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: RC capacitor and energy

  1. Mar 27, 2010 #1
    1. The problem statement, all variables and given/known data
    At what time (in ms) after the discharge begins is the energy stored in the capacitor reduced to half its initial value?

    RC=7 ms

    2. Relevant equations
    RC=7ms
    E=.5CV^2
    E=.5Q^2/C
    E=.5QV
    Q_f = Q_i * e^(-t/RC)

    3. The attempt at a solution
    I don't even know where to go from here. I know that since the problem involves RC, the equation to solve the problem must have some form of Q_f = Q_i * e^(-t/RC) but I'm stuck.
     
  2. jcsd
  3. Mar 27, 2010 #2
    Since energy is proportional to the square of charge, by what fraction of the maximum charge should the final charge be to produce half the energy stored with maximum charge?
     
    Last edited: Mar 27, 2010
  4. Mar 27, 2010 #3
    Ah! I see now! When one works the math, the final charge after half the energy is gone is q/2^.5. the initial charge is q. From there it is just plugging in to get an the answer 2.43 ms.

    Thanks!! :D
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook