# Homework Help: Electrostatic equilibrium and Gauss' law

1. Feb 16, 2006

### Kenny Lee

A conducting spherical shell of ineer radius a and outer radius b carries a net charge Q. A point charge q is placed at the center of this shell. Determine the surface charge densit on (a) the ineer surface of the shell and (b) the outer surface of the shell.

I'm not sure of my reasoning. For part (a), I argued that at the inner surface
E = q/ (4*pi*a^2*epsilon) from Gauss' law.

And equated that to the formula for electric field, right outside a conductor in electro-static equilibrium i.e.
E = (surface charge density)/ epsilon.

So I get q/ (4*pi*a^2). Is that correct? If not, what would the correct method be.

PS I apologize for the ugly equations. Still learning how to use the equation editor...

2. Feb 17, 2006

### phucnv87

http://photo-origin.tickle.com/image/69/3/6/O/69362752O719777534.jpg [Broken]

Last edited by a moderator: May 2, 2017
3. Feb 17, 2006

### Astronuc

Staff Emeritus
A charge q would induced an equal and opposite charge on the inner surface of a 'conducting' sphere, and the same charge on the outer surface. The areas are different.

However, the sphere also has a net charge Q. How would this be distributed.

And then think of superposition.

4. Feb 17, 2006

### Kenny Lee

Thanks. So my answer was wrong? Yea, come to think of it, I only accounted for the charge q and not the sphere...I'll have another go at it.