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Homework Help: RL Circuit

  1. Sep 21, 2015 #1
    1. The problem statement, all variables and given/known data
    Assume that the switch in the figure has been closed for a long time and opens after in t=0.
    Find Ix in a) 0- b)0+ c) 40ms

    3. The attempt at a solution
    Switch is opened in t=0, current flows only in the 10Ω resistor, Ix=3v/10Ω= 0.3A-> 0-
    0+ --> 3v/10=0.3A
    Not sure if both currents are the same, and for the 40ms, when solving the equation (τ=L/R) τ (tau) results in a small number.
    Last edited: Sep 21, 2015
  2. jcsd
  3. Sep 21, 2015 #2
    Do you think it's possible for the current through an inductor to change instantaneously?

    It might help to see your work for part c in order to check it.
  4. Sep 21, 2015 #3
    No couldt change instantaneously.
    this is what i have for part c
    τ=L/R -> (500mH)/70Ω=7.14ms

    i(t)= i(∞)-(i(∞)-i(0+))e^(-t/τ)
    i(40)= ???

    That where I am stuck
  5. Sep 21, 2015 #4


    User Avatar

    Staff: Mentor

    Hi jeuceda. :welcome:

    What assumptions allow you to say this?

    How did you calculate this?

    You mention an equation. What is your equation, exactly?

    It is clearer when you include a phrase such as "at the moment before the switch is closed ...", or "once the switch has been open for a long time ...", to confirm precisely what you are talking about.

    BTW, the diagram is ambiguous; it is not clear from the diagram whether at time t=0 the switch is opened after having already been closed for a long time, or whether it is being closed at t=0 having prior to this been open for a long time. The textbook's double-headed arrow would be better drawn with only one arrow head.
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