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Homework Help: Power dissipated in an RL circuit

  1. Apr 7, 2014 #1
    1. The problem statement, all variables and given/known data

    a) A circuit contains two elements, but it is not known if they are L, R or C. The current in this circuit when connected to a 2.16×102 V, 60.0 Hz source is 6.10 A and lags the voltage by 3.60×10^1 degrees. What are the two elements?

    c) What is the power dissipated in this circuit?

    2. Relevant equations


    3. The attempt at a solution

    I determined that it was an LR circuit and calculated the impedance Z for part b) of the question but I'm stuck on the power. I did a simple P=IR to get an answer of 1.32E3 W but that was wrong, and I also tried dividing the I and V given by √2 to get rms values, then multiplying, which gave me 659 W but it's still wrong. Any insights or nudges in the right direction would be much appreciated!

    Edit: I just realized my angle is 36 degrees, not 360 like I originally thought. That means I need to have a cos36 in there somewhere right? I'm still unsure as to whether I'm supposed to be using rms values or not though.
  2. jcsd
  3. Apr 7, 2014 #2


    User Avatar

    Staff: Mentor

    You can assume that the voltage and current values given are RMS values unless otherwise indicated.

    Note that the given current has a phase angle associated with it, which implies a power factor for the circuit. When they ask for power they want the real component of the complex power, which is the magnitude of the total power multiplied by the power factor.
  4. Apr 15, 2014 #3
    I got it, thank you!
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