# Homework Help: Conducting hollow sphere

1. Feb 20, 2010

### reising1

1. The problem statement, all variables and given/known data

Consider a conductor in the shape of a hollow sphere with inner radius A and outer radius B. The sphere has a net positive charge +q.

A negative point charge of value -2q is placed at the center of the sphere (r=0). Determine the electric field in the three regions of space:
i) r < A
ii) A < r < B
iii) r > B

3. The attempt at a solution

Since this is a conductor, I thought part (i) and (ii) both have 0 Electric Field because the charge on the inside of the sphere will move to the surface, since this is a conductor.

But this is wrong. I need some help here.

2. Feb 20, 2010

### xcvxcvvc

There is charge placed in the center of the hollow sphere.

3. Feb 20, 2010

### reising1

Yes, but since it is a conductor, would the charge not immediately move to the inner surface, thus giving no charge on the inside of the conductor?

4. Feb 20, 2010

### xcvxcvvc

we're dealing with electrostatics here - statics as in not moving. But even if you wanted to think of charge as moving, the point charge is placed inside of a conductor - and you've already told me the E-field inside of a conductor is zero. From where would the force to move it come from?

5. Feb 20, 2010

### reising1

Okay, I understand. So in terms of electrostatics, the answer to part i would be
E = (-2q)/(4pi * epsilon not * r^2)

Now how would I approach part (ii)

6. Feb 20, 2010

### xcvxcvvc

Well, in part two it asks for the E-field inside of a solid piece of metal. If there were any E-field, a current would flow to transport the charge until the E-field ceased. Therefore, I'd conclude it to be zero. I could be wrong here, however.