# Homework Help: Find the net electric field at the center of the foil

1. Feb 2, 2014

### Lee33

1. The problem statement, all variables and given/known data
A large, thin plastic disk with radius R = 1.1 meter carries a uniformly distributed charge of Q = -7e–5 C. A circular piece of aluminum foil is placed d = 3 mm from the disk, parallel to the disk. The foil has a radius of r = 6 cm and a thickness t = 1 millimeter.

a. Find the net electric field at the center of the foil.
b. Calculate the magnitude q of the charge on the left circular face of the foil.

2. The attempt at a solution

How can I solve this problem? If I have a disk uniformly charged then I know how to calculate the electric field from an observation location but I don't know how to calculate the electric field when there is a foiled disk next to the plastic disk.

2. Feb 2, 2014

### jackarms

This looks like Gauss' Law.

For a it's asking for the electric field inside the foil, and what do you know about the field inside of an isolated conductor?

For b try putting a Gaussian surface into the problem (let's say a cylinder) such that one end passes through the middle of the foil and the other end passes through the charged disk to the other side. Then you can relate the flux through that surface to the charge enclosed by it, which will be charge of the disk that lies inside the cylinder, and the charge on the inner surface of the foil.

3. Feb 2, 2014

### Lee33

I can't use Gauss law. We haven't been taught that yet.