Homework Assignment 4 Solution Set
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1 Homework Assignment 4 Solution Set PHYCS February, 24 Problem (Griffiths 2.37 If the plates are sufficiently large the field near them does not depend on d. The field between the plates is zero (the fields from each plate are equal in strength but oppositely directed, and the field just outside either plate is the sum of the contributions from each plate. Therefore, the pressure on each plate is P 2 σ( E outside + E between 2 σ(2 σ 2ɛ + ˆd σ2 2ɛ ˆd Q2 2ɛ A 2 ˆd. Of course, this is the same as considering the force on one plate due to an electric field E σ 2ɛ ˆd due to the other plate. Problem 2 (Grifffiths optional Let s consider the sphere to be centered at the origin with its equator in the x y plane. The electrostatic pressure may be thought of as being due to the average field surrounding the charge on any given element of surface. The field is zero inside the sphere, and outside the field is E Q 4πɛ R ˆr. So, for any 2 given element of surface the pressure will be d P σ 2 ( E outside + E inside σ 2 ( Q ˆr + 4πɛ R2 4πɛ 8πR 4 ˆr 32π 2 ɛ R 4 ˆr.
2 Since the pressure is in the radial direction we know that only the components in the z direction will contribute to the overall force. Integrating the z components of the pressure over the upper hemisphere gives F P da z hemisphere π 2 2π dθ 32π 2 ɛ R 2 π 32πɛ R 2. dφ( 32π 2 ɛ R 4 ˆr (R2 sin θˆr cos θ Problem 3 (Griffiths 2.34(a The field due to this configuration is zero everywhere except for between the two shells. Thus, U ɛ E 2 dτ 2 allspace ɛ b ( 2 q 2 a 4πɛ r 2 4πr 2 dr ( q2 2 4πɛ a. b Problem 4 (Griffiths 3.26 To find the approximate potential (in the context of this chapter, anyway we should use the multipole expansion and keep only the first non-vanishing term. The monopole gives V ( r 4πɛ r 4πɛ r 2π 4πɛ r π 2 4πɛ r. V ρ( r dτ dr dθ dφ k R r 2 (R 2r sin θ r 2 sin θ π dr dθ kr(r 2r sin 2 θ dr kr(r 2r The total charge is zero, so the monopole term in the potential is zero. Thus, we should next check the dipole term. We have V ( r 4πɛ r 2 r cos Θρ( r dτ V 2
3 where Θ is the angle between r and r. In this case, we have been asked to find the potential at a point on the z axis, so Θ θ. Therefore V (z 4πɛ z 2 2π 4πɛ z 2. dr dθ dφ r cos θ k R r 2 (R 2r sin θ r 2 sin θ π dr dθ kr R(R 2r sin 2 θ cos θ This, too, is zero (this time because of the θ integral! So, we continue with V 2 (z 4πɛ z 3 4πɛ z 3 2πkR 5 4πɛ 6z 3 πkr5 92ɛ z 3. ( 3 dr dθ dφ r 2 2 cos2 θ 2 ( 3 dr dθ dφ 2 cos2 θ 2 π sin 2 θ dθ ( 3 2 cos2 θ 2 k R r 2 (R 2r sin θ r 2 sin θ kr(rr 2 2r 3 sin 2 θ This, then, is the approximate potential for points far from our charge distribution. Problem 5 (Griffiths 3.3 For a pure dipole the potential and field are given in the book, and they are exact (they are due to the dipole moment only, since all other terms in the multipole expansion vanish. We can easily calculate a F q (a,, q E(a,, qp 4πɛ a 3 (2 cos π 2 ˆr + sin π 2 ˆθ qp 4πɛ a 3 ˆθ. Note: at the point (a,,, ˆθ ẑ. b F q (,, a q E(,, a Note: at the point (,, a, ˆr ẑ. qp (2 cos ˆr + sin ˆθ 4πɛ a3 qp 2πɛ a 3 ˆr. 3
