutions to Jackson Problems: Chapter 3

3

Problem 3.2 A spherical surface of radius R has charge uniformly distributed over its surface with a density Q/4πR2 , except for a spherical cap at the north pole, defined by the cone θ = α. (a) Show that the potential inside the spherical surface can be expressed as Φ=

∞ rl Q X 1 [Pl+1 (cos α) − Pl−1 (cos α)] l+1 Pl (cos θ) 8π0 2l + 1 R l=0

where, for l = 0, Pl−1 (cos α) = −1. What is the potential outside? (b) Find the magnitude and direction of the electric field at the origin. (c) Discuss the limiting forms of the potential (part a) and electric field (part b) as the spherical cap becomes (1)very small, and (2) so large that the area with charge on it becomes a very small cap at the south pole.

(a) Let’s denote the charge density on the sphere by σ(θ). At a point infinitesimally close to the surface of the sphere, the electric field is F = −∇Φ = − so

σ ˆ r 0

σ ∂Φ = . ∂r r=R 0

(2)

The expression for the potential within the sphere must be finite at the origin, so the Bl in (1) are zero. Differentiating that expansion, (2) becomes ∞

X ∂ Φ(r, θ) = lAl rl−1 Pl (cos θ) ∂r l=1

Multiplying by Pl0 and integrating at r = R gives Z 1 1 2l σ(θ)Pl (cos θ)d(cos θ) = Al Rl−1 0 −1 2l + 1 so

2l + 1 Al = · 2lRl−1



Q 4πR2 0

Z

cos α

Pl (x)dx. −1

To evaluate the integral we use the identity (eq. 3.28 in the text) Pl (x) =

d 1 [Pl+1 (x) − Pl−1 (x)] (2l + 1) dx

Homer Reid’s Solutions to Jackson Problems: Chapter 3

so

Z

cos α

Pl (x)dx = −1

4

1 [Pl+1 (cos α) − Pl−1 (cos α)] . 2l + 1

(We used the fact that Pl+1 (−1) = Pl−1 (−1) for all l.) With this we have Al =

Q [Pl+1 (cos α) − Pl−1 (cos α)] 8π0 lRl+1

so the potential expansion is Φ(r, θ) =

∞ Q X1 rl [Pl+1 (cos α) − Pl−1 (cos α)] l+1 Pl (cos θ). 8π0 l R l=1

Within the body of the sum, I have an l where Jackson has a 2l + 1. Also, he includes the l = 0 term in the sum, corresponding to a constant term in the potential. I don’t understand how he can determine that constant from the information contained in the problem; the information about the charge density only tells you the derivative of the potential. There’s nothing in this problem that fixes the value of the potential on the surface beyond an arbitrary constant. (b) The field at the origin comes from the l = 1 term in the potential: ∂Φ 1 ∂Φ ˆ E(r = 0) = −∇Φ|r=0 = − ˆ r+ θ ∂r r ∂θ r=0   d Q ˆ [P (cos α) − 1] P (cos θ)ˆ r + P (cos θ) θ = − 2 1 1 8π0 R2 dθ  h i Q 3 3 = − cos θˆ r − sin θ θˆ cos2 α − 2 8π0 R 2 2 =

3Q sin2 α ˆ k. 16π0 R2

The field points in the positive z direction. That makes sense, since a positive test charge at the origin would sooner fly up out through the uncharged cap than through any of the charged surface.

5

Homer Reid’s Solutions to Jackson Problems: Chapter 3

Problem 3.3 A thin, flat, conducting, circular disk of radius R is located in the x − y plane with its center at the origin, and is maintained at a fixed potential V . With the information that the charge density on a disc at fixed potential is proportional to (R2 − ρ2 )−1/2 , where ρ is the distance out from the center of the disc, (a) show that for r > R the potential is ∞

2V R X (−1)l Φ(r, θ, φ) = π r 2l + 1 l=0

R 2l r

!

P2l (cos θ)

(b) find the potential for r < R. (c) What is the capacitance of the disk? We are told that the surface charge density on the disk goes like σ(r)

= K(R2 − r2 )−1/2   1  r 2 K 3 · 1  r 4 5 · 3 · 1  r 6 1+ = + + +··· R 2 R (2!)(2 · 2) R (3!)(2 · 2 · 2) R ∞   K X (2n − 1)!! r 2n (3) = R n=0 n! · 2n R

for some constant K. From the way the problem is worded, I take it we’re not supposed to try to figure out what K is explicitly, but rather to work the problem knowing only the form of (3). At a point infinitesimally close to the surface of the disk (i.e., as θ → π/2), the component of ∇Φ in the direction normal to the surface of the disk must be proportional to the surface charge. At the surface of the disk, the normal direction is the negative θˆ direction. Hence σ 1 ∂ =± . Φ(r, θ) (4) r ∂θ  0 θ=(π/2)

with the plus (minus) sign valid for Φ above (below) the disc. For r < R the potential expansion is Φ(r, θ) =

∞ X

Al rl Pl (cos θ).

(5)

l=0

Combining (3), (4), and (5) we have ∞ ∞ X K X (2n − 1)!!  r 2n d =± Al rl−1 Pl (cos θ) . dθ R0 n=0 n! · 2n R cos θ=0 l=0

(6)

Homer Reid’s Solutions to Jackson Problems: Chapter 3

6

For l even, dPl /dx vanishes at x = 0. For l odd, I used some of the Legendre polynomial identities to derive the formula (2l − 1)!! d P2l+1 (x) . = (−1)l (2l + 1) dx l! · 2l x=0 This formula reminds one strongly of expansion (3). Plugging into (6) and equating coefficents of powers of r, we find A2l+1 = ± so Φ(r, θ) = A0 ±

(−1)l K (2l + 1)R2l+1 0

∞ K X (−1)l  r 2l+1 P2l+1 (cos θ). 0 2l + 1 R l=1

I wrote A0 explicitly because we haven’t evaluated it yet–the derivative condition we used earlier gave no information about it. To find A0 , observe that, on the surface of the disk (cos θ = 0), all the terms in the above sum vanish ( because Pl (0) is 0 for odd l) so Φ = A0 on the disk. But Φ = V on the disk. Therefore, A0 = V . We have Φ(r, θ) = V ±

∞ K X (−1)l  r 2l+1 P2l+1 (cos θ) 0 2l + 1 R

(7)

l=1

where the plus (minus) sign is good for θ less than (greater than)π/2. Note that the presence of that ± sign preserves symmetry under reflection through the z axis, a symmetry that is clearly present in the physical problem. (a) For r > R, there is no charge. Thus the potential and its derivative must be continuous everywhere–we can’t have anything like the derivative discontinuity that exists at θ = π/2 for r < R. Since the physical problem is symmetric under a sign flip in cos θ, the potential expansion can only contain Pl terms for l even. The expansion is Φ(r, θ) =

∞ X

B2l r−(2l+1) P2l (cos θ).

l=0

At r = R, this must match up with (7): V ±

∞ ∞ X K X (−1)l P2l+1 (cos θ) = B2l R−(2l+1) P2l (cos θ). 0 2l + 1 l=1

l=0

Multiplying both sides by P2l (cos θ) sin(θ) and integrating gives   Z 0 Z 1 Z 1 ∞ K X (−1)l 2R−(2l+1) P2l+1 (x)Pl (x)dx Pl (x)dx + P2l+1 (x)P2l (x)dx + B2l = V − 4l + 1 0 2l + 1 0 −1 −1 l=1 Z ∞ 2K X (−1)l 1 = 2V δl,0 + P2l+1 (x)P2l (x)dx. 0 2l + 1 0 l=1

Homer Reid’s Solutions to Jackson Problems: Chapter 3

7

but I can’t do this last integral.

