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RC Circuit 2

  1. Oct 1, 2009 #1
    Three resistors and a capacitor are in a circuit with a battery:
    R1 = 23 Ω
    R2 = 13 Ω
    R3 = 15 Ω
    C = 6.2 µF
    V = 17 V
    The switch has been closed for a long time. What will the charge be on the capacitor 195 μs after the switch is opened?

    picture
    https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys212/fall09/homework/06/11_paired/image.gif [Broken]
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 1, 2009 #2

    kuruman

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    WHat are your thoughts on the matter? Is this a charging or discharging capacitor 195 μs after the switch is opened? What are the relevant equations to describe this?
     
  4. Oct 4, 2009 #3
    ok well the capacitor is fully charged, so the voltage of the capacitor is the same as the battery, from there i can find the max charge. then i used the equation Qmax(1-e^(t/CR))
    where i used 28 for the resistance. but thats not giving me the right answer
     
  5. Oct 4, 2009 #4

    kuruman

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    Try again. The voltage across the capacitor is the same as across the series combination R2+R3. That's not the voltage across the battery. What is it?
     
  6. Oct 5, 2009 #5
    That's not the only problem. The current through C is not the current through either R1 or R2+R3 so you can't use those in Qmax(1-e^(t/CR) either.

    I recommend replacing V, R1, R2 and R3 with their Thevenin equivalent.
     
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