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Homework Help: RC circuit final charge

  1. Feb 10, 2009 #1
    1. The problem statement, all variables and given/known data
    Consider an RC circuit in which a capacitor is being charged by a battery connected in a circuit. Initially the capacitor is uncharged. Then the switch is closed.
    After a time equal to 4.5 time constants, what percent of the final charge is present on the capacitor?


    2. Relevant equations
    Q = Qo [1 - e^ (-t/RC)]



    3. The attempt at a solution

    I picked a value of 100 for Qo
    so
    Q= 100 [1 - e^ (-t/4.5)]
    my approach was going to be to find Q and then see wht percent that is of the initial charge Qo
    but I dont know wht to put in for the time "t"
    please help me out
     
  2. jcsd
  3. Feb 10, 2009 #2

    LowlyPion

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    I think you would take t = 4.5RC
     
  4. Feb 10, 2009 #3
    so for the expression e^(-t/RC) I do e^(-4.5) ?
     
  5. Feb 10, 2009 #4

    Delphi51

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    4.5 time constants means a time of 4.5*R*C.
    R*C comes out in units of time.
     
  6. Feb 10, 2009 #5

    LowlyPion

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    Yes that should do it.
     
  7. Feb 10, 2009 #6
    Awsome! Thank you so much!
     
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