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Homework Help: Power Dissipated By Resistor is LRC Circuit

  1. Jul 27, 2016 #1
    [Note by mentor: this post does not use the homework template because it was originally posted in a non-homework forum.]


    What is the average power dissipated by a 25-Ω resistor in an LRC series ac circuit for which the power factor is equal to 0.25 and the maximum voltage of the ac source is 8.0 V?

    Answer: .08W

    Attempt at Solution:
    We know that Pav = (1/2)(V)(I)cos(phi) where cos(phi) is the power factor. So if we substitute I for V/I (ohms law) we get Pav = (1.2)((V^2)/R)cos(phi). Plugging in all the values I get that Pav = .32W which is clearly incorrect...

    What did I do wrong?
    Thanks for the advice!
    Last edited by a moderator: Jul 27, 2016
  2. jcsd
  3. Jul 28, 2016 #2


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    Homework Helper

    Check your formulas. What is Ohm's Law for an AC circuit? How do you get the average power in terms of Vrms and R?
  4. Jul 28, 2016 #3
    ok i understand i made a typo by saying substitute I in for V/I meant V/R as you can see in the next step, but why do we need to use the Vrms in this situation rather than V?
  5. Jul 28, 2016 #4


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    Homework Helper

    Well, you can use the maximum voltage and current if you like, but I=V/R is valid for the voltage across the resistor which is not the same as the given 8.0 V.
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