Problem 3.4 The surface of a hollow conducting sphere of inner radius a is divided into an even number of equal segments by a set of planes; their common line of intersection is the z axis and they are distributed uniformly in the angle φ. (The segments are like the skin on wedges of an apple, or the earth’s surface between successive meridians of longitude.) The segments are kept at fixed potentials ±V , alternately. (a) Set up a series representation for the potential inside the sphere for the general case of 2n segments, and carry the calculation of the coefficients in the series far enough to determine exactly which coefficients are different from zero. For the nonvanishing terms exhibit the coefficients as an integral over cos θ. (b) For the special case of n = 1 (two hemispheres) determine explicitly the potential up to and including all terms with l = 3. By a coordinate transformation verify that this reduces to result (3.36) of Section 3.3.

(a) The general potential expansion is Φ(r, θ, φ) =

∞ X l h X l=0 m=−l

i Alm rl + Blm r−(l+1) Ylm (θ, φ).

(8)

For the solution within the sphere, finiteness at the origin requires Blm = 0. ∗ Multiplying by Yl0m0 and integrating over the surface of the sphere we find Z 1 ∗ Alm = Φ(a, θ, φ) Ylm (θ, φ) dΩ al Z Z n π 2kπ/n V X ∗ k Ylm (θ, φ) sin θ dφ dθ = (−1) al 0 2(k−1)π/n k=1 (Z )  1/2 Z 1 X n 2kπ/n V 2l + 1 (l − m)! k −imφ m = Pl (x) dx (−1) e dφ . al 4π (l + m)! −1 2(k−1)π/n k=1

(9)

The φ integral is easy: Z 2kπ/n i 1 h −2imkπ/n e − e−2im(k−1)π/n . e−imφ dφ = − im 2(k−1)π/n

This is to be summed from k = 1 to n with a factor of (−1)k thrown in: i X 1 h −2mπi(1/n) = − (e − 1) − (e−2mπi(2/n) − e−2mπi(1/n) ) + · · · − (1 − e−2mπi((n−1)/n) ) im o 2 n = 1 − e−2mπi/n + e2(−2mπi/n) − e3(−2mπi/n) + · · · + e(n−1)(−2mπi/n) . (10) im

Homer Reid’s Solutions to Jackson Problems: Chapter 3

8

Putting x = − exp(−2mπi/n), the thing in braces is 1 + x + x2 + x3 + · · · + xn−1 =

1 − xn 1 − e−2mπi = , 1−x 1 + e−2imπ/n

Note that the numerator vanishes. Thus the only way this thing can be nonzero is if the denominator also vanishes, which only happens if the exponent in the denominator equates to -1. This only happens if m/n = 1/2, 3/2, 5/2, · · · . In that case, the 2mπi/n term in the exponent of the terms in (10) equates to πi, so all the terms with a plus sign in (10) come out to +1, while all the terms with a minus sign come out to -1, so all n terms add constructively, and (10) equates to ( 2n X , m = n/2, 3n/2, 5n/2, · · · = im 0, otherwise. Then the expression (9) for the coefficients becomes Alm

1/2 Z 1  2nV 2l + 1 (l − m)! = Plm (x)dx, imal 4π (l + m)! −1

m=

n 3n , , · · · = 0, otherwise. 2 2

(b) As shown above, the only terms that contribute are those with m = n/2, m = 3n/2, et cetera. Of course there is also the constraint that m < l. Then, with n = 2, up to l = 3 the only nonzero terms in the series (9) are those with l = 1, m = ±1, and l = 3, m = ±1 or ±3. We need to evaluate the θ integral for these terms. We have Z 1 Z 1 1 P1 (x) dx = − (1 − x2 )1/2 dx = −π Z

Z

−1 1

P31 (x) dx −1 1

−1

P33 (x) dx

= −

Z

−1 1

−1

= −15

Z

(1 − x2 )1/2 1

−1



 3π 15 2 3 dx = − x − 2 2 8

(1 − x2 )3/2 dx = −

15π . 4

Using these results in (??), we have A1±1 A3±1 A3±3

 1/2 3 4πV i a 4π · 2  1/2 3πV i 7 · 2 = ± 2a3 4π · 4!  1/2 5πV i 7 = ± 3 a 4π · 6! = ±

Now we can plug these coefficients into (8) to piece together the solution. This involves some arithmetic in combining all the numerical factors in each

9

Homer Reid’s Solutions to Jackson Problems: Chapter 3

coefficient, which I have skipped here. h r 7  r 3 sin θ(5 cos2 θ − 1) sin φ sin θ sin φ + Φ(r, θ, φ) = V 3 a 16 a  7  r 3 3 + sin θ sin 3φ + · · · 144 a

Problem 3.6 Two point charges q and −q are located on the z azis at z = +a and z = −a, respectively. (a) Find the electrostatic potential as an expansion in spherical harmonics and powers of r for both r > a and r < a. (b) Keeping the product qa = p/2 constant, take the limit of a → 0 and find the potential for r 6= 0. This is by definition a dipole along the z azis and its potential. (c) Suppose now that the dipole of part b is surrounded by a grounded spherical shell of radius b concentric with the origin. By linear superposition find the potential everwhere inside the shell. (a) First of all, for a point on the z axis the potential is   1 q 1 Φ(z) = − 4π0 |z − a| z + a     a   a 2  a   a 2 q +··· − 1− ··· = 1+ + + 4π0 z z z z z      q a a 3 = + +··· 2π0 z z z P for z > a. Comparing this with the general expansion Φ = Bl r−(l+1) Pl (cos θ) at θ = 0 we can identify the Bl s and write     a 3 a q P3 (cos θ) + · · · P1 (cos θ) + Φ(r, θ) = 2π0 r r r for r > a. For r < a we can just swap a and r in this equation.

(b) Φ(r, θ)

= = →

qa 2π0 r2 p 4π0 r2 p 4π0 r2

 

P1 (cos θ) + P1 (cos θ) +

cos θ

 a 2

P3 (cos θ) + · · ·

r

P3 (cos θ) + · · ·

r  a 2

as a → 0.

