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Energy Stored in an Inductor

  1. Nov 27, 2013 #1
    1. The problem statement, all variables and given/known data

    Not really relevant here.

    2. Relevant equations

    U = LI^2 -- maybe?

    3. The attempt at a solution

    http://i.imgur.com/Pq4dOex.png

    The picture is there, as well as the answer. Why is that the answer? How do inductors work when completely disconnected, and not in a circuit? Thanks.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 27, 2013 #2

    mfb

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    Staff: Mentor

    The inductor is in a circuit.

    1/2 LI^2 (note the prefactor) is indeed the energy stored in an inductor.
     
  4. Nov 27, 2013 #3
    I'm an idiot. The question asks what happens when the switch is switched over to position B, after being in position A for "a very long time"
     
  5. Nov 28, 2013 #4

    rude man

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    That sine curve in the illustration tells you what happens to UL.

    Hint: the instantaneous sum of stored energies in C and L is a constant.
     
  6. Nov 28, 2013 #5
    I guess I'm just confused as to why the energy in the inductor decreases, but then increases again?
     
  7. Nov 28, 2013 #6

    rude man

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    I can't give you a good verbal explanation. A publication like the ARRL Handbook can.

    Mathematically, the integro-differential equation 1/C∫0t i(t') dt' = -L di/dt is solved with initial condition i(0+) = E/R

    where i is the current flowing out of the inductor and into the capacitor. Each term is the voltage at the capacitor and inductor.
     
  8. Nov 28, 2013 #7

    NascentOxygen

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    Because the L-C elements represent a resonant circuit (having no resistive losses). Just as a child's swing oscillates when you release it from some height, so does the energy in the analogous L-C circuit.
     
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