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RL circuits - finding resistance and inductance

  1. Feb 12, 2017 #1
    1. The problem statement, all variables and given/known data
    I am practicing questions from the book in preparation for an exam, and I am having trouble with this one

    2. Relevant equations

    3. The attempt at a solution
    According to my professor, the answers are: R1=12ohm, R2=9ohm, L=34.28 H
    So far this is what I have done:
    Voc = 6V
    τ = 20/7
    Vo = 3V
    This is just from analyzing the equation that was given in the question. I don't know if my logic is correct, and I am not sure what the next steps will be.
  2. jcsd
  3. Feb 12, 2017 #2
    Start with your equation for v(t). What is its value when t equals 0?

    Since the current through L can't change instantaneously, prior to closing the switch, what is the current flowing in the circuit? That is the same current that flows through the 3 ohm resistor, so the v(0) value should help.

    See if this helps any.
  4. Feb 12, 2017 #3


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    Heed @magoo's clues. The important thing here is to note that the current through the inductor doesn't (or can not) change instantaneously, but voltage across it can.
  5. Feb 12, 2017 #4


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    What do these symbols (Rt, τ, Voc) stand for? Which method are you using?

    Given the voltage equation, what will be the equation of current for t>0?
  6. Feb 12, 2017 #5
    τ is the time constant
    Voc is open circuit voltage
    and Rt is thevenin equivalent resistance
    (I think)
  7. Feb 12, 2017 #6


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    You need not use Thevenin for this circuit.
    What is the current through the circuit at t=0+?
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