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Energy and pd of capacitor

  1. Jan 6, 2008 #1
    1. The problem statement, all variables and given/known data

    A capacitor of capacitance C stores an amount of energy E when the pd across it is V. Which line, A to D, gives the correct energy and pd when the charge is increased by 50%?

    Energy pd
    A) 1.25E 1.5V
    B) 2.25E 1.5V

    2. Relevant equations

    E= 0.5 CV^2
    E= 0.5 QV

    C= Q/V

    3. The attempt at a solution

    The answer is B).

    Do I have to find two simultaneous equations because I have two unknowns? But how?

    Thanks :)
  2. jcsd
  3. Jan 6, 2008 #2

    Doc Al

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    Staff: Mentor

    You know that C doesn't change (it's the same capacitor) and you are told how Q changes. Use the last of your three equations to figure out what happens to the potential difference. Then use either of the other equations to see what happens to the energy.
  4. Jan 6, 2008 #3
    thanks! :D
  5. Jan 9, 2008 #4
    When charges increase by 50%

    then by equation Q = C*V we can know that V increases by 50% so, it changes from V to 1.5V

    then by using equation E = 1/2*C*V^2
    then V is changed to 1.5V so, use that in the above equation (1.5)^2 = 2.25
    hence it becomes 2.25E
  6. Jan 9, 2008 #5

    Doc Al

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    Staff: Mentor

    You are several days too late. :wink:
  7. Jan 9, 2008 #6
    Hi Doc Al - I just joined the forum & picked up a question that I first saw!

    Isn't it appropriate?
  8. Jan 10, 2008 #7

    Doc Al

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    Staff: Mentor

    Actually, no. :wink:
    (1) You are more than welcome to chime in, but read the thread first before doing so. If you did you'd see that the OP had already solved the problem.
    (2) Never just post full solutions to a problem. See our rules posted at the top of each page and the Homework Help FAQ.

    Nonetheless, welcome to PF! :smile:
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