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Homework Help: How to find capacitance

  1. Feb 8, 2004 #1
    A capacitor, in a series with a 720 ohm resistor, is being charged. At the end of 10 ms its charge is half the final value. The capacitance is about:

    a)9.6 uF
    b)14 uF
    c)20 uF
    d)7.2 uF

    Now how in the heck and I suppose to know this one. I know ohms law and I know how to find capacitance. But what does either of these has to do with time?
  2. jcsd
  3. Feb 8, 2004 #2
    The rate at which a capacitor is charged in an RC circuit is:

    [tex]q(t) = Q_{max}(1-e^\frac{-t}{RC}) [/tex]

    You know t and R.
    You don't know q(t) or Qmax but you do know the ratio of each to the other, and that's all you need to solve for C.
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