Calculate the power of a microwave that uses 520.0 J of energy in 10.0 s.

What this problem is asking

You are given how much energy the microwave transfers (in joules) and how long it takes (in seconds). Power tells you the rate of energy use, so you divide the energy by the time.

Using the power equation

The definition of power is

$$P = \frac{E}{t}$$

where $P$ is power in watts (W), $E$ is energy in joules (J), and $t$ is time in seconds (s).

Substituting the numbers and calculating

$$P = \frac{520.0\ \text{J}}{10.0\ \text{s}} = 52.0\ \text{J/s} = 52.0\ \text{W}$$

Checking units and significant figures

• $\text{J/s}$ is the same as watts, so the unit is correct.
• Both inputs have 3 significant figures, so the result is reported as $52.0\ \text{W}$.