What is the Surface Charge Density for a Spherical Cavity?

[Clancy]

Homework Statement

A sphere with excess charge of 1.5 x10 -7 Coulombs, An outer radius of 2.5cm and inner radius of 1.5 cm ( a cavity ). The question is to find the surface charge density on the inside surface.

I'm totally at a loss on this one and couldn't find how to solve it.

Homework Equations

Well surface charge density would be C/area of inner sphere.

The Attempt at a Solution

I couldn't at all figure out how this is done and didnt fiond anythig on it in HRW any help please?

 

[rl.bhat]

If the sphere is made of good conductor, there will be no charge on the inner surface of the sphere.

 

[Millimole]

Is this an insulating sphere or a conducting sphere?