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Homework Help: Hard problemT_T Charge on capacitor?

  1. Dec 7, 2009 #1
    Hard problemT_T Charge on capacitor??

    1. The problem statement, all variables and given/known data
    A 6.0 uF capacitor is charged up to 12V and subsequently connected through a witch to a 100ohm resistor. At t=0, the switch is closed. What's the charge on the capacitor 6.0ms(=.0006sec which is RC) after the switch is closed??

    2. Relevant equations
    Q= Qmax e^(-t/RC)

    Please tell me some more equations like this(Q= Qmax e^(-t/RC), I =Imax e^(-t/RC)...etc..
    do i use these when what is discharging??!or charging?!)

    3. The attempt at a solution
    -t/RC is -1 ?? is it right?!

    I don't know what to do T_T/...

    Help needed ASAP Thank you very much
  2. jcsd
  3. Dec 7, 2009 #2
    Re: Hard problemT_T Charge on capacitor??

    Charging: q = CV(1 - exp(-t/RC))
    Discharging: q = q0 * exp(-t/RC)

    In this case, i think you should use discharging eq and there is miscalculation in t/RC.
  4. Dec 7, 2009 #3
    Re: Hard problemT_T Charge on capacitor??

    hmmm.. the answer is 3.3 x 10^-9C

    I have no idea how to get this answer....

    I do know that I should use Discharging Equation...

    But I don't get the answer...

    Please help me again! Thank you soo much!
  5. Dec 7, 2009 #4
    Re: Hard problemT_T Charge on capacitor??

    6 ms = 0.006 s (you have an extra zero).
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