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Homework Help: Check my work? LC circuit oscillations.

  1. Apr 23, 2006 #1
    I'm not too confident in my work for this problem, so I was wondering if someone could check it over for me.

    Consider a circuit with 4 elements, C1=100micro farads, C2=50micro farads, L1=20mH, and L2=10mH. At t=0, the capacitors are charged with Q=0.01 Coulomb. There is initially no current. (all 4 are connected in series)

    a) What are the voltages across the capacitors?
    V1=(1/C1)Q=(1/100micro farads)0.01= 100 Volts
    V2=(1/50micro farads)0.01= 200 Volts
    b)How much electrical and magnetic energy, respectively is stored in the circuit initially?
    Ue=electric energy=q^2/(2C)
    1/C=1/C1 +1/C2 C=33.3micro Farads
    0.01^2/(2*33.3micro farads)=1.50 J
    Ub=magnetic energy=(L*i^2)/2
    i(initial)=0 so magnetic energy=0
    c)What is the total inductance of the circuit.
    L=L1+L2=20mH+10mH= 30mH
    d)What is the total capacitance?
    1/C=1/C1 + 1/C2 C=33.3micro farads
    e)What is the frequency of oscillations in the circuit?
    w=sqrt[1/(LC)] = sqrt[1/(30mH * 33.3micro farads)] = 1000 rad/s
    f)What is q(t)? Make sure it satisfies q(0)=Q
    q(t) = Qcos(wt + phi)
    q(t) = (0.01)Cos(1000t) <---is the phase constant zero?
    g)Compute i(t)= dq/dt
    dq/dt= -10Sin(1000t)
    h)Compute di/dt=d^2(q)/dt^2
    di/dt= -10000Cos(1000t)

    Thanks a lot to anyone who can do me this favor.
  2. jcsd
  3. Apr 23, 2006 #2


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    Homework Helper

    yes, the phase constant is zero since qharge on Cap's is maximum at t=0.
    This is verified by the current being zero at t=0 (from the sine).
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