 

Homer Reid’s Solutions to Jackson Problems: Chapter 3

10

(c) When we put the grounded sphere around the two charges, a surface charge distribution forms on the sphere. Let’s denote by Φs the potential due to this charge distribution alone (not including the potential of the dipole) and by Φd the potential due to the dipole. To calculate Φs , we pretend there are no charges within the sphere, in which case we have the general expansion (1), with Bl = 0 to keep us finite at the origin. The total potential is just the sum Φs + Φd : ∞

Φ(r, θ) =

X p cos θ + Al rl Pl (cos θ). 2 4π0 r l=0

The condition that this vanish at r = b ensures, by the orthogonality of the Pl , that only the l = 1 term in the sum contribute, and that p . A1 = − 4π0 b3 The total potential inside the sphere is then r p  P1 (cos θ). 1 − Φ(r, θ) = 4π0 b2 b

Problem 3.7 Three point charges (q, −2q, q) are located in a straight line with separation a and with the middle charge (−2q) at the origin of a grounded conducting spherical shell of radius b, as indicated in the figure. (a) Write down the potential of the three charges in the absence of the grounded sphere. Find the limiting form of the potential as a → 0, but the product qa2 = Q remains finite. Write this latter answer in spherical coordinates. (b) The presence of the grounded sphere of radius b alters the potential for r < b. The added potential can be viewed as caused by the surface-charge density induced on the inner surface at r = b or by image charges located at r > b. Use linear superposition to satisfy the boundary conditions and find the potential everywhere inside the sphere for r < a and r > a. Show that in the limit a → 0,   Q r5 Φ(r, θ, φ) → 1 − 5 P2 (cos θ). 2π0 r3 b (a) On the z axis, the potential is   q 2 1 1 Φ(z) = − + + 4π0 z |z − a| z + a       a   a 2  a   a 2 q ··· + 1− +··· = −2 + 1 + + + 4π0 r z z z z    q a 2  a 4 = + +··· . 2π0 z z z

Homer Reid’s Solutions to Jackson Problems: Chapter 3

11

As before, from this result we can immediately infer the expression for the potential at all points:     a 4 a 2 q P2 (cos θ) + P4 (cos θ) + · · · Φ(r, θ) = 2π0 r r r     2 2 qa a = P (cos θ) + · · · P (cos θ) + 4 2 2π0 r3 r Q → P2 (cos θ) as a → 0 (11) 2π0 r3 (b) As in the previous problem, the surface charges on the sphere produce an extra contribution Φs to the potential within the sphere. Again we can express Φs with the expansion (1) (with Bl = 0), and we add Φs to (11) to get the full potential within the sphere: ∞

Φ(r, θ) =

X Q P2 (cos θ) + Al rl Pl (cos θ) 3 2π0 r l=0

From the condition that Φ vanish at r = b, we determine that only the l = 2 term in the sum contributes, and that A2 = −

Q . 2π0 b5

Then the potential within the sphere is   r 5  Q Φ(r, θ) = 1 − P2 (cos θ). 2π0 r3 b

Problem 3.9 A hollow right circular cylinder of radius b has its axis coincident with the z axis and its ends at z = 0 and z = L. The potential on the end faces is zero, while the potential on the cylindrical surface is given as V (φ, z). Using the appropriate separation of variables in cylindrical coordinates, find a series solution for the potential anywhere inside the cylinder. The general solution of the Laplace equation for problems in cylindrical coordinates consists of a sum of terms of the form R(ρ)Q(φ)Z(z). The φ function is of the form Q(φ) = A sin νφ + B cos νφ

Homer Reid’s Solutions to Jackson Problems: Chapter 3

12

with ν an integer. The z function is of the form Z(z) = Cekz + De−kz . In this case, Z must vanish at z = 0 and z = L, which means we have to take k imaginary, i.e. πn , n = 1, 2, 3, · · · Z(z) = C sin(kn z) with kn = L With this form for Z, R must be taken to be of the form R(ρ) = EIν (kn ρ) + F Kν (kn ρ). Since we’re looking for the potential on the inside of the cylinder and there is no charge at the origin, the solution must be finite as ρ → 0, which requires F = 0. Then the potential expansion becomes Φ(ρ, φ, z) =

∞ X ∞ X

[Anν sin νφ + Bnν cos νφ] sin(kn z)Iν (kn ρ).

(12)

n=1 ν=0

Multiplying by sin ν 0 φ sin kn0 z and integrating at r = b, we find Z L Z 2π πL Iν (kn b)Anν V (φ, z) sin νφ sin(kn z) dφ dz = 2 0 0 so Z L Z 2π 2 Anν = V (φ, z) sin(νφ) sin(kn z) dφ dz. πLIν (kn b) 0 0 Similarly, Z L Z 2π 2 Bnν = V (φ, z) cos(νφ) sin(kn z) dφ dz. πLIν (kn b) 0 0

(13)

(14)

Problem 3.10 For the cylinder in Problem 3.9 the cylindrical surface is made of two equal halfcylinders, one at potential V and the other at potential −V , so that  V for −π/2 < φ < π/2 V (φ, z) = −V for π/2 < φ < 3π/2 (a) Find the potential inside the cylinder. (b) Assuming L >> b, consider the potential at z = L/2 as a function of ρ and φ and compare it with two-dimensional Problem 2.13. The potential expansion is (12) with coefficients given by (13) and (14). The relevant integrals are Z L Z 2π V (φ, z) sin(νφ) sin(kn z) dφ dz 0

0

13

Homer Reid’s Solutions to Jackson Problems: Chapter 3

= V = 0

(Z

L

sin(kn z) dz 0

Z = V = =

(Z

L 0

Z

) (Z

π/2 −π/2

sin(νφ) dφ −

Z

3π/2

sin(νφ) dφ π/2

)

2π

V (φ, z) cos(νφ) sin(kn z) dφ dz 0

L

sin(kn z) dz 0

) (Z

π/2 −π/2

cos(νφ) dφ −

Z

3π/2

cos(νφ) dφ π/2

)

o 2V n π/2 3π/2 (n odd) |sin νφ|−π/2 − |sin νφ|π/2 νkn  0 , n or ν even  8V /kn ν , n odd, ν = 1, 5, 9, · · ·  −8V /kn ν , n odd, ν = 3, 7, 11, · · ·

Hence, from (13) and (14), Anν Bnν

= 0 = 0, = (−1)(ν−1)/2 · 16V /(nνπ 2 Iν (kn b)),

n or ν even n and ν odd

The potential expansion is Φ(ρ, θ, z) =

16V X (−1)(ν−1)/2 cos(νφ) sin(kn z)Iν (kn ρ) π 2 n,ν nνIv (kn b)

(15)

where the sum contains only terms with n and ν odd. (b) At z = L/2 we have Φ(ρ, θ, L/2) =

Iν (kn ρ) 16V X (−1)(n+ν−2)/2 cos(νφ) . 2 π n,ν nν Iν (kn b)

As L → ∞, the arguments to the I functions become small. Using the limiting form for Iν quoted in the text as equation (3.102), we have Φ(ρ, θ) =

 ρ ν 16V X (−1)(n+ν−2)/2 cos(νφ) . π 2 n,ν nν b

The sums over n and ν are now decoupled: (∞ )( ∞ )  ρ ν X (−1)ν 16V X (−1)n Φ(ρ, θ) = cos(νφ) π2 2n + 1 2ν + 1 b n=0 ν=0 (∞ )  ρ ν 16V n π o X (−1)ν cos(νφ) = 2 π 4 2ν + 1 b ν=0   4V 2ρb cos φ = tan−1 π b2 − ρ 2

Homer Reid’s Solutions to Jackson Problems: Chapter 3

14

This agrees with the result of Problem 2.13, with V1 = −V2 = V . The first series is just the Taylor series for tan−1 (x) at x = 1, so it sums to π/4. The second series can also be put into the form of the Taylor series for tan−1 (x), using tricks exactly analogous to what I did in my solution for Problem 2.13.

