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Homework Help: Alternating RLC circuit with an additional capacitor

  1. Mar 9, 2017 #1
    1. The problem statement, all variables and given/known data
    part b, specifically finding the maximum charge for C1.
    The question that boggles me is whether Imax changes on the left side of the circuit, after the switch closes.

    2. Relevant equations
    Xc = 1/(wC) XL = wL

    3. The attempt at a solution
    I was able to find max charge on C2 because I know the voltage on both sides of the current is the same. Vmax must be 120, and multiplying that by the capacitance will give me the maximum charge.

    To find the maximum charge on C1, I need the maximum voltage on C1, which equals Imax*Xc.

    Imax with the switch closed was found in the previous question, however, after the switch closes, (what I think) the maximum current on the left side must changes since there will be current going to the capacitor on the right.

    Some of my classmates did the problem assuming that the maximum current on the left side does not change, though they do not have a satisfying explanation.

    Heres how I got C2
    Q = CV
    V = Imax X = 120/5*(1/((1/3)*3)) = 24V
    24V*3 = 72C
    Last edited by a moderator: Mar 9, 2017
  2. jcsd
  3. Mar 9, 2017 #2


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    Homework Helper

    Will the voltage change across the RLC circuit?
  4. Mar 9, 2017 #3
    No, it will not, since the two branches are in parallel!
    So the current wouldn't change!
    Thanks for the hint!

    However, if it does not change, then the current out from the source after the switch is closed will be higher than when the switch is opened.
    Does that mean the impedance of the entire circuit decreased?(if we can think of impedance as analogous to resistance, I am kind of the confused as to the way impedance stacks up)

    My first year engineering physics class does not cover AC in parallels, however, I am fairly interested in the subject and wouldn't mind any terms that would require some googling for myself.
  5. Mar 9, 2017 #4


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    Staff: Mentor

    The total current that the source provides will be higher, yes. The load impedance that the voltage source sees will decrease, yes.

    Impedances combine in the same way that resistances do. The difference is that impedances vary with frequency.
  6. Mar 9, 2017 #5


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    Homework Helper

    Yes, two branches are connected in parallel to the same source. The reciprocal of the complex impedances add up. The complex impedance have both magnitude and phase, so it is not sure that two impedances in parallel mean smaller impedance then any of them. For example, an inductor and capacitor in parallel have an equivalent impedance with higher magnitude than that of the inductor or the capacitor. If you are familiar with complex numbers, it will not be difficult!
  7. Mar 9, 2017 #6
    Got it! Thanks a lot!
  8. Mar 9, 2017 #7
    We have not done complex numbers, but I understand the statement!
    Thanks so much for the help!
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