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What is the charge on the capacitor?

  1. Aug 11, 2009 #1
    1. The problem statement, all variables and given/known data

    https://www.physicsforums.com/attachment.php?attachmentid=20071&stc=1&d=1250025514

    The switch in the figure has been closed for a very long time.
    https://www.physicsforums.com/attachment.php?attachmentid=20071&stc=1&d=1250025514

    (a) What is the charge on the capacitor?

    (b) The switch is opened at t = 0 s. At what time has the charge on the capacitor decreased to 10% of its initial value?


    2. Relevant equations

    E-iR1-iR2=0
    emf (delta)Vi=0
    Q=Qe^-t/RC (possibly)

    3. The attempt at a solution
    So, I know that the circuit is initially closed, and I don't need current to solve. Also that I probably need to combine resistors because they are in series. Yet, I'm a little confused as how to set up the problem.

    Any type of help or insight would be deeply appreciated.
     
    Last edited: Aug 11, 2009
  2. jcsd
  3. Aug 11, 2009 #2

    tms

    User Avatar

    Re: Circuits

    You'll have to at least describe the circuit in words, if you can't post a graphic of it.
     
  4. Aug 11, 2009 #3
    Re: Circuits

    If we could see the picture it might help. If you can not load describe the circuit and value of the components. This would help a lot.
     
  5. Aug 11, 2009 #4

    tms

    User Avatar

    Re: Circuits

    The link does not work.
     
  6. Aug 11, 2009 #5
    Re: Circuits

    It looks something like this:


    ____WWW______________
    I..........60 OHM.....I................I
    I...........................I.........…
    _ 100V.................W...........W
    -.............40 OHM W............W 10 OHM
    I............................I........…
    I............................I........… I_
    I............................I........… 2 micro F
    I............................I........…
    I________________I_________I
     
  7. Aug 11, 2009 #6
    Re: Circuits

    Your picture is still not clear, I couldn't find where is the switch but general suggestions:
    1) Convert your circuit to thevenian with capacitor at output (with one voltage source and one thevenian resistor)
    2) 1/RC it T(time constant)
    3) Use x(inf) - [x(inf) - x(0)] exp (-t/T) formula for finding voltage across capacitor as a function of t.
    4) For steady state (when long time), make capacitor open and solve the circuit
     
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