Problem Set 2

Problem Set 2

Due Thursday, September 17, by 5pm

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Problem 1 [10 points]
Prove this mean value theorem: For charge-free space in the electrostatic limit, the value of the electrostatic potential φ at any point in space is equal to the average of the potential over the surface of any sphere centered on that point.
Hint: Use the fact that where there are no charges ∇²φ=0. Then relate the averge φ over the surface of a shpere to the flux of electric field through the surface.
Functions which satisfy Laplace's equation are called harmonic functions; harmonic functions obey the above mean value theorem (in other words, you have to prove this mean value theorem, you cannot simply say it is true because harmonic functions obey it!).
Problem 2 [10 points]
a) In lecture we solved the problem of the electric field from a spherical shell of radius R with uniform surface charge density σ=q/(4πR²). Consider now the problem where this shell is of finite thickness d. That is, there is a uniform charge density ρ in a spherical shell of finite thickness from radius R to radius R+d, such that the total charge on this shell is q. Find the potential φ(r) by solving Poisson's equation (there may be easier ways to do it, but do it this way!), then take the gradient to get E(r). Sketch φ(r) and E(r) vs r. Now take the limit d→0 keeping ρd=σ constant. Compare your result with the case of the infinitesmally thin shell done in lecture.
b) Consider an infinitesmally thin spherical shell of radius R with a total charge q uniformly distributed over its surface, and a concentric infinitesmally thin spherical shell of radius R+d with total charge -q uniformly distributed over its surface. Find the potential φ(r) by solving Poisson's equation for this geometry, then take the gradient to get E(r). Sketch φ(r) and E(r) vs r. Now take the limit d→0 keeping qd constant. What do you find? This is the limit of an infinitesmally thin dipole layer.