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Homework Help: Decay in an RC Circuit

  1. Oct 11, 2007 #1
    1. The problem statement, all variables and given/known data

    A capacitor is charged to 1 coulomb; the capacitance is 9.00×10-5 farads.
    Then a switch is closed which puts the capacitor in a closed circuit with a resistor; the resistance is 9.0x10^1 ohms.

    Calculate the charge on the capacitor after 4.00 milliseconds. (1 ms = 0.001 s).

    I know some equations relating capacitance, charge, and voltage, but I don't understand how to approach this problem

    2. Relevant equations

    Q(t) = Q0[e^-t/RC]

    3. The attempt at a solution

    No clue. I have no idea how to approach this problem and no idea what the value for V is.
    Last edited: Oct 11, 2007
  2. jcsd
  3. Oct 11, 2007 #2


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    Homework Helper

    Just use: Q(t) = Q0[e^-t/RC]

    where are you getting stuck?
  4. Oct 11, 2007 #3
    lol... ugh I was using 9 ohms as a resistance instead of 90! Thanks.
  5. Dec 17, 2007 #4
    I'd like to ask for help in this thread as my question is directly related.

    Here is my problem.

    Q = 37.1

    Q0 = 37.44

    e = 2.718

    t = 0.25

    C = 15.6x10-9

    R = ?

    How do I rearrange the equation to find R?

    Q = Q0[e^-t/RC]

    I don't have any idea how to go about it and would appreciate any help. :smile:
  6. Dec 17, 2007 #5


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    Homework Helper


    divide by Q0


    take ln

    RC*ln(Q/Q0)=-t and so forth
  7. Dec 18, 2007 #6
    Thanks for the reply. Its a great help. :smile:
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