# Homework Help: What's the problem asking me to find?

1. Sep 14, 2011

### pyroknife

1. The problem statement, all variables and given/known data
A conducting spherical shell of inner radius b and outer radius c is concentric with a small metal sphere of radius a<b. The metal sphere has a positive charge Q while the total charge on the conducting spherical shell is -Q. Assume the potential is 0 very far from all charges.

a. What is the electric potential of the metal spherical shell?
b. what is the electric potential of the metal sphere?

I'm a little confused about what they're asking from the questions. for (a) am i supposed to make 3 electric potential equations at a<r<b, b<r<c, and r>c? and for just r>a for part b?

2. Sep 14, 2011

### pyroknife

I did part a can someone check my answer

Note this is only V for the shell only. Since the sphere inside is charged +Q that means the inner radius of the shell is -Q and the outer radius thus has to be 0.
for r>c V=kqinside/r + kqoutside/r = k(-Q)/r + 0= -kQ/r
for b<r<c V=kqinside/r+kqoutside/r=k(-Q)/r
for r<b V=-kQ/r

3. Sep 15, 2011

### Redbelly98

Staff Emeritus
For r>b, think carefully about what qinside is. It is the sum of all the charge within r.

4. Sep 15, 2011

### pyroknife

ohh so it would be 0 inside the conducting shell of -Q charge and 0 within a<r<b b/c the +Q and -Q charge cancels them out?

5. Sep 15, 2011

### Redbelly98

Staff Emeritus
The +Q and -Q charges cancel for r>b, not a<r<b.

They want the potential at the spherical shell (b<r<c), and also at the inner metal sphere (r≤a).