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Whos right? - capacitor to capacitor discharge.

  1. Jan 21, 2009 #1
    I read on another forum, some person says (read below) when you charge one cap, then discharge into another, there is 50% efficiency loss?

    I think this is wrong, as the only losses I can think of, are due to the caps ESR when there is a voltage difference between the caps. Can someone help show his\her Math is wrong..

    Also you lose 1/2 the energy in this process.

    I can illustrate with a little math

    Given capacitors C1 and C2 of the same size (C). One charged to V and one uncharged then the energy stored is

    1/2 C * V *V

    If you hook these two capacitors in parallel then they will settle (after a current pulse) to a voltage in each of 1/2 V. The energy in each will then be

    1/2 C * 1/2 V * 1/2 V

    or 1/4 of the original energy in each capacitor for 1/2 of the original energy.
    Last edited: Jan 21, 2009
  2. jcsd
  3. Jan 21, 2009 #2
    This is the famous Two Capacitor Problem. You have two identical capacitors separated by an open switch. One is charged to V. Then the switch is closed. After the voltage has equalized what is the resultant voltage on both capacitors?

    The answer depends on conservation of charge or conservation of energy.

    Charge (coulombs), Q = C*V
    Energy (joules), E = C*V^2/2

    Obviously they can't both apply.
  4. Jan 21, 2009 #3
    Last edited: Jan 21, 2009
  5. Jan 21, 2009 #4


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  6. Jan 21, 2009 #5
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