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Homework Help: RC circuit and energy!

  1. Apr 11, 2009 #1
    1. The problem statement, all variables and given/known data
    The switch in the figure (Intro 1 figure) has been in position a for a very long time. It is suddenly flipped to position b for 1.25 ms, then back to a. How much energy is dissipated by the resistor?

    the circuit for a is 50V battery, 125 ohm resistor and 20uF capacitor. The circuit for b is the 20uF capacitor and the 50 ohm resistor.

    2. Relevant equations

    Time constant=RC, Ucap=0.5CV^2, V=IR, Q=CV, i dunno I've tried others too

    3. The attempt at a solution

    I know the capacitor will be at 50V. I found time constant and current and charge and tried so see if i=i(max)e^(-t/RC) would work and tried to relate it back to energy but it didn't. The answer is 23 mJ.
  2. jcsd
  3. Apr 12, 2009 #2


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    Homework Helper

    First of all find the total charge in the first capacitor when the switch in position a.
    Find the total charge transferred to second capacitor in 1.25 ms when the switch is in b position. dQ/dt will give you the current in the resistance. Find the average current I and I^2R will give you the power dissipation.
    Last edited: Apr 12, 2009
  4. Apr 13, 2009 #3
    To get answer integrate I^2R dt. Integrating the average current squared is an approximation...
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