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RL circuits: Find the voltage through the resistors

  1. Feb 14, 2016 #1
    1. The problem statement, all variables and given/known data
    Find the voltage through the resistors.

    2. Relevant equations
    V=IR
    P=IV


    3. The attempt at a solution
    Am I doing these correctly? Can I find the voltage across the inductor the way I did it?
    Screen Shot 2016-02-14 at 5.19.03 PM.png
     
  2. jcsd
  3. Feb 14, 2016 #2

    SammyS

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    You state that the current through the resistor is zero. Then what is the voltage drop across the resistor?
     
  4. Feb 14, 2016 #3
    0 volts right? Did I flip those 2 things? I thought inductors after a long time act like wires with no resistance. Is that not true?
     
  5. Feb 14, 2016 #4

    SammyS

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    Yes. zero.

    The resistor is in parallel with the inductor, thus it has the same voltage drop. (Can also arrive at this because the inductor has zero resistance.)
     
  6. Feb 14, 2016 #5
    Ahhh. Ok that makes sense. But then how can we find how much power the current source would supply. P = IV = I^2R = V^2/R. Would we have to discharge the inductor and then use V=IR = (1A)(1000ohms)=1000V?
     
  7. Feb 14, 2016 #6

    SammyS

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    How do get 1000 ?

    Any way you calculate this, you multiply by either the resistance of the inductor, or the voltage drop or some other zero.
     
  8. Feb 14, 2016 #7
    Woops. I used the wrong equation. I meant to use P = I^2R = (1A)^2*(1000ohms) = 1000V. But is that wrong?
    How would I get the resistance of the inductor? Are you talking about the reactance? Xl=wL? And isn't the voltage drop across the inductor 0? I'm not sure how that helps us find the power that the current source is supplying.
     
  9. Feb 14, 2016 #8

    SammyS

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    Yes, voltage drop across the inductor is indeed zero. Therefore, V⋅I = ?
     
  10. Feb 14, 2016 #9
    Power is 0? How can the power be 0? Does that mean the current source is just off?
     
  11. Feb 14, 2016 #10

    SammyS

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    no.

    simply that to maintain current needs no voltage if there is no resistance.
     
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