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RC DC Series and Parallel Circuit

  1. Aug 21, 2013 #1
    Hi!

    1. The problem statement, all variables and given/known data

    attachment.php?attachmentid=61046&stc=1&d=1377062905.jpg


    2. Relevant equations

    V = IR
    Kirchhoff Current and Voltage Law.


    3. The attempt at a solution

    I am facing difficulty with this RC circuit.
    I tried looking up solutions but all I could find were either exclusively series or parallel.

    My first guess was to take Total Current = 28/(6+18). However, that ended up being wrong.
    I worked backwards from the answer provided and realised that Current through R2 = 28/(6+18) instead of what I thought earlier. However, I am unable to relate this to any of the formula I can find.


    I would be a great help if someone were able to explain to me how to arrive and the solutions to the question.
    Thanks.
     

    Attached Files:

    Last edited: Aug 21, 2013
  2. jcsd
  3. Aug 21, 2013 #2

    ehild

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    Homework Helper
    Gold Member

    How did you proceed from here?

    Does any current flow through the capacitors after reaching the equilibrium voltage?
    The "total current" is the same as the current through either R2 and R1.

    ehild
     
  4. Aug 21, 2013 #3
    Can you calculate the voltages across the capacitors?
     
  5. Aug 21, 2013 #4
    Oh my. It was a mathematical error on my part. I deeply apologise.

    After taking Total Current = V/R= 28/(6+18) = 1r1r6 A
    Voltage across R1 = IR = 1r1r6 * 6 = 7 V
    Therefore voltage across C1 + C2 = 28 - 7 = 21 V
    Capacitance total = (1/12 + 1/24)^-1 = 8 μF
    Therefore Charge across C1 and C2 = CV = (8*10^-6)(21) = 168 μC (FOUND)

    However there is one thing I would like to ask.
    So, this means that no current flows through the capacitors?
     
  6. Aug 21, 2013 #5

    ehild

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    Gold Member

    It flows at the beginning when the circuit is connected. After a while, the capacitors are charged, till the voltage gets the same across them as the voltage across R2. After that, no more potential difference to drive charges to the capacitor plates, so no current flows after the steady state is settled.


    ehild
     
  7. Aug 21, 2013 #6
    Ahhh, I understand now. Thanks!

    Thread closed then :)
     
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