# Determining charge from electric potential

1. Feb 18, 2013

### rmcgovern

1. The problem statement, all variables and given/known data
A metal sphere with radius ra = 1.20 cm is supported on an insulating stand at the center of a hollow, metal, spherical shell with radius rb = 9.00 cm. Charge +q is put on the inner sphere and charge -q on the outer spherical shell. The magnitude of q is chosen to make the potential difference between the spheres 500 V, with the inner sphere at higher potential. Calculate q.

2. Relevant equations

ΔV=q/K [(1/ra)-(1/rb)]

3. The attempt at a solution

Solving for q from the eqn above I have reached q= ΔV/[(1/ra)-(1/rb)] * K. By simply plugging in the numbers given I reached a soln of q= 6.23*10^10 C. This answer seemed far too large to be right, and indeed it is not correct. Is this the proper equation to use in this situation? I double checked the math and it appears everything is right, does anyone see where I may have gone wrong? Any help is greatly appreciated.

2. Feb 18, 2013

### TSny

Hi rmcgovern. Welcome to PF!

Did you put the constant K in the correct location in your equation for ΔV?

3. Feb 18, 2013

### rmcgovern

ah! I forgot that K=1/4∏ε0. a rather trivial error. thank you for your help.