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Homework Help: Power absorbed in a circuit

  1. Jul 19, 2011 #1
    I wish someone could tell me what I am doing wrong. I have tried using P=VI then integrating P from t1 to t2 where I get 225uJ but it keeps telling me I am wrong. Seems like a rather simple problem.


    2. Relevant equations

    3. The attempt at a solution
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 19, 2011 #2


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    Staff: Mentor

    Welcome to the PF.

    I think you left out a factor of 1/2 in the first part of the integration. I get half of your answer.
  4. Jul 19, 2011 #3


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    Science Advisor

    I think he's out by a factor of 6.

    If you normalize everything it just comes down to [itex]\int_0^1 x(1-x) dx = 1/6[/itex]

    Doing it properly with V, I and tr in the equation you get [tex]\frac{V \, I \, t_r}{6}[/tex]
  5. Jul 19, 2011 #4


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    Staff: Mentor

    Interesting approach. Did you try it just the standard way, though? The number for t is so small that the t^3 term after the integration is negligible (if I did it right), and only the t^2 term contributes to the answer...
  6. Jul 19, 2011 #5
    I ended up figuring it out. I decided to write equations for the slopes of each line. So

    for the voltage

    -30/250x + 30

    For current

    [itex]\int[/itex](-30/250x+30)(30/250x)dx from 0 to 250

    Then multiply by 10^-3 to get in micro jewel

    Comes to 37.5uJ

    Crazy little problem but I am ME so not used to this sort of thing.
  7. Jul 19, 2011 #6
    I just noticed everyone's approach worked, geniuses. Thanks for everything, its nice to know there are still people out there willing to help others. I will definitely write these approaches down in my book for later use.
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