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Homework Help: RC Circuit find capacitor charge

  1. Feb 10, 2010 #1
    1. The problem statement, all variables and given/known data
    The capacitor in an RC circuit (R = 130 \Omega, C = 40uF) is initially uncharged.
    1) Find the charge on the capacitor in the circuit one time constant (tau = RC) after the circuit is connected to a 9.0- V battery.
    2) Find the current in the circuit one time constant (\tau = RC) after the circuit is connected to a 9.0-V battery.
    2. Relevant equations
    R = V/I
    Q = qo[1-e^-t(RC)] ???


    3. The attempt at a solution
    Im actually not sure I even have the correct equations and I'm not sure what to use for qo
     
  2. jcsd
  3. Feb 11, 2010 #2

    ehild

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    Q=qo[1-e^(-t/(RC))]

    ehild
     
  4. Feb 11, 2010 #3

    Lok

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    Qo=C*V
     
  5. Feb 11, 2010 #4
    I'm so confused
     
  6. Feb 11, 2010 #5

    rl.bhat

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    In the problem C and V is given. When the capacity is fully charged, the maximum charge Qo = C*V. Find this one.
    In the relevant equation, put t = RC and find Q.
     
  7. Feb 11, 2010 #6
    i got the first part but i got the second part wrong. i did V/R = 6.9e-2. then i multiplied that by 1-e^-1 and got 4.4e-2 then multiplied that by 1000 to go from A to mA and got 44. where did i go wrong?
     
  8. Feb 11, 2010 #7

    Lok

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    There is a similar formula to the Q=qo[1-e^(-t/(RC))] for I(ntensity). It is a must have.
     
  9. Feb 11, 2010 #8
    yeah I used that and substituted qo for V/R and worked it out the same way i did the first one
     
  10. Feb 11, 2010 #9

    rl.bhat

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    The current I = dQ/dt = (qo/RC)[1-e^(-t/(RC))]
    Now find the current.
     
  11. Feb 12, 2010 #10
    i cant wrap my mind around this !!!
     
  12. Feb 12, 2010 #11

    rl.bhat

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    The current I = dQ/dt = (qo/RC)[e^(-t/(RC))]
    Now find the current.
     
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