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Homework Help: Charge on cylindrical capacitor

  1. Oct 3, 2009 #1
    1. The problem statement, all variables and given/known data

    A cylindrical air capacitor with a length of 12.2 m stores an amount of energy equal to 3.00E-8 J when the potential difference between the conductors is 4 V. Calculate the magnitude of the charge on each conductor.

    2. Relevant equations


    energy density=1/2*epsilon_0*E^2

    3. The attempt at a solution

    I know the energy, length, and potential. I'm not given either the charge on the cylinder or the ratio of the radii of the inner/outer conductors, both of which are necessary in the formula. I suspect the additional information needs to come from the equation involving the energy, some new input from knowing the electric field. Any help would be greatly appreciated.
    Last edited: Oct 4, 2009
  2. jcsd
  3. Oct 4, 2009 #2


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    You don't need the dimensions of the capacitor. What relations do you know about the energy stored in a capacitor?
  4. Oct 4, 2009 #3
    U=1/2Q*V gives me a charge. but I need the magnitude of charge on each conductor. Is this the correct value?
  5. Oct 4, 2009 #4
    yes it is. thank you :) was overthinking it.
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