# Potential question

Gold Member

## Homework Statement

Electric charges are distributed on a spherical surface of radius a so as to produce the potential

$$\Phi(\vec r)=A(x^2-y^2)+Bx$$

in the region r<a. Find the potential in the region r>a (hint: use the table of spherical harmonics).

## Homework Equations

I am unsure, but I think I should start with the following general potential expression (solution to laplace's eqn in terms of the spherical harmonics Y).

$$\Phi(r, \theta, \phi)= \sum_{l=0}^{\infty} \sum_{m=-l}^l \left[ A_{lm}r^l+B_{lm}r^{-(l+1)} \right] Y_l^m(\theta, \phi)$$

## The Attempt at a Solution

If I am to use the above potential eqn, I need to utilize boundary conditions to find the coefficients A and B, but the surface potential is not specified so I'm not sure where to start...can someone point me in the right direction?

First write out the coefficients in that expansion for r<a. This can be done easily by expanding out the x and y in their angular components and finding combinations of spherical harmonics that match them.

Once you have that, you will need BC's. You know at the surface of the sphere the potential is continuous. You also know how the potential behaves at infinity. This should be enough information to solve for that expansion in the region r>a.

Last edited:
Gold Member
i'm having trouble finding the combination of spherical harmonics which describes:

$$\Phi(\vec r)=A(x^2-y^2)+Bx=Ar^2(cos^2 \theta -sin^2 \theta )+Brcos \theta=Ar^2cos 2 \theta + Brcos \theta$$

This isn't 2 dimensions. So there should be a $$\phi$$ term in there. Remember:

$$x = r cos(\phi)sin(\theta)$$
$$y = r sin(\phi)sin(\theta)$$
$$z = r cos(\theta)$$

Also this might be useful:

$$sin(\phi) = \frac{e^{i\phi}-e^{-i\phi}}{2i}$$
$$cos(\phi) = \frac{e^{i\phi}+e^{-i\phi}}{2}$$