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Homework Help: Instantaneous currents in RLC Circuit!

  1. Dec 7, 2014 #1
    1. The problem statement, all variables and given/known data

    I am completely stumped on this problem:

    In this circuit: 2e5m6ap.png

    v= 30.0
    R = 400 ohms
    C = 2.50 micro coulombs
    L = .300 H

    The switch (pretend you see one) is close at t=0

    Current i1 flows through L and current i2 flows through C

    QUESTION: At what time (t) does i1=i2 ?

    2. Relevant equations

    i(t) = I cos (omega t)

    3. The attempt at a solution

    I'm not sure where to even start.....I don't know how to find two different functions (of time t) for i1 and i2 to set equal to each other to solve for t.

    Can anyone suggest what these functions may be?

    Thanks in advance!!
  2. jcsd
  3. Dec 7, 2014 #2


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    Science Advisor
    Homework Helper
    2017 Award

    ##i(t) = I \cos (\omega t)## isn't applicable here: you are not driving this circuit with an alternate current but with a battery.
    So back to the textbook for relationships between I, V, and their time derivatives, and C, L
  4. Dec 7, 2014 #3
    well...the only thing I can think of is using Kirchoffs rules.

    I used the loop rule and got:

    R( i1+i2) + L(di/dt) = EMF

    R(i1+i2) + q/c = EMF

    ...not sure how these can help me find a function for i(t)....
  5. Dec 7, 2014 #4


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    Homework Helper

    q is the charged on the capacitor. What is i? How is the current flowing through the capacitor related to this q?
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