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Parallel-Plate Vacuum Capacitor

  1. Oct 3, 2005 #1
    If a parallel-plate vacuum capacitor, with its plates separated by a distance of x1 has an amount of energy equal to U stored in it. The separation is then decreased to x2 .

    What is the energy now stored if the capacitor was disconnected from the potential source before the separation of the plates was changed? I know that I am suppose to use the formula C= Q/V but how do I put U in terms of x1 and x2?
  2. jcsd
  3. Oct 4, 2005 #2


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    As the battery is disconnected, the charge on the capacitor plates becomes constant. Now by changing capacitance the potential difference will change and hence it is batter to use the formula Q^2 /2C of the energy.

    Capacitance in the two situations are to be calculated and Q is calculated by first situation only.
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