Question

An alarm clock is plugged into a 120 volt outlet and has a resistance of 15,000 ohms. How much power does it use?

Answer 1

Answer:

The power used is 0.96 watts.

Explanation:

Recall the formula for electric power (P) as the product of the voltage applied  times the circulating current:

$P=V\,\,I$

and recall as well that the circulating current can be obtained via Ohm's Law as the quotient of the voltage applied divided the resistance:

$V=I\,\,R\\I=\frac{V}{R}$

Then we can re-write the power expression as:

$P=V\,\,I=V\,\,\frac{V}{R} =\frac{V^2}{R}$

which in our case becomes:

$P=\frac{V^2}{R}=\frac{120^2}{15000} =0.96\,\,watts$