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Homework Help: Capacitor/Inductor Problem

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    A capacitor is charged with a 12 volt battery until there is 3 μc of charge on the capacitor. The capacitor is then connected to an inductor. The angular frequency of the resulting oscillation is 1.25 x 105 rad/s. What is the value of the inductor?

    2. Relevant equations
    LC: ω = 1/sqrt(LC)

    3. The attempt at a solution
    rearranging the equation gives: L=1/(ω2C)=1/(1.25E5rad/sec)(3E-6C)

    This gave me an answer of 2.67 H, which is incorrect. Am I not using the correct equation for this type of problem?
  2. jcsd
  3. Oct 27, 2014 #2


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    Homework Helper

    What do you think C is in the formula for the angular frequency ω = 1/sqrt(LC)??
  4. Oct 27, 2014 #3


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    Staff: Mentor

    Hi! Your answer is going to be wrong for two reasons, I think. You've forgotten there's a squared term? The C in the formula is capacitance, not charge.

    I'm curious, though --- does your textbook give you its correct answer??
  5. Oct 27, 2014 #4
    Sorry I know C is for capacitance I don't know why I did that, I still can't figure this out though. What term is supposed to be squared in this equation? It does give me the correct answer, it is supposed to be 256 μH.
  6. Oct 27, 2014 #5
    Nevermind I figured it out thanks guys.
  7. Oct 27, 2014 #6


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    Staff: Mentor

    The problem says the capacitor is given a charge of 3 μC from the battery; though it doesn't state that the capacitor is fully charged to 12V. This tells us that the capacitance is at least a certain minimum value---it could be larger and just not charged right up to 12V. We aren't told exactly.

    So IMHO the correct answer is that L256μH

    This would make a good trick question, allocating 1 mark for 256 uH and 1 mark for the inequality.
    Last edited: Oct 27, 2014
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