Solutions to Problems in Jackson, Classical Electrodynamics, Third Edition Homer Reid June 15, 2000

Chapter 3: Problems 11-18 Problem 3.11 A modified Bessel-Fourier series on the interval 0 ≤ ρ ≤ a for an arbitrary function f (ρ) can be based on the ”homogenous” boundary conditions: At

ρ = 0,

At ρ = a,

d Jν (k 0 ρ) = 0 dρ λ d ln[Jν (kρ)] = − dρ a

ρJν (kρ)

(λ real)

The first condition restricts ν. The second condition yields eigenvalues k = yνn /a, where yνn is the nth positive root of x dJν (x)/dx + λJν (x) = 0. (a) Show that the Bessel functions of different eigenvalues are orthogonal in the usual way. (b) Find the normalization integral and show that an arbitrary function f (ρ) can be expanded on the interval in the modified Bessel-Fourier series f (ρ) =

∞ X

n=1

A n Jν

y

νn

a



with the coefficients An given by 2 An = 2 a

"

ν2 1− 2 yνn



Jν2 (yνn )

+

1



dJν (yνn ) dyνn

2 #−1 Z

a

f (ρ)ρJν 0

y ρ νn dρ. a

Homer Reid’s Solutions to Jackson Problems: Chapter 3

(a) The function Jν (kρ) satisfies the equation     d ν2 1 d 2 ρ Jν (kρ) + k − 2 Jν (kρ) = 0. ρ dρ dρ ρ Multiplying both sides by ρJν (k 0 ρ) and integrating from 0 to a gives      Z a d ν2 d 0 2 0 ρ Jν (kρ) + k ρ − Jν (k ρ)Jν (kρ) dρ = 0. Jν (k ρ) dρ dρ ρ 0 The first term on the left can be integrated by parts:   Z a d d 0 Jν (k ρ) ρ Jν (kρ) dρ dρ dρ 0 a Z a    d d d ρ Jν (k 0 ρ) Jν (kρ) dρ. = ρJν (k 0 ρ) Jν (kρ) − dρ dρ dρ 0 0

2

(1)

(2)

(3)

One of the conditions we’re given is that the thing in braces in the first term here vanishes at ρ = 0. At ρ = a we can invoke the other condition: d 1 λ d ln[Jν (kρ)] = Jν (kρ) =− dρ Jν (kρ) dρ a ρ=a ρ=a → a

d Jν (ka) = −λJν (ka). dρ

Plugging this into (3), we have   Z a d d ρ Jν (kρ) dρ Jν (k 0 ρ) dρ dρ 0   Z a  d d 0 0 Jν (k ρ) Jν (kρ) . ρ = −λJν (k ρ)Jν (kρ) − dρ dρ 0

(4)

This is clearly symmetric in k and k 0 , so when we write down (2) with k and k 0 switched and subtract from (2), the first integral (along with the ν 2 /ρ term) vanishes, and we are left with Z a 02 2 (k − k ) ρJν (k 0 ρ)Jν (kρ) dρ = 0 0

proving orthogonality. (b) If we multiply (1) by ρ2 J 0 (kρ) and integrate, we find Z a Z a Z a d 0 2 0 2 0 2 ρJν (kρ) [ρJν (kρ)]dρ+k Jν (kρ)Jν0 (kρ)dρ = 0. ρ Jν (kρ)Jν (kρ)dρ−ν dρ 0 0 0 (5)

3

Homer Reid’s Solutions to Jackson Problems: Chapter 3

R The first and third integrals are of the form f (x)f 0 (x)dx and can be done immediately. In the second integral we put f (ρ) = ρ2 Jν (kρ), g 0 (ρ) = Jν0 (kρ) and integrate by parts: Z a Z a Z a 2 2 a 2 2 0 ρJν (kρ)dρ − ρ2 Jν (kρ)Jν0 (kρ)dρ ρ Jν (kρ)Jν (kρ)dρ = ρ Jν (kρ) 0 − 2 0

0

→

Z

a

0

ρ2 Jν (kρ)Jν0 (kρ)dρ =

0

1 2 2 a Jν (ka) − 2

Using this in (5), a2 02 (ak)2 2 Jν (ka) + aJν (ka) − k 2 2 2 so Z

a 0

ρJν2 (kρ)dρ

= =



ν2 a2 − 2 2 2k (

a2 2

Z

Z

a

0

ρJν2 (kρ)dρ.

a 0

ρJν2 (kρ)dρ −

ν2 2 J (ka) = 0 2 ν

a2 02 J (ka) 2k 2 ν   2 ) ν2 d 2 1− Jν (ka) + Jν (ka) (ka)2 d(ka) 

Jν2 (ka) +

This agrees with what Jackson has if you note that k is chosen such that ka = ynm .

Problem 3.12 An infinite, thin, plane sheet of conducting material has a circular hole of radius a cut in it. A thin, flat, disc of the same material and slightly smaller radius lies in the plane, filling the hole, but separated from the sheet by a very narrow insulating ring. The disc is maintained at a fixed potential V , whilc the infinite sheet is kept at zero potential. (a) Using appropriate cylindrical coordinates, find an integral expression involving Bessel functions for the potential at any point above the plane. (b) Show that the potential a perpendicular distance z above the center of the disc is   z Φ0 (z) = V 1 − √ a2 + z 2 (c) Show that the potential a perpendicular distance z above the edge of the disc is   kz V K(k) 1− Φa (z) = 2 πa where k = 2a/(z 2 + 4a2 )1/2 , and K(k) is the complete elliptic integral of the first kind.

4

Homer Reid’s Solutions to Jackson Problems: Chapter 3

(a) As before, we can write the potential as a sum of terms R(ρ)Q(φ)Z(z). In this problem there is no φ dependence, so Q = 1. Also, the boundary conditions on Z are that it vanish at ∞ and be finite at 0, whence Z(z) ∝ exp(−kz) for any k. Then the potential expansion becomes Z ∞ Φ(ρ, z) = A(k)e−kz J0 (kρ) dk. (6) 0

To evaluate the coefficients A(k), we multiply both sides by ρJ0 (k 0 ρ) and integrate over ρ at z = 0: Z ∞  Z ∞ Z ∞ ρΦ(ρ, 0)J0 (k 0 ρ) dρ = A(k) ρJ0 (kρ)J0 (k 0 ρ) dρ dk 0

0

0

A(k 0 ) k0

= so A(k)

= k

Z

= kV Plugging this back into (6), Z Φ(ρ, z) = V

∞ 0

Z

∞

ρΦ(ρ, 0)J0 (kρ) dρ 0

Z

a

ρJ0 (kρ)dρ. 0

a

kρ0 e−kz J0 (kρ)J0 (kρ0 ) dρ0 dk.

