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Homework Help: Finding Expressions for Electric Fields with Radius

  1. Feb 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A solid nonconducting sphere of radius a has a has a total charge +Q uniformly distributed throughout its volume. The surface of the sphere is coated witha avery thing (negligable thickness) conducting layer of gold. A total charge of -2Q is placed on this conducting layer.
    Use Gauss's Law to do the following:
    a. Find the expression for the electric field E(r) for r<a (inside the sphere, up to and excluding the gold layer).
    b. Find the expression for the electric field E(r) for r>a (outside the coated sphere, beyond the sphere and the gold layer).

    2. Relevant equations

    Eflux = ∫E dot dA = Q/ε0
    A sphere = 4∏r^2

    3. The attempt at a solution
    I got part a. like this:

    Eflux = ∫E dot dA (electric field is constant because uniformly distributed) = EA = E*4∏r^2 = Q/ε0.
    solving for E = Q/ 4∏r^2ε0

    is this right?

    for part b. I am lost....
  2. jcsd
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