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Homework Help: Finding field from charge

  1. Jan 24, 2008 #1
    1. The problem statement, all variables and given/known data
    There is an electrical field causing a charge distribution of +Q and -Q on a thin square sheet of conductor with area A. Find the field

    3. The attempt at a solution

    I was wondering how you can find the electric field if you don't know the thickness of the thin metal sheet? I thought about using Gauss's law but I don't know over what surface you could find a constant E.
  2. jcsd
  3. Jan 25, 2008 #2

    Doc Al

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    Staff: Mentor

    Assume that the charges are uniformly distributed over the surfaces of the conductor. (What's the field from a sheet of charge?) What field would such a charge distribution create in the space between them? What must be the actual field within the conductor? So what additional field must be present?
  4. Jan 25, 2008 #3
    Wouldn't the field inside the sheet be 0 since it is a conductor?
  5. Jan 25, 2008 #4

    Doc Al

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    Staff: Mentor

    Right. The total field within the conducting sheet will be zero. What's the contribution from the surface charges?
  6. Jan 25, 2008 #5
    Alright I see where this is going. The field from the "two" plates creates a field equal and opposite to the electric field that is on the outside.

    The thing that had me confused was that in the problem they give you the size of the plates (15cm), so I thought you can't assume they are infinite plates. But now I realize they just tell you that so you can get charge density.

    Is the reason you can assume that the plates are infinite in size that they are so close together that any charge you put between would be so close to the plate compared to the size of the plate?


    I solved the problem at least I'm pretty sure. I did it using the fact that the field away from an infinite plane is density/(2*Enot)

    The other way I did it was using Gauss's Law and putting a box through the plane.

    Thanks for the help
    Last edited: Jan 25, 2008
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