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Two capacitors connected in series

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data
    Two capacitors, capacitance of 30 μF and 60 μF respectively, each has charge of 50 μF and 70 μF. The capacitors then connected in series. Find the potential difference on each capacitor now


    2. Relevant equations
    Q = CV


    3. The attempt at a solution
    I know that the charge should be same for series circuit but I don't know how to analyse this question. Each capacitors has its own charge and voltage and after connected in series, their charge should be the same. How to find it?
     
  2. jcsd
  3. Sep 30, 2012 #2

    phinds

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    uF is a measure of capacitance, not charge, so I don't understand what you are saying here.
     
  4. Sep 30, 2012 #3

    CWatters

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    Think about what "The capacitors then connected in series" means. Presumably only one plate of each is connected together otherwise they would be in parallel. If only one side of each capacitor is connected together how much current can flow?
     
  5. Sep 30, 2012 #4
    sorry, I meant 50 μC and 70 μC

    Sorry I really don't know the concept here of how to determine the current when only one side of each capacitor is connected...I assume that one positive plate is connected to other negative plate. Will the case be the same if the series circuit connects positive plate to positive plate?

    Thanks
     
  6. Oct 2, 2012 #5

    CWatters

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    If the only one end of each capacitor is connected to anything there is no closed circuit so no current flow. Therefore the capacitors retain whatever charge and voltage they had before they were connected.
     
  7. Oct 4, 2012 #6
    Sorry, actually there are choices for this question:
    a. 4 and 8 Volt
    b. 8 and 4 Volt
    c. 5 and 5 Volt
    d. 3 and 6 Volt
    e. 6 and 3 Volt

    If the capacitors retain their voltage, their voltage should be 5/3 and 7/6 Volt.

    So there is no correct choice?

    Thanks
     
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