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Find electric field

  1. Aug 13, 2011 #1
    1. The problem statement, all variables and given/known data

    see below.

    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Aug 13, 2011
  2. jcsd
  3. Aug 13, 2011 #2
    See the attachment for the visual, but ignore the negative charges. They aren't supposed to be there.

    The image is of two large non-conducting sheets with identical distributions of positive charge. What is the value of the electric field above the sheets? sigma is the symbol for the identical distribution of positive charge.

    I know the answer: [itex]\sigma[/itex]/epsilon[itex]\epsilon0[/itex]

    I know that you need to add to use the principle of superposition.

    However I still don't get the answer. The top sheet adds [itex]\sigma[/itex]/2epsilon[itex]\epsilon0[/itex] upwards. The downwards from the top sheet is canceled out by the upwards (electric field) from the bottom sheet. The bottom sheet has an electric field that also points in the negative y direction but with the same magnitude. So everything cancels out. So the electric field is zero.

    This is wrong. However, I still do not see why and why the right answer is right.
     

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