4 c The path one takes to move the charge from one point to another should not change the final answer. The fact that we are using an approximate potential does not change the fact that we still must satisfy Laplace s equation, and the field is still a conservative one. Therefore, W U U (,,a U (a,, qp cos 4πɛ a 2 qp cos π 2 4πɛ a 2 qp 4πɛ a 2. Problem 6 (Griffiths optional For such a distribution of point charges the integrals in the multipole expansion become sums over the point charges because ρ( r q δ( r r + q 2 δ( r r 2 + q 3 δ( r r 3. Therefore, the first term in the multipole expansion is The second term is V ( r 4πɛ r (q + q 2 + q 3 4πɛ q r. V ( r 4πɛ r 2 (q r cos Θ + q 2 r 2 cos Θ 2 + q 3 r 3 cos Θ 3. Here the only tricky thing is figuring out how to write Θ i. However, we can exploit the fact that r r i rr i cos Θ i to write V ( r 4πɛ r 2 ˆr (q r + q 2 r 2 + q 3 r 3 (sin θ cos φ, sin θ sin φ, cos θ (q(,, a q(, a, q(, a,. 4πɛ r2 Therefore, including only the two lowest terms in the multipole expansion, the potential from this three charge configuration is approximately V ( r V + V 2 [ q 4πɛ r + q ] r 2 (a cos θ a sin θ sin φ + a sin θ sin φ ( q 4πɛ r + q r 2 a cos θ. Apparently, the dipole contribution from the two charges on the y axis exactly cancel and we are left with only the dipole term from the charge on the z axis. 4
5 To find the field we simply take the gradient. Thus, E( r V ( r ( V r ˆr + r q 4πɛ V θ ˆθ + [( r 2 + a cos θ r 3 r sin θ V φ ˆφ ˆr + a sin θ ˆθ r 2 ] Problem 7 (Griffiths 3.6 We have seen that the potential (and, therefore, the field of such a configuration may be modeled by the placement of image charges that are the mirror-images (across the x y plane of the existing charges. Thus, the charge +q at z 3d experiences a force due to the field of three other point charges. It may be written as F ( q 2q 4πɛ (2d ( q 2 ẑ 72 4πɛ d 2q (4d 2 q (6d 2 ẑ Problem 8 (Griffiths 3. As seen in class, the image charges should be located as though they were the images in mirrors in the x and y planes. Thus, there are three images in all: an image q at the points (a, b and ( a, b, and an image q at the point ( a, b. The potential, then, is the potential of four point charges: V (x, y q ( 4πɛ (x a2 + (y b + 2 (x + a2 + (y + b 2 (x + a2 + (y b 2 (x a2 + (y + b 2 The force on the charge q at the point (a, b will be the same as if it were due to the three image charges only. So, F q (a, b [( q 2 ( 4πɛ (2a 2 + 2a ˆx + ((2a 2 + (2b (2b 2 + 2b ((2a 2 + (2b The potential due to the three images charges after q is brought to the point (a, b is [ ] V (a, b q 4πɛ 2a 2b +. (2a2 + (2b 2 So, the work required to bring the charge in from infinity is half the energy due to this potential (because the image charges accumulate for free. Therefore, W q 2 [ 4πɛ 4 a ] b +. a2 + b 2 5 ] ŷ.
6 This method does not work for any arbitrary angle of intersection of two conducting planes. In order for the reflections to terminate at some finite number of image charges, the set of the real charge and all of its reflections in the conducting planes must form an abelian group. This is fancy math-talk for saying that you should be able to get from the charge to any of it s images by successive application of a discreet set of operators. In our case, the operator that takes you from the real charge to one of its images is more than spacial. It should consist of a rotation about the point of intersection of the planes as well as a reversal of charge (the first image charge has the same sign as the real charge, but the next has opposite sign, etc.. This is a very formal proof, but your reasoning may be more straight forward and less rigorous. The short story is, only those situations in which the angle of intersection of the planes is an even divisor of 36 will be solvable via the image charge method. So, for the image method to work we require θ intersection 36 ; n {, 2, 3, }. 2n 6
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