(7)

0

The ρ0 integral can be done right away. To do it, I appealed to the differential equation for J0 : 1 J000 (u) + J00 (u) + J0 (u) = 0 u so Z x Z x Z x uJ0 (u) du = − uJ000 du − J00 (u) du 0 0 0 Z x Z x x 0 0 = − |uJ0 (u)|0 + J0 (u) du − J00 (u) du x

0

0

= − |uJ00 (u)|0 = −xJ00 (x) = xJ1 (x).

(In going from the first to second line, I integrated by parts.) Then (7) becomes Z ∞ Φ(ρ, z) = aV J1 (ka)J0 (kρ)e−kz dk. (8) 0

5

Homer Reid’s Solutions to Jackson Problems: Chapter 3

(b) At ρ = 0, (7) becomes Z

a

Z

∞ −kz

0



ke J0 (kρ )dk dρ0 Φ(0, z) = V J0 (0) ρ 0 0  Z a  Z ∞ ∂ 0 −kz 0 = V ρ − e J0 (kρ )dk dρ0 ∂z 0 0 !) Z a ( ∂ 1 p dρ0 = V ρ0 − ∂z ρ02 + z 2 0 Z a zρ0 = V dρ0 02 2 )3/2 (ρ + z 0 0

Here we substitute u = ρ02 + z 2 , du = 2ρ0 dρ: Z 2 2 V zJ0 (0) a +z −3/2 Φ(0, z) = u du 2 z2 2 2 1 a +z = −V z 1/2 u z2   1 1 −√ = Vz z z2 + z2   z = V 1− √ a2 + z 2 (b) At ρ = a, (8) becomes Φ(a, z) = aV

Z

∞

J1 (ka)J0 (ka)e−kz dk 0

Problem 3.13 Solve for the potential in Problem 3.1, using the appropriate Green function obtained in the text, and verify that the answer obtained in this way agrees with the direct solution from the differential equation. For Dirichlet boundary value problems, the basic equation is Z I 0 1 0 0 0 0 ∂G(x; x ) 0 G(x; x )ρ(x ) dV + Φ(x ) Φ(x) = − 0 dA . 0 V ∂n S x

(9)

Here there is no charge in the region of interest, so only the surface integral contributes. The Green’s function for the two-sphere problem is G(x; x0 ) = −

∞ X l ∗ X Ylm (θ0 , φ0 ) Ylm (θ, φ) Rl (r; r0 ) 2l + 1 l=0 m=−l

(10)

6

Homer Reid’s Solutions to Jackson Problems: Chapter 3

with

1

0

Rl (r; r ) =  1−

a b


2l+1



l r


b2l+1



.

(11)

Actually in this case the potential cannot have any Φ dependence, so all terms with m 6= 0 in (10) vanish, and we have G(x; x0 ) = −

∞ 1 X Pl (cos θ)Pl (cos θ0 )Rl (r; r0 ). 4π l=0

In this case the boundary surfaces are spherical, which means the normal to a surface element is always in the radial direction: ∞ ∂ 1 X ∂ 0 Pl (cos θ)Pl (cos θ0 ) Rl (r; r0 ). G(x; x ) = − ∂n 4π ∂n l=0

The surface integral in (9) has two parts: one integral S1 over the surface of the inner sphere, and a second integral S2 over the surface of the outer sphere: Z π Z 2π  ∞ ∂Rl 1 X 0 0 2 0 0 Pl (cos θ) S1 = − Φ(a, θ )Pl (cos θ )a sin θ dφ dθ 4π ∂n r0 =a 0 0 l=0 Z 1  ∞ ∂Rl V X 2 a Pl (cos θ) P (x) dx = − l 2 ∂n r0 =a 0 l=0 ∞ ∂Rl V X 2 a γl Pl (cos θ) · = − 2 ∂n r0 =a l=0

where

γl

=

Z

1

Pl (x) dx 0

(l − 2)!! 1 , = (− )(l−1)/2 2 2[(l + 1)/2]! = 0,

l odd l even.

A similar calculation gives S2

Z 0  ∞ ∂Rl V X 2 b Pl (cos θ) P (x) dx l 2 ∂n r0 =b −1 l=0 ∞ ∂Rl V X 2 b γl Pl (cos θ) 2 ∂n r0 =b

= − =

l=0

because Pl is odd for l odd, so its integral from -1 to 0 is just the negative of the integral from 0 to 1. The final potential is the sum of S1 and S2 : 0 ∞ 02 ∂Rl r =b V X Φ(r, θ) = (12) γl Pl (cos θ) r 2 ∂n r0 =a l=0

Homer Reid’s Solutions to Jackson Problems: Chapter 3

7

Since the point of interest is always between the two spheres, to find the normal derivative at r = a we differentiate with respect to r< , and at r = b with respect to r> . Also, at r = a the normal is in the +r direction, while at r = b the normal is in the negative r direction.   1 al−1 rl 2 ∂ 0 2 a Rl (r; r ) = (2l + 1)a  − 2l+1
2l+1 ∂n rl+1 b 1 − ab r 0 =a   b−(l+2) a2l+1 0 2 l 2 ∂ Rl (r; r ) = (2l + 1)b  b
2l+1 r − l+1 ∂n r 1 − ab r 0 =b

Combining these with some algebra gives Φ(r, θ) =

  ∞ (ab)l+1 (bl + al )r−(l+1) − (al+1 + bl+1 )rl V X (2l + 1)γl Pl (cos θ) 2 b2l+1 − a2l+1 l=0

in agreement with what we found in Problem 3.1.

Problem 3.14 A line charge of length 2d with a total charge Q has a linear charge density varying as (d2 − z 2 ), where z is the distance from the midpoint. A grounded, conducting spherical shell of inner radius b > d is centered at the midpoint of the line charge. (a) Find the potential everywhere inside the spherical shell as an expansion in Legendre polynomials. (b) Calculate the surface-charge density induced on the shell. (c) Discuss your answers to parts a and b in the limit that d

−

l r>

b2l+1



.

Since the potential vanishes on the boundary surface, the potential inside the sphere is given by Z 1 G(r, θ; r0 , θ0 )ρ(r0 , θ0 )dV. Φ(r, θ) = − 0 V In this case ρ is only nonzero on the z axis, where r = z. Also, Pl (cos θ)=1 for z > 0, and (−1)l for z < 0. This means that the contributions to the integral from the portions of the line charge for z > 0 and z < 0 cancel out for odd l, and add constructively for even l: # " Z ∞ d X 1 Rl (r; z)ρ(z) dz Pl (cos θ) 2 Φ(r, θ) = 4π0 0 l=0,2,4,...

