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RC circuit energy dissipation

  1. Jul 29, 2013 #1
    1. The problem statement, all variables and given/known data

    A capacitor C=8.00*10^-8F which has an initial charge q=1.4*10^-6C starts to discharge through a resistor R=3.00*10^6Ω at time t=0. Determine the total energy W dissipated as thermal energy in the resistor. Hint: use energy conservation.


    3. The attempt at a solution

    I already found U(0)=1.225e-5J (energy stored in capacitor at t=0), but how do I use energy conservation to find W?
     
  2. jcsd
  3. Jul 29, 2013 #2
    When the capacitor is completely discharged, where has the stored energy gone?
     
  4. Jul 29, 2013 #3
    are you saying all the energy goes to heat so U=W?
     
  5. Jul 29, 2013 #4
    Yes! ...are you certain this was the whole question? The value of the resistance does not affect the energy transfer...do you know what it will affect?
     
  6. Jul 29, 2013 #5
    yes, thank you, it was the whole question, but there were parts before it.
    This was part e I believe.

    I was a little skeptical about U=W because this part is worth 7 points, which makes U=W seem too simple.
     
  7. Jul 29, 2013 #6
    What was the whole question?
     
  8. Jul 29, 2013 #7
    (it was actually part d. my bad!)

    A capacitor C=8.00*10^-8F which has an initial charge q=1.4*10^-6C starts to discharge through a resistor R=3.00*10^6Ω at time t=0. Determine the following:

    a. (3points) the time constant (tau) of the circuit.
    b. (5points) the current i at t=(tau)
    c. (4points) the energy U(0) at t=0
    d. (7points) the total energy W dissipated as thermal energy in the resistor
    e. (6points) the time which the energy U is equal to 25% of U(0)

    I figured out the other parts, but d looked suspicious since it is worth more than all other questions but apparently has the same answer as c.
     
  9. Jul 29, 2013 #8
    oh, and for your question there, I believe resistance affects the time constant RC which affects the rate of the capacitor's discharging.
     
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