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Homework Help: Capacitor Problem

  1. Oct 11, 2007 #1
    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 9.00 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.
     
    Last edited: Oct 11, 2007
  2. jcsd
  3. Oct 11, 2007 #2

    Doc Al

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    Staff: Mentor

    Look up "RC time constant" and the associated equation for a discharging capacitor.
     
  4. Oct 11, 2007 #3
    Time Constant is T=RC;

    but I don't know how to use this equation to solve my problem.
     
  5. Oct 11, 2007 #4

    Doc Al

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    That's just the definition of the time constant, not the capacitor discharge equation that uses it. Read this: Charging a Capacitor (Charging and discharging are inverse operations.)
     
  6. Oct 11, 2007 #5
    Still not making sense. I don't know the value of Q0 or V. Someone please help.
     
  7. Oct 11, 2007 #6

    Doc Al

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    Q0 is given in your first post; you don't need V. Don't you have a textbook?

    [tex]Q = Q_0 e^{{-t}/{RC}}[/tex]
     
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