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Homework Help: Determine the magnitude of the charge on the inner and outter surface of sphere

  1. Feb 4, 2013 #1
    1. The problem statement, all variables and given/known data
    A hollow conducting spherical shell has an inner radius of 8 cm and an outer radius of
    10 cm. The electric field at the inner surface of the shell, Ei, has a magnitude of 90 N/C
    and points toward the centre of the sphere, and the electric field at the outer surface, Eo,
    has a magnitude of 80 N/C and points away from the center of the sphere (see Figure A).
    Determine the magnitude of the charge on the inner surface and the outer surface of the
    spherical shell.

    2. Relevant equations
    For the inner surface: E =kqt/r1r^{3}
    For the outer surface E = kqt/r2^{2}

    3. The attempt at a solution
    This uses Gauss's law. For the inner surface, set gaussian surface, with r1<r, r is the inner radius.
    For outer surface, gaussian surface outside the sphere. r2>rs which is radius of shell.
    I'm stupid I don't know what to do.
  2. jcsd
  3. Feb 4, 2013 #2


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    Science Advisor
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    Gold Member

    For there to be a field inside the conducting shell, there must be some isolated charge inside it. By symmetry, you can consider that to be at the centre. Let that charge be qc. There are also charges spread over the inside surface of the sphere, qi say, and on the outside surface, qo.
    Can you write down an expression for the potential at distance r from the centre? To start with, just consider a single shell of charge at some radius a. You need to consider r < a and r > a separately. When you've got that, you can write down the total potential from the three charges just by adding them up.
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