We have Z

d

Rl (r; z)ρ(z) dz = λ 0

Z

d 0

l r


l r>

b2l+1



(d2 − z 2 ) dz

This is best split up into two separate integrals: =λ

Z

d 0

l r< λ (d2 − z 2 ) dz − 2l+1 l+1 b r>

Z

d 0

l l r< r> (d2 − z 2 ) dz

The second integral is symmetric between r and r 0 , so we may integrate it directly: −

λ b2l+1

Z

d 0

l l r< r> (d2

2

− z ) dz

Z d λrl = − 2l+1 z l (d2 − z 2 ) dz b 0   dl+3 λrl dl+3 − = − 2l+1 b l+1 l+3 = −

λrl dl+3 (l + 1)(l + 3)b2l+1

The first integral must be further split into two: λ

Z

d 0

l r< (d2 − z 2 ) dz l+1 r>

(14)

9

Homer Reid’s Solutions to Jackson Problems: Chapter 3

= λ

(

1 rl+1

Z

r 0

z l (d2 − z 2 ) dz + rl

Z

d r

d2 − z 2 dz z l+1

)

d ) 2 d r r 1 1 l d − + r − l + = λ l+1 l−2 r l+1 l+3 lz (l − 2)z r  2   r l r2 d2 r2 d 2 2 − + − + = λ d l+1 l+3 d l(l + 2) l l+2    r l r2 2 d2 2 d = λ − + (l + 2)(l + 3) l(l + 1) d l(l + 2) (



2 l+1

l+3



Combining this with (14), we have d

  r l r2 2 d2 rl dl+3 d2 − + − (l + 2)(l + 3) l(l + 1) d l(l + 2) (l + 1)(l + 3)b2l+1 0 (15) But something is wrong here, because with this result the final potential will contain terms like r 0 Pl (cos θ) and r2 Pl (cos θ), which do not satisfy the Laplace equation.

Z

Rl (r; z)ρ(z) dz = λ



Homer Reid’s Solutions to Jackson Problems: Chapter 3

10

Problem 3.15 Consider the following “spherical cow” model of a battery connected to an external circuit. A sphere of radius a and conductivity σ is embedded in a uniform medium of conductivity σ 0 . Inside the sphere there is a uniform (chemical) force in the z direction acting on the charge carriers; its strength as an effective electric field entering Ohm’s law is F . In the steady state, electric fields exist inside and outside the sphere and surface charge resides on its surface. (a) Find the electric field (in addition to F ) and current density everywhere in space. Determine the surface-charge density and show that the electric dipole moment of the sphere is p = 4π0 σa3 F/(σ + 2σ 0 ). (b) Show that the total current flowing out through the upper hemisphere of the sphere is I=

2σσ 0 · πa2 F σ + 2σ 0

Calculate the total power dissipation outside the sphere. Using the lumped circuit relations, P = I 2 Re = IVe , find the effective external resistance Re and voltage Ve . (c) Find the power dissipated within the sphere and deduce the effective internal resistance Ri and voltage Vi . (d) Define the total voltage through the relation Vt = (Re + Ri )I and show that Vt = 4aF/3, as well as Ve + Vi = Vt . Show that IVt is the power supplied by the “chemical” force.

(a) What’s going on in this problem is that the conductivity has a discontinuity going across the boundary of the sphere, but the current density must be constant there, which means there must an electric field discontinuity in inverse proportion to the conductivity discontinuity. To create this electric field discontinuity, there has to be some surface charge on the sphere, and this charge gives rise to extra fields both inside and outside the sphere. Since there is no charge inside or outside the sphere, the potential in those two regions satisfied the Laplace equation, and may be expanded in Legendre polynomials:

11

Homer Reid’s Solutions to Jackson Problems: Chapter 3

for r < a,

Φ(r, θ) = Φin (r, θ) =

∞ X

Al rl Pl (cos θ)

l=0 ∞ X

for r > a,

Φ(r, θ) = Φout (r, θ) =

Bl r−(l+1) Pl (cos θ)

l=0

Continuity at r = a requires that Al al = Bl a−l+1

Bl = a2l+1 Al

→

so Φ(r, θ) =

(

Φin (r, θ) =

P∞

Φout (r, θ) =

l=0

P∞

Al rl Pl (cos θ),

l=0

Al a

r a.

(16)

Now, in the steady state there can be no discontinuities in the current density, because if there were than there would be more current flowing into some region of space than out of it, which means charge would pile up in that region, which would be a growing source of electric field, which would mean we aren’t in steady state. So the current density is continuous everywhere. In particular, the radial component of the current density is continuous across the boundary of the sphere, i.e. Jr (r = a− , θ) = Jr (r = a+ , θ). (17) Outside of the sphere, Ohm’s law says that J = σ 0 E = −σ 0 ∇Φout . Inside the sphere, there is an extra term coming from the chemical force: ˆ = σ(−∇Φ + F k). ˆ J = σ(E + F k) in Applying (17) to these expressions, we have   ∂ 0 ∂ + F cos θ = −σ Φout σ − Φin ∂r ∂r r=a r=a

Using (16), this is

F P1 (cos θ) −

∞ X l=0

lAl al−1 Pl (cos θ) =



σ0 σ

X ∞ (l + 1)Al al−1 Pl (cos θ). l=0

Multiplying both sides by Pl0 (cos θ) and integrating from −π to π, we find  0 σ 2A1 (18) F − A1 = σ

Homer Reid’s Solutions to Jackson Problems: Chapter 3

12

for l=1, and −lAl =



σ0 σ



(l + 1)Al

(19) (20)

for l 6= 1. Since the conductivity ratio is positive, the second relation is impossible to satisfy unless Al = 0 for l 6= 1. The first relation becomes σ A1 = F. σ + 2σ 0 Then the potential is Φ(r, θ) =

(

σ σ+2σ 0 F r cos θ,

ra

cos θ,

(21)

The dipole moment p is defined by Φ(r, θ) →

1 p·r 4π0 r3

as r → ∞.

(22)

The external portion of (21) can be written as Φ(r, θ) =

F a3 z σ σ + 2σ 0 r3

and comparing this with (22) we can read off σ ˆ F a3 k. p = 4π0 σ + 2σ 0 The electric field is found by taking the gradient of (21): ( σ ˆ − σ+2σ ra σ+2σ 0 F r

The surface charge σs (θ) on the sphere is proportional to the discontinuity in the electric field: σs (θ) = 0 [Er (r = a+ ) − Er (r = a− )] 30 σ = F cos θ. σ + 2σ 0 (b) The current flowing out of the upper hemisphere is just Z Z ˆ · dA J · dA = σ (Ein + F k)  Z π/2 Z 2π  σ F cos θ sin θ a2 dφ dθ =σ 1− σ + 2σ 0 0 0 σσ 0 2 · πa F =2 σ + 2σ 0

(23)

13

Homer Reid’s Solutions to Jackson Problems: Chapter 3

The Ohmic power dissipation in a volume dV is dP = σE 2 dV

(24)

To see this, suppose we have a rectangular volume element with sides dx, dy, and dz. Consider first the current flowing in the x direction. The current density there is σEx and the cross-sectional area is dydz, so I = σEx dydz. Also, the voltage drop in the direction of current flow is V = Ex dx. Hence the power dissipation due to current in the x direction is IV = σEx2 dV . Adding in the contributions from the other two directions gives (24). For the power dissipated outside the sphere we use the expression for the electric field we found earlier: Z ∞ Z π Z 2π Pout = σ 0 E 2 (r, θ, φ)r2 sin θ dφ dθ dr a

