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Energy in capacitor

  1. Aug 6, 2008 #1
    1. A 20 pF capacitor is charged to 3.0 kV and then removed from the charger and connected to an uncharged
    50 pF capacitor.
    (a) what is the new charge on each capacitor?
    (b) Find the initial energy stored in the 20 pF capacitor and the final energy stored in the two capacitors. Is electrostatic potential energy conserved, lost or gained when the two capacitors are connected together?

    2. Relevant equations
    u=1/2 cv^2
  2. jcsd
  3. Aug 6, 2008 #2


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    Homework Helper

    Well what have you tried so far?
  4. Aug 7, 2008 #3
    hehe, google search found me this, I'm guessing a fellow UQ-er.

    For a), I wasn't sure whether to reason:
    1) that because both capacitors are conductors, the charge will spread evenly between them
    2) that is will spread proportionately between them based on their capacitance.

    as for b), I've been unable to find a pertinent equation. But I would imagine that electrostatic potential should be conserved.
    is u electrostatic potential....?
  5. Aug 7, 2008 #4


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    Homework Helper

    The second one is right if you're thinking along the lines of conservation of charge and common potential difference. But you need to be more explicit as to how exactly are the charges distributed.

    This requires the first part. Use the formula for energy in capacitors and sum both up, then you'll be able to see if it is conserved or not.
  6. Aug 7, 2008 #5

    thankyou sir.
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