# Electric Fields (Uniformly Charged Plates)

## Homework Statement

In the figure two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have excess surface charge densities of opposite signs and magnitude 7.76 × 10-22 C/m2. What is the magnitude of the electric field at points (a) to the left of the plates, (b) to the right of them, and (c) between them?

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Gauss' Law

## The Attempt at a Solution

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So I understand how to solve this problem and I am getting the correct solution but I was hoping for some clarification When using the pillbox method for gauss' law you consider two areas, the top and bottom part of the cylinder that is penetrating the plane. In parts a and b we still consider both areas but why wouldn't we just consider the area to the left or area to the right? Hopefully this makes sense!

gneill
Mentor

Sorry about that! Here it is.

gneill
Mentor
So I understand how to solve this problem and I am getting the correct solution but I was hoping for some clarification When using the pillbox method for gauss' law you consider two areas, the top and bottom part of the cylinder that is penetrating the plane. In parts a and b we still consider both areas but why wouldn't we just consider the area to the left or area to the right? Hopefully this makes sense!
You need a volume that encloses the charge. If you placed one face so that it is coincident with the charge, geometrically the charge would not be enclosed by the volume.