0

0

2 Z ∞Z π 1 σ 2 6 0 F a (4 cos2 θ + sin2 θ) sin θ dθ dr = 2πσ 4 σ + 2σ 0 r 0 a  2 8π 0 σ = σ F 2 a3 3 σ + 2σ 0 

Dividing by (23), we find the effective external voltage Ve : Ve = Pout /I =

4 σ aF · 3 σ + 2σ 0

and the effective external resistance: 2 . Re = Pout /I 2 = 3πaσ 0 (c) The power dissipated inside the sphere is Pin = σ

Z

0

ˆ 2 dV = (E + F k)

4σσ 2 F2 (σ + 2σ 0 )2 0

Z

dV

16σσ 2 = πa3 F 2 3(σ + 2σ 0 )2

Since we’re in steady state, the current flowing out through the upper hemisphere of the sphere must be replenished by an equal current flowing in through the lower half of the sphere, so to find the internal voltage and resistance we can just divide by (23): 8 σ0 Vi = Pin /I = aF 3 σ + 2σ 0 4 Ri = Pin /I 2 = . 3πaσ

14

Homer Reid’s Solutions to Jackson Problems: Chapter 3

(c) (Re + Ri )I =

2 3πa

(Vi + Ve ) =



1 2 + σ0 σ



·

2σσ 0 4 πa2 F = aF σ + 2σ 0 3

4aF 4 σ + 2σ 0 = aF 3(σ + 2σ 0 ) 3

Problem 3.17 The Dirichlet Green function for the unbounded space between the planes at z = 0 and z = L allows discussion of a point charge or a distribution of charge between parallel conducting planes held at zero potential. (a) Using cylindrical coordinates show that one form of the Green function is G(x, x0 ) = − ∞ ∞ X X

e

im(φ−φ0 )

n=1 m=−∞

sin

 nπz  L

sin



1 πL

nπz 0 L

× 

Im

 nπρ0 


L

(b) Show that an alternative form of the Green function is G(x, x0 ) = − Z ∞ X

m=−∞

1 × 2π

∞ 0

dk eim(φ−φ ) Jm (kρ)Jm (kρ0 ) 0

sinh(kz< ) sinh[k(L − z> )] . sinh(kL)

In cylindrical coordinates, the solutions of the Laplace equation look like linear combinations of terms of the form Tmk (ρ, φ, z) = eimφ Z(kz)Rm (kρ).

(25)

There are two possibilities for the combination Z(kz)Rm (kρ), both of which solve the Laplace equation: Z(kz)Rm (kρ) = (Aekz + Be−kz )[CJm (kρ) + DNm (kρ)]

(26)

Z(kz)Rm (kρ) = (Aeikz + Be−ikz )[CIm (kρ) + DKm (kρ)].

(27)

or

The Green’s function G(x; x0 ) must be a solution of the Laplace equation, and must thus take one of the above forms, at all points x0 6= x. At x0 = x, G must be continuous, but have a finite discontinuity in its first derivative.

.

Homer Reid’s Solutions to Jackson Problems: Chapter 3

15

Furthermore, G must vanish on the boundary surfaces. These conditions may be met by dividing space into two regions, one on either side of the source point x, and taking G to be different linear combinations of terms T (as in (25)) in the two regions. The question is, in which dimension (i.e., ρ, z, or φ) do we define the two “sides” of the source point? (a) The first option is to imagine a cylindrical boundary at ρ0 = ρ, i.e. at the radius of the source point, and take the inside and outside of the cylinder (i.e., ρ0 < ρ and ρ0 > ρ) as the two distinct regions of space. Then, within each region, the entire range of z must be handled by one function, which means this one function must vanish at z = 0 and z = L. This cannot happen with terms of the form (26), so we are forced to take Z and R as in (27), with B = −A and k restricted to the discrete values kn = nπ/L. Next considering the singularities of the ρ functions in (27), we see that, to keep G finite everywhere, for the inner region (ρ0 < ρ) we can only keep the Im (kρ) term, while for the outer region we can only keep the Km (kρ) term. Then G(x; x0 ) will consist of linear combinations of terms T as in (25) subject to the restrictions discussed above: (P imφ0 sin(kn z 0 )Im (kn ρ0 ), ρ0 < ρ 0 mn Amn (x)e G(x; x ) = P imφ0 0 0 sin(kn z )Km (kn ρ ), ρ0 > ρ. mn Bmn (x)e

Clearly, to establish continuity at ρ0 = ρ, we need to take Amk (x) = γmk (z, φ)Km (kρ) and Bmk (x) = γmk (z, φ)Im (kρ), where γmk is any function of z and φ. Then we can write G as X 0 γmk (z, φ)eimφ sin(kz 0 )Im (kρ< )Km (kρ> ). G(x; x0 ) = mk

The obvious choice of γmk needed to make this a delta function in z and φ is γmk = (4/L)e−imφ sin(kz). Then we have G(x; x0 ) =

4 X im(φ0 −φ) e sin(kz) sin(kz 0 )Im (kρ< )Km (kρ> ). L mk

What I don’t quite understand is that this expression already has the correct delta function behavior in ρ, even though I never explicitly required this. To obtain this expression I first demanded that it satisfy the Laplace equation for all points x0 6= x, that it satisfy the boundary conditions of the geometry, and that it have the right delta function behavior in z 0 and φ0 . But I never demanded that it have the correct delta function behavior in ρ0 , and yet it does. I guess the combination of the requirements that I did impose on this thing is already enough to ensure that it meets the final requirement. (b) The second option is to imagine a plane boundary at z 0 = z, and take the two distinct regions to be the regions above and below the plane. In other words, the first region is that for which 0 ≤ z 0 ≤ z, and the second region that for which z ≤ z 0 ≤ L. In this case, within each region the entire range of ρ0 (from 0 to ∞) must be handled by one function. This requirement excludes terms of the form

Homer Reid’s Solutions to Jackson Problems: Chapter 3

16

(27), because Km is singular at the origin, while Im is singular at infinity, and there is no linear combination of these functions that will be finite over the whole range of ρ0 . Hence we must use terms of the form (26). To ensure finiteness at the origin we must exlude the Nm term, so D = 0. To ensure vanishing at z 0 = 0 we must take A = −B, so the z 0 function in the region 0 ≤ z 0 ≤ z is proportional to sinh(kz 0 ). To ensure vanishing at z 0 = L we must take A = −Be−2kL , so the z 0 function in the region z ≤ z 0 ≤ L is proportional to sinh[k(z 0 − L)]. With these restrictions, the differential equation and the boundary conditions are satisfied for all terms of the form (25) with no limitation on k. Hence the Green’s function will be an integral, not a sum, over these terms:  P∞ R ∞ imφ0 sinh(kz 0 )Jm (kρ0 ) dk, 0 ≤ z0 ≤ z 0 m=0 R0 Am (k, ρ, φ, z)e P∞ G(x ; x) = 0 ∞ imφ sinh[k(z 0 − L)]Jm (kρ0 ) dk, z ≤ z0 ≤ L m=0 0 Bm (k, ρ, φ, z)e

