Electric field problem - Plastic Slab

1. Jan 20, 2005

meteorologist1

A plastic slab of thickness t has a uniform free charge density, +rho, embedded inside, and also one surface has a surface charge of -sigma. Find the electric fields here and sketch as a function of distance from one surface. Also find the potential as a function of distance, sketch results. [Ignore issues of dielectric polarizability, the function of the plastic is simply to fix the charges in place]

Here is what I did: I draw a Gaussian pillbox with area A. Only the top and bottom caps contribute where E and dA are parallel. The charge enclosed is: (rho)(A)(t) + (-sigma)(A) = A((rho)(t) - sigma)

By Gauss's Law, we have: 2EA = [A((rho)(t) - sigma)] / epsilon, so therefore, E = ((rho)(t) - sigma) / 2epsilon

I'm not sure what I did wrong here since the question asks to plot E as a function of r. In my case I would have a constant line. What did I do wrong? Thanks.

2. Jan 20, 2005

meteorologist1

Maybe I also have to find the E field within the slab... That answer would depend on r. Any thoughts?

3. Jan 20, 2005

learningphysics

Yes. You've found the field outside the slab. The field inside depends on distance from edge.