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Homework Help: Parallel plate capacitors

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data
    A capacitor C1 (the one on top in the diagram) of capacitance 4 mu F is charged until its charge is 200 mu C and another capacitor C2 (the one below) of capacitance 3 mu F is charged until its charge is 300 mu C . Then both capacitors are connected in the circuit as shown in the figure . What is the total enerygy loss in the capacitors ?

    2. Relevant equations

    E=1/2CV^2 or E=1/2 QV

    3. The attempt at a solution

    I an not sure its asking for energy loss as a whole or energy loss for each capacitors .

    so i know how to calculate the energy stored in each capacitors using the formula , what should i do after that >

    Attached Files:

  2. jcsd
  3. Mar 15, 2010 #2


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    Homework Helper

    Find the common potential difference across the capacitors.
    They are connected in parallel.
    find the equivalent capacitor.
    Find the final energy stored in the combination of the capacitors.
  4. Mar 15, 2010 #3
    ok let me try

    the effective pd would be 100+50=150 V

    Effective capacitance = 7 mu

    energy loss = 1/2 x 7 mu x 150^2 = 0.07875 J ?

    but the answer given is 0.002 J

    btw , is energy stored in the capacitors the same as energy loss in the capacitors
  5. Mar 15, 2010 #4


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    Common voltage V = (Q1 + Q2)/(C1 + C2)
  6. Mar 15, 2010 #5
    thanks , so let me retry ,

    E= Q^2/2C

    =[(500 x 10^(-6))^2]/2(7 x 10^(-6))

    =0.018 which is approximately 0.02 J

    Am i correct nw ?
  7. Mar 15, 2010 #6


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    Homework Helper

    That is correct.
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