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Homework Help: Insulating charged sphere in conducting shell and electric field

  1. Sep 11, 2012 #1
    1. A solid sphere of radius a = 12.8 cm is concentric with a spherical conducting shell of inner radius b = 37.1 cm and outer radius c = 39.1 cm. The sphere has a net uniform charge q1 = 9.00×10-6 C. The shell has a net charge q2 = -q1. Find expressions for the electric field, as a function of the radius r, between the sphere and the shell (a < r < b). Evaluate for r = 25.0 cm.

    2. ∫E.dA = Q/ε0
    ρ = Q/Volume

    3. OK, so I have the charge density of the insulting sphere, which I'm calling ρ. My Gaussian surface is a sphere, so it's area in this case would be 4∏(0.25m)^2
    I know how to find the electric field inside of the insulating sphere, but not between the insulating sphere and the conducting shell, which is what this problem is asking for. I tried ignoring the conducting shell and just using the equation:
    E = (ρa^2)/(3ε0r^2)
    It's telling me that the answer is wrong. What am I doing wrong? I assume it has something to do with the charge on the inner surface of the conducting shell, but I don't know what to do with that.
  2. jcsd
  3. Sep 11, 2012 #2


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    Homework Helper

    Hi Kyle,welcome to PF.

    The equation in red is wrong. Why do you use the charge density instead of Gauss Law with the enclosed charge q1=9.00×10-6 C?

  4. Sep 11, 2012 #3
    Thanks! I guess I was overthinking. Instead of using the whole Q = (4/3)πr^2*ρ, I just had to use the charge given to me.
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