Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    e)10F

    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook