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Homework Help: Electrical Engineering- Simple AC circuit problem

  1. Sep 14, 2010 #1
    1. The problem statement, all variables and given/known data
    I was able to find the solution to this problem but I don't understand where it comes from.

    A series RL circuit is connect to a 110-V ac source. If the voltage across the resistor is 85 V, find voltage across inductor.

    2. Relevant equations

    Not sure.

    3. The attempt at a solution

    Solution is given as

    V_s^2 = V_r^2 + V_l^2

    then solve for V_l, I don't understand were that equation comes from or how it applies here. We are currently starting to learn how to solve circuits with the phasor method.
  2. jcsd
  3. Sep 14, 2010 #2
    The impedance of an inductor is called inductive reactance. It is called reactance to indicate that it is an imaginary number with no real part. (In reality all inductors have a small real part.) Because inductive reactance is at right angles to a resistor, the problem you are given amounts to a right triangle in which you are given the hypotenuse and one side. Now do you recognize the equation? The 110 VAC is the hypotenuse and the 85 VAC is the resistive voltage. Can you find the other side?
  4. Sep 14, 2010 #3
    I understand they are at right angles to each other but I'm not sure why the hypotenuse is 110 V. Isn't the 110 V in phase with the V_r? so it points in the same direction as V_s? Isn't 85 V only the projects of the Voltage on the real axis and not the amplitude itself? What am I misunderstanding?
  5. Sep 14, 2010 #4
    All set! For some reason the section that explained how that worked was the one after which this problem was presented.
  6. Sep 14, 2010 #5
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