Problem 3.18 The configuration of Problem 3.12 is modified by placing a conducting plane held at zero potential parallel to and a distance L away from the plane with the disc insert in it. For definiteness put the grounded plane at z = 0 and the other plane with the center of the disc on the z axis at z = L. (a) Show that the potential between the planes can be written in cylindrical coordinates (z, ρ, φ) as Z ∞ sinh(λz/a) Φ(z, ρ) = V dλJ1 (λ)J0 (λρ/a) . sinh(λL/a) 0 (b) Show that in the limit a → ∞ with z, ρ, L fixed the solution of part a reduces to the expected result. Viewing your result as the lowest order answer in an expansion in powers of a−1 , consider the question of corrections to the lowest order expression if a is large compared to ρ and L, but not infinite. Are there difficulties? Can you obtain an explicit estimate of the corrections? (c) Consider the limit of L → ∞ with (L − z), a and ρ fixed and show that the results of Problem 3.12 are recovered. What about corrections for L  a, but not L → ∞? (a) The general solution of the Laplace equation in cylindrical coordinates with angular symmetry that vanishes at z = 0 is Z ∞ Φ(ρ, z) = A(k)J0 (kρ) sinh(kz) dk. (28) 0

17

Homer Reid’s Solutions to Jackson Problems: Chapter 3

Multiplying both sides by ρJ0 (k 0 ρ) and integrating at z = L yields Z ∞  Z ∞ Z ∞ 0 0 ρJ0 (k ρ)Φ(ρ, L) dρ = A(k) sinh(kL) ρJ0 (k ρ)J0 (kρ) dρ dk 0 0 0   Z ∞ 1 0 δ(k − k ) dk A(k) sinh(kL) = k 0 1 = 0 A(k 0 ) sinh(k 0 L) k so Z ∞ k ρJ0 (kρ)Φ(ρ, L) dρ sinh(kL) 0 Z a Vk = ρJ0 (kρ) dρ sinh(kL) 0 Z ka V uJ0 (u) du. = k sinh(kL) 0

A(k) =

(29)

I worked out this integral earlier, in Problem 3.12: Z x uJ0 (u) du = xJ1 (x). 0

Then (29) becomes A(k) =

V · (ka)J1 (ka) k sinh(kL)

and (28) is ∞

sinh(kz) dk sinh(kL) Z0 ∞ sinh(λz/a) dλ. =V J1 (λ)J0 (λρ/a) sinh(λL/a) 0

Φ(ρ, z) = V

(b) For x  1,

Z

aJ1 (ka)J0 (kρ)

(30)

1 J0 (x) → 1 − x2 + · · · 4

and for x  1 and y  1,

  x + 16 x3 + · · · x sinh(x) 1 2 2 = = (x − y ) + O(x4 ) 1 + sinh(y) y 6 y + 16 y 3 + · · ·

With these approximations we may expand the terms containing a in (30): !  2 !    2 sinh(λz/a) 1 λρ z 1 λ J0 (λρ/a) ≈ 1− (x2 − y 2 ) (31) 1+ sinh(λL/a) 4 a L 6 a " #  2   1 2 z λ 1 2 2 = (L − z ) + ρ + · · · 1− (32) L a 6 4

Homer Reid’s Solutions to Jackson Problems: Chapter 3

Then the potential expansion (30) Z ∞ 1 Vz J1 (λ) dλ − 2 Φ(ρ, z) = L a 0

18

becomes   Z ∞ 1 2 1 2 2 2 λ J1 (λ) dλ + · · · (L − z ) + ρ 6 4 0

The first integral evaluates to 1, so for a infinite the potential becomes simply Φ(z) = V z/L. This is just what we expect to get for the potential between two infinite sheets, one grounded and the other at potential V. The second integral, unfortunately, has a bit of an infinity problem. It’s not hard to see where the problem comes: I derived the expansion above based on the premise that λ/a is small, but the integral goes over all λ up to ∞, so for any finite a the expansions eventually become invalid in the integral. I’m still trying to work out a better procedure for estimating corrections for finite a. (c) In this part we’re interested in taking L → ∞ and looking at the potential a fixed distance away from the plane with the circular insert. Calling the fixed distance z 0 , the z coordinate of the point we’re interested in is L − z 0 . We have sinh k(L − z 0 ) sinh(kL) cosh(−kz 0 ) + cosh(kL) sinh(−kz 0 ) = sinh kL sinh kL = cosh(kz 0 ) − coth(kL) sinh(kz 0 )

(33)

Now, coth(kL) differs significantly from 1 only for kLa . 1, in which region kz 0 . z/L  1, so cosh(kz 0 ) ≈ 1 and sinh(kz 0 ) ≈ 0. By the time k gets big enough that kz 0 is starting to get significant, coth(kL) has long since started to look like 1, so the two terms in (33) add directly. The result is that, for all k, (33) can be approximated as exp(−kz 0 ). Then (30) becomes Z ∞ 0 Φ(ρ, z) = aV J1 (ka)J0 (kρ)e−kz dk 0

as we found in Problem 3.12.

Solutions to Problems in Jackson, Classical Electrodynamics, Third Edition Homer Reid August 6, 2000

Chapter 3: Problems 19-27 Problem 3.19 Consider a point charge q between two infinite parallel conducting planes held at zero potential. Let the planes be located at z = 0 and z = L in a cylindrical coordinate system, with the charge on the z axis at z = z0 , 0 < z0 < L. Use Green’s reciprocation theorem of Problem 1.12 with Problem 3.18 as the comparison problem. (a) Show that the amount of induced charge on the plate at z = L inside a circle of radius a whose center is on the z axis is given by QL (a) = −

q Φ(z0 , 0) V

(b) Show that the induced charge density on the upper plate can be written as Z ∞ sinh(kz0 ) q dk σ(ρ) = − kJ0 (kρ) 2π 0 sinh(kL) (c) Show that the charge density at ρ = 0 is σ(0) =

 πz  −πq 0 sec2 2 8L 2L

(a) Green’s reciprocation theorem says that Z Z Z Z σΦ0 dA. ρΦ0 dV + σ 0 Φ dA = ρ0 Φ dV + V

V

S

1

S

(1)

2

Homer Reid’s Solutions to Jackson Problems: Chapter 3

We’ll use the unprimed symbols to refer to the quantities of Problem 3.18, and the primed symbols to refer to those of Problem 3.19. Then ρ(r, z) = 0 σ(r, z) =? Φ(r, z) = 0,

z=0

= 0, = V,

z = L and r > a z = L and r < a

=V

Z

∞

dk aJ1 (ak)J0 (rk) 0

sinh(kz) sinh(kL)

0 < 0 0 G(ρ, φ; ρ , φ ) = sin − sin 2mπ a2 ρ> β β m I seem to be off by a factor of 2 here, but I can’t find where.


 
  

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