# Homework Help: Finding electric field

1. Nov 9, 2009

### du_uk

1. The problem statement, all variables and given/known data
A spherical hole is located inside a uniformly charged sphere of charge density p. The centre of the hole is at a distance a from the centre of the sphere, and the radii of the sphere and the hole are given by R and R' respectively. Determine the electric field strength E inside the hole.

2. Relevant equations

3. The attempt at a solution
I think I need to use Gauss's law the find the electric field around this red surface:
http://img20.imageshack.us/img20/6870/electroqy.jpg [Broken]
and since there is symmetry, integrate around 0 to 2(pi) wrt the extra (third dimensional) coordinate.
Then minus the electric field for a sphere outside the charge.

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

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: May 4, 2017
2. Nov 10, 2009

### gabbagabbahey

Hi du_uk, welcome to PF!

I'm not sure exactly what you mean here, but no, you are not going in the right direction. What exactly is the symmetry you are referring to here?

Instead, take advantage of the superposition principle...what happens if you place an object of charge density $-\rho$ inside a larger object of charge density $+\rho$?

Last edited by a moderator: May 4, 2017
3. Nov 10, 2009

### Phrak

More to the point, place a spherical charge density $-\rho$ inside a larger spherical charge density $+\rho$. What are the forces inside the smaller sphere?