# Homework Help: Find electric Field through 3 plates using Gausses Law

1. Feb 13, 2013

### Broem

1. The problem statement, all variables and given/known data
The figure shows three very thin parallel planes of charge with equal separation 8 cm between them. The charge density of the middle plane is +4 nC/m2 (nC = "nano Coulomb") and that of the two outer planes is -2nC/m2. Use Gauss's law to solve the following questions. We choose positive electric field along the y-direction.
What is the Electric Field at point C?

2. Relevant equations
E=ρD/ε Electric Field at position D

3. The attempt at a solution
I've tried to Calculate the E at distance between The top Positive ρ and the point C and from the bottom ρ to point C. Should these two be equal?
From top:
(4e-9c/m^2 * 0.004m) / ε to give me field to point C

From bottom:
(2e-9c/m^2 * 0.004m) / ε to give field to point C

I know the field results in a - I've tried to add them to get the answer and subtract.
What am I to do from this point?
Am I using the wrong equation?

Thanks!

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2. Feb 13, 2013

### TSny

The question asks you to use Gauss' law to find the answer. Since you want the field at point C, you should try to construct a Gaussian surface that goes through C. Choose the shape of the Gaussian surface so that you can take advantage of any symmetry in the problem.