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Homework Help: RC circuit problem

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data
    What is the charge on the capacitor?


    2. Relevant equations
    Kirchoff's rule
    change in V =q/c


    3. The attempt at a solution
    I want to use Kirchoff rule but I don't know if this is the right way to go.
     

    Attached Files:

  2. jcsd
  3. Mar 28, 2010 #2

    rl.bhat

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    When the capacitor is completely charges, there is no current in that branch. The voltage across the capacitor is the potential difference across 40 ohm resistor.
     
  4. Mar 28, 2010 #3
    Ya, I was able to figure out that the charge on the resistor would be 80 microCoulombs . The voltage difference between the plates of the capacitor was 40 volts and from there it was plugging in. Now I face another problem where I can't find how long it will take the capacitor to discharge to 25% of 80 microCoulombs. I derived that it should be -RC*ln(1/4)=t. I plug in R= 50, c = 2 and get the wrong answer :/
     
  5. Mar 28, 2010 #4

    rl.bhat

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    t = 2 μF.
     
  6. Mar 28, 2010 #5
    How does t = 2 MICROFARADS? Don't you mean seconds or milliseconds? I don't quite understand your logic though. Doesn't the capacitor discharge through 2 resistors that become 50 ohms?
     
  7. Mar 28, 2010 #6

    rl.bhat

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    Sorry.It is typo. I mean C = 2 μF. Your R is correct. You didn't mention your answer.
     
  8. Mar 28, 2010 #7
    Well then it would be -RC*ln(1/4) = -(50)(2)*ln(1/4)= 138.63 ms but that is somehow wrong.. :/
     
  9. Mar 29, 2010 #8
    Oh stupid me! I freakin' forgot to convert the units... it should be 138.63 microseconds or .1386 milliseconds.

    Thanks for the help! :P
     
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