# Spherical capacitor in a uniform magnetic field

1. Feb 26, 2013

1. The problem statement, all variables and given/known data
Two concentric spherical shells carrying uniformly distributed charges +Q and -Q at radii a and b, respectively (b>a). Now, they are immersed in a uniform magnetic field pointing along the z-axis. Find the angular momentum of the fields.

2. Relevant equations
No need. (I know them; don't need help using them; I can post them if somebody really wants).

3. The attempt at a solution
I understand the math and the equations that must be used. I simply need to find the E and B fields. I am confused as to what happens when placing the spheres in the uniform magnetic field. I know that typically for a spherical capacitor, the only non-zero charge will be a < r < b. I don't believe this will change when placed in the uniform magnetic field, and I think the E field will remain as E=k*Q/r^2 pointing radially outward. However, what will the B-field be within the spherical capacitor? It doesn't seem likely that it would remain the same (in the z-direction); but I am not sure how it would change. Also, since E is zero at r<a and r>b, I don't need to worry about the B-field in those locations to calculate the angular momentum of the fields/

My confusion is simply on the physical principles. Can somebody help clear up what is happening for the fields at a<r<b? If any of my understanding above is incorrect, please let me know that as well.

I appreciate any attempt to help (and apologize if this is the wrong section).

2. Feb 27, 2013

### BruceW

Yep, sounds good. (Except I guess you mean that the only non-zero electric field is within that region).

It will depend on what material is between the concentric shells. But since they don't mention what that material is, you can assume that it is effectively vacuum.

I agree.