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Homework Help: Time required to fully discharge the capacitor where did i go wrong?

  1. Jul 22, 2011 #1
    1. The problem statement, all variables and given/known data

    A cube of gold (resistivity ρR = 2.44 10-8 Ω m, density ρD = 1.93 104 kg / m3, and specific heat c = 129 J / kg °C) that is 3.5 mm on a side is connected across the terminals of a 40-µF capacitor that initially has a potential difference of 340.0 V between its plates.


    What time is required to fully discharge the capacitor? (Assume the capacitor is fully discharged when the charge is less than 0.01%.)

    3. The attempt at a solution

    R = pL / A = (2.44 * 10^-8) * (3.5 * 10^-3m) / (3.5*10^-3m)^2 = 6.97 * 10^-6 m^2

    C = 40 * 10^-6 F

    Q = Q0 e (-t/(RC))


    t = 2.56 e -9

    which is not the right answer for question a
  2. jcsd
  3. Jul 22, 2011 #2


    User Avatar

    Staff: Mentor

    Your units for the resistance of the gold block should be Ohms, not square meters :smile:

    I can't find a fault with your calculation (although the result when I calculated it rounded to 2.57 x 10-9 seconds).

    Does the marker require any particular units to be used for the result (picoseconds? nanoseconds?)
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