Power Dissipated By Resistor is LRC Circuit

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1. Jul 27, 2016

mj23

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

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Problem:
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?

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?

Last edited by a moderator: Jul 27, 2016
2. Jul 28, 2016

ehild

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?

3. Jul 28, 2016

mj23

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?

4. Jul 28, 2016

ehild

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.