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Homework Help: Current through an inductor after a switch closes

  1. Apr 29, 2016 #1
    1. The problem statement, all variables and given/known data

    Let's say I have a circuit such as the one above, and lets say the circuit has been
    open for a long time.

    How would I find the current through the inductor, the instant after closing the switch?

    and the voltage potential across the 40 mh inductor?

    2. Relevant equations
    I(t) = E/R ( 1 - e^(-R/L*t) )

    3. The attempt at a solution
    I'm a bit lost would the current just be the voltage/resistor = 100/120 = .83333 amps??

    how would I find the current right after the switch closes?
    and then how would I use that to find the voltage across the 30 mH inductor?

    Could someone give me an explanation on how to find the current through the inductor and voltage potential, I'm quite lost and confused : /
  2. jcsd
  3. Apr 29, 2016 #2


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    Gold Member

    No. Put t=0 in this equation.
  4. Apr 29, 2016 #3
    So the current would just then be 0 ? I(t) = E/R ( 1 - 1) (0) = (0)
  5. Apr 29, 2016 #4


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    Homework Helper
    Gold Member

    Yes. Inductor acts as an open switch at this instant. What will be the voltage across this open switch?
  6. Apr 29, 2016 #5
    It's 100 volts, i'm not gonna bother writing all the work out here since I just derived it on paper.
    My confusion just stemmed from the behavior of the inductor itself...
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