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Electric Fields (Uniformly Charged Plates)

  1. Jun 5, 2016 #1
    1. The problem statement, all variables and given/known data

    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?


    2. Relevant equations

    Gauss' Law

    3. The attempt at a solution

    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!
  2. jcsd
  3. Jun 5, 2016 #2


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    Staff: Mentor

    Your image isn't visible; it may be behind a required login. Can you UPLOAD it instead?
  4. Jun 5, 2016 #3
    Sorry about that! Here it is.

  5. Jun 5, 2016 #4


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    Staff: Mentor

    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.
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