# Power in an AC Circuit

1. The problem statement, all variables and given/known data

The average power dissipated by a resistor is 4.0 W. What is the average power if the resistance R is doubled?

2. Relevant equations

$$P_{rms}=I_{rms}^2R=I_{rms}V_{rms}=\frac{V_{rms}^2}{R}$$

3. The attempt at a solution

I am told the answer is 2.0 W. But here’s what I am confused about…it is valid to use all of the power equations above for this problem, correct? Well if I use

$$P_{rms}=I_{rms}^2R$$ I get the new average power to be 8.0 W.

But if I use the

$$P_{rms}=\frac{V_{rms}^2}{R}$$ I get the new average power to be 2.0 W.

How do I know which equation to use?

Thanks.

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