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Homework Help: Electrostatic Potential Energy stored outside a shell.

  1. Mar 2, 2017 #1
    1. The problem statement, all variables and given/known data

    A point charge +Q is placed at the centre of an isolated conducting shell of radius R. Find the electrostatic potential energy stored outside the spherical shell if the shell also contains a charge +Q distributed uniformly over it.

    2. Relevant equations


    dU/dV=(1/2)εoE2, where V is volume, U is potential energy.

    3. The attempt at a solution

    The charge +Q inside the conductor will induce -Q charge on the inside surface, which further leads to +2Q charge on the outer surface of the shell. Therefore E=2kQ/r2.

    My question is, why have we used the formula dU/dV=(1/2)εoE2. How did we derive it? Isn't it equal to the energy of a charged capacitor.

    Sorry if this a bad question, I just want to know how did we derive this formula.

  2. jcsd
  3. Mar 2, 2017 #2


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    Science Advisor
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    Gold Member

    For the same reason that we use mass density ρ to find the mass of an extended object by integration.
    That you can find in any intermediate-level book on Electricity and Magnetism.
    No, it is not. It is an energy density, i.e. energy per unit volume (Joules/m3 in SI units). To do this problem you need to use Gauss's law to find the electric field outside the shell, find the energy density and integrate that over all space outside the shell.
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