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Voltage across capacitor

  1. May 5, 2014 #1
    1. The problem statement, all variables and given/known data
    here's my question and the working in the same photo.. my question is to find to find the effective capacitance in the circuit i use (1/c1 +1/c2)^-1 , why should cant just take effective c =
    (1/c1 +1/c2)^-1 but must times 2 ? since (1/c1 +1/c2)^-1 is already effective capacitance. please ignore the irrelevant part . sorry for the messy work

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. May 5, 2014 #2

    rude man

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    Why should effective capacitance be the criterion for obtaining the resultant voltage across the capacitors?

    One way:
    You can write 3 equations in 3 unknowns: Qb, Q1 and Q2 where
    Qb is the charge on each plate before reconnection,
    Q1 is the charge on plate 1 after reconnection
    Q2 is the charge on plate 2 after reconnection

    Then V after reconnection is obvious.
     
    Last edited: May 5, 2014
  4. May 5, 2014 #3

    mfb

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    Can you explain what you calculated? It is hard to understand that with those unexplained formulas.

    By the way, as this is a multiple choice question, there is a shorter way to find the right answer: three out of four answers do not have the correct units.
     
  5. May 5, 2014 #4

    rude man

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    Ho ho, good point! Lousy choices!
     
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