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Homework Help: LC circuit

  1. Sep 5, 2010 #1
    LC circuit!!

    1. The problem statement, all variables and given/known data

    A 550 microfarad capacitor is connected across a 1.35 H inductor. At a certain time, the charge on the capacitor is zero and the current is 0.362 A.

    How much later will the capacitor charge reach its peak?
    What's the total energy in the circuit?
    What is the peak charge on the capacitor?

    i tried Q=Q0cos(wt) with Q=0 then solving for t, which gives 2.45s but i dunno what to do from there. the only equation i have for LC circuits is that one just there and w=sqrt(1/LC)

    helpppp? :)
  2. jcsd
  3. Sep 5, 2010 #2
    That looks like a good equation to have for the first part. I didn't check your answer, but the approach seems correct.

    As far as energy and charge, remember that the energy is being alternately stored in the capacitor and the coil. At the start, all energy is in the coil. Do you know the formulae for energy in a capacitor and in a coil?
  4. Sep 5, 2010 #3
    Re: LC circuit!!

    for the first part this is what i did:


    at a particular time t, Q =0... which means cos(wt) =0 so (wt)=90. so t = 90/w
    w = sqrt(1/LC) = 36.7 (pretty sure the unit is Hz)

    so t=90/36.7 =2.45seconds.

    but it says its wrong...

    as for part two i only know the formula for energy stored in an inductor... buttt..

    its U=0.5LI^2.... so then if thats all the energy at that time t, then the energy in the capacitor is equal to that when the capacitor charge is at its peak yeah?

    and one i have those two answers part 3 is easy... i just cant seem to get part a...
  5. Sep 5, 2010 #4
    Re: LC circuit!!

    i figured some out...

    for part 1 i needed to use pi/2 instead of 90 degrees... that got the answer..

    then for part 2 i found energy in inductor and then that was the answer at peak charge...

    now for part 3 im lost...

    obviously using Q=Q0cos(wt) equation to find peak charge Q0 but i dunno how... coz peak charge occurs at cos(wt)=1 which wt=0???
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