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Homework Help: Capacitor Discharge

  1. May 6, 2009 #1
    1. The problem statement, all variables and given/known data

    A 10 microfarad capacitor is initally charged to 15 volts. If the capacitor then discharges through a 12kohm resistor how long before the charge stored has falled to 0.8 x 10^-4 C?

    2. Relevant equations


    Q= Q_0 x e ^(-t/RC)

    3. The attempt at a solution

    Calculate initial charge using Q=CV

    Q= 15x 10x10^-6

    Qinital = 1.5x10^-4

    Then using Q= Q_0 * e^(-t/RC)

    Q= Q_0 x e ^(-t/RC)
    0.8 x10^-4 = 1.5x10^-4 * e^(-t/12000*10x10^-6)

    ln(0.8/1.5) = -t/12000*10x10^-6

    ln(0.8/1.5)*12000*10x10^-6 = t

    which when calculated comes out as 0.0754, or 75 ms.

    However, the answer in the practise test is 120ms.... anyone see what i am doing wrong?

    Many thanks, Andy
  2. jcsd
  3. May 6, 2009 #2


    User Avatar
    Homework Helper
    Gold Member

    Your calculations look fine to me, so I can only assume that there is a typo in either the answer key or the question sheet.
  4. May 6, 2009 #3
    Thankyou for taking a look. I'd hoped that was the case and i wasn't going mad ;)

  5. May 6, 2009 #4
    I punched those number real quick, and they are fine.
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