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Affect on stored energy if charge is doubled

  1. Aug 22, 2007 #1
    The problem statement, all variables and given/known data

    A parallel plate capacitor is air filled and assumed to be ideal. It is connected to a battery so it has constant potential difference. By what factor does stored energy change if the charge is doubled..

    The attempt at a solution

    Using the formula U = 0.5QV..

    If charge is doubled, then U is doubled correct?

    I say the answer is stored energy is doubled, but the answer at the back reckons quadrupled. Who is right?
  2. jcsd
  3. Aug 22, 2007 #2
    E = Q^2 / 2C is another formula. That would yield a 4 times greater energy being stored.

    Also, if E = (0.5)QV, then if Q = 2Q

    Q = CV
    C = Q/V

    C = 2Q/2V, as C cannot change, so V must change in order to keep it constant.

    E = (0.5).2Q.2V

    I think.
  4. Aug 22, 2007 #3
    well I'm given two formulas, U = (0.5)QV and U = 1/2CV^2

    The second formula is not relevant as charge is not a variable, hence my above reasoning...
  5. May 8, 2009 #4
    In case the charge doubles, the voltage would also get doubled. Hence stored energy would get quadrupled. All the formulas would yield the same result.
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