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Help with Gauss' Law?

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

    A point charge, q, is at the center of an insulating hollow sphere with inside radius, a, and outside radius, b. The spherical shell has charge per unit volume, rho. You may assume that q and rho are positive.

    Using Gauss' law, find the magnitude of the electric field for a < r < b.

    2. Relevant equations

    Gauss' Law ----> E * dA = Q_encl/epsilon_0

    3. The attempt at a solution

    I know that the electric field for r < a is E = (rho*r)/(3*epsilon_0).

    And the electric field for r > b is E = (rho*b^3)/(3*epsilon_0*r^2).

    But I can't seem to figure out the electric field for a < r < b. I've tried using the same method, but I get really confused. I'm thinking that it might just be zero, but I'm not sure.

    Any suggestions would be greatly appreciated!
  2. jcsd
  3. Sep 13, 2011 #2


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    Actually, neither of those answers is correct. For one thing, where's q? And the first answer shouldn't depend on ρ.
    Show us what you've tried so we can see where you're getting confused.
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