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Homework Help: RC circuits

  1. Apr 10, 2010 #1
    1. The problem statement, all variables and given/known data
    In an RC discharge circuit, R= 2.5 X10^4 ohms and C=40 microfarad. The initial potential difference across C is 25 V. Find the rate at which energy is being lost by C at 0.5 seconds.

    2. Relevant equations

    3. The attempt at a solution
    Like in this case, I probably have to use Q = Q(initial)e^(-t/RC) and U= 0.5 Q^2/C. But then we need dU/dt...i'm not sure what to do...like i can solve for the charge at t=0.5 seconds and also the current if that matters...but like how am i supposed to use dt?...I've been stuck on this problem for like an hour...
  2. jcsd
  3. Apr 10, 2010 #2


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    Put Q(t) that you know in the equation for U, then take the derivative. Evaluate the expression that you get at t = 0.5 s.
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