Two students carry identical boxes up the same stairs. One jogs, one strolls. They do the same work, but one does it faster. Power measures how fast energy is transferred.
Key idea
Power = energy transferred ÷ time: $$P = \dfrac{W}{t}$$ in watts (W), where $1\,\mathrm{W} = 1\,\mathrm{J/s}$. A 60 W bulb uses 60 joules every second.
Why it works
Joules measure how much energy; watts measure how fast. It is like liters and liters per second for water: a bathtub holds liters, while a faucet delivers liters per second. A more powerful motor does the same job in less time.
Worked example: Climbing stairs
A 600 N student climbs stairs 3 m high in 4 s. What is her power?
- Work against gravity: $W = F\,d = 600 \times 3 = 1{,}800$ J.
- $P = \dfrac{1{,}800\,\mathrm{J}}{4\,\mathrm{s}} = 450\,\mathrm{W}$.
Answer: 450 W
Worked example: Energy from power
A 1,000 W microwave runs for 90 s. How much energy does it use?
- Rearrange: $W = P\,t = 1{,}000\,\mathrm{W} \times 90\,\mathrm{s} = 90{,}000\,\mathrm{J}$.
Answer: 90,000 J
Common mistake
Using watts and joules as if they meant the same thing. A watt is a joule per second. "This heater is 1,500 W" tells you its rate; how much energy it uses depends on how long it runs. Check the units: J ÷ s = W, and W × s = J.
Engines are often rated in horsepower: 1 horsepower is about 746 W.