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Homework Help: LC Circtuit finding Q across an element for given time

  1. Nov 11, 2012 #1
    1. The problem statement, all variables and given/known data
    A circuit is constructed with two capacitors and an inductor as shown. The values for the capacitors are: C1 = 513.0 μF and C2 = 68.0 μF. The inductance is L = 265.0 mH. At time t =0, the current through the inductor has its maximum value IL(0) = 106.0 mA and it has the direction shown.

    2) What is Q1(t1), the charge on the capacitor C1 at time t = t1 = 14.6 ms? The sign of Q1 is defined to be the sameas the sign of the potential difference Vab = Va - Vb at time t = t1.

    3) What is Vbc(t1)= Vb - Vc, the voltage across the inductor at time t1 = 14.6 ms? Note that this voltage is a signed number.

    2. Relevant equations

    w0 = 1/sqrt(LC)
    I(t) = -wQmax*sin(wt+phi)
    Q(t) = Qmax*cos(wt+phi)

    3. The attempt at a solution
    can't solve this for a week...

    plugging in given... w = 250,
    I(0) = I max = .106 = -wQ(1)
    so Qmax = 4.24E-4
    Also, sin(wt+phi) = 1 @ t=0, so w(0)+phi = pi/2

    for t=.0146s,
    Q(.0146) = Qmax*cos(wt+phi)...
    plugged in to get 2.0639E-4, but it's wrong...

    As for 3, once i solve for Q in question 2, i can use that in Q/C = Vinductor...
  2. jcsd
  3. Nov 11, 2012 #2


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    Gold Member

    The circuit is not shown.
  4. Nov 11, 2012 #3
    Woops, meant to mention the 3 elements (2 capacitors and the inductor) are just in series so no diagram was needed.

    It prob happened since I exited this once as I was typing the question, so forgot to do so second time around D:

    and I STILL need help on this
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