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Homework Help: Surface charge of a cube?

  1. Sep 26, 2013 #1
    1. The problem statement, all variables and given/known data

    A cube of side a has a cube of side a/2 centered within it. The inner cube has a total charge Q that is uniformly distributed over its surface. A) For the surface of the outer cube, find:

    s E * dA

    B) Is this sufficient information to find the electric fields at points on the surface of the outer cube? Explain.

    2. Relevant equations

    E = F/Q


    Not sure with a cube otherwise.

    3. The attempt at a solution

    Not sure how to start it. Drew out diagram of situation. See attached document.

    Attached Files:

  2. jcsd
  3. Sep 26, 2013 #2
  4. Sep 26, 2013 #3
    Thanks for your response! Ok so looking at it again:

    ∫E * dA

    Since E is a constant, it can be pulled out:

    E ∫dA = (a) (a/2) = a2/2

    If I'm doing this right, then I just need to find the integral of a2/2, right? So I would then get:

    E * a2x/2

    As for the second question, if the above is correct, I would say then you have enough information to find the electric field at points on other areas of the surface of the cube too then.
  5. Sep 26, 2013 #4
    Use the Gauss's law! It directly gives the required integral without any integration!
    ##\int E.dA=Q/ \epsilon##
    Note:-If all you have got is the electric flux through the surface you cannot find the electric field at all the points.
    To do this you will have to integrate due to the lack of symmetry of the surface.
  6. Sep 26, 2013 #5
    Ok, that makes more sense since the outer surface is not symmetric as you said. You have to integrate at along different points to find each electric field. Thanks for your help with this problem!
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