Charge on a cavity wall and Gauss' law

[Jrlinton]

Homework Statement

An isolated conductor has a net charge of +14.0 × 10- 6 C and a cavity with a particle of charge q = +4.30 × 10-6 C. What is the charge (a) on the cavity wall and (b) on the outer surface?

Homework Equations

The Attempt at a Solution

So I understand that B is just adding the charge of the particle and conductor together
=1.8E-5C
I am lost on the solution to part A. I thought that the charge on or within the cavity wall of a gaussian shell was zero but that doesn't seem to be the answer they are looking for

 

[Doc Al]

I am lost on the solution to part A. I thought that the charge on or within the cavity wall of a gaussian shell was zero but that doesn't seem to be the answer they are looking for

Imagine a Gaussian surface within the conducting material that encloses the cavity. What's the total charge?

 

[Jrlinton]

the net charge is zero so the charge on the wall is the negative of the particle charge within

 

[Doc Al]

the net charge is zero so the charge on the wall is the negative of the particle charge within

Exactly.