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Homework Help: Electric Field Inside a Conductor Using Gauss's Law

  1. Jan 30, 2013 #1
    1. The problem statement, all variables and given/known data

    A hollow metal sphere has 7cm and 9cm inner and outer radii, respectively. The surface charge density on the inside surface is - 300nC/m^2</units> . The surface charge density on the exterior surface is + 300nC/m^2</units> .

    What is the strength of the Electric Field at a point 4cm from the center.

    2. Relevant equations
    ∫EXda= Qin/εo

    3. The attempt at a solution

    I'm pretty lost on how to solve this question. I initially tried to follow steps my prof did on a previous, similar example, but he was solving for charge while this is clearly asking for Electric Field strength.

    I thought perhaps I could integrate to get E.A= Qin/εo and then rearrange for E= Qin/(Axεo) and then solve for E but I got a ridiculous number. Any ideas?
  2. jcsd
  3. Jan 30, 2013 #2


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    Homework Helper

    Yeah, that's the right idea. What Gaussian surface are you using? And from that, what is the enclosed charge?

    p.s. welcome to physicsforums :)
  4. Jan 30, 2013 #3


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    Science Advisor
    Homework Helper

    That sounds a little strange. If there is a surface charge on the inside of the sphere then there must also be another charge around that they haven't mentioned. I don't think you are going to be able to determine the E field at 4cm unless you make some assumptions about how that charge is distributed.
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