What a learner can do afterwards
- States the definition of power and gives its unit (watt)
- Uses P = E/t to calculate power, energy, or time given the other two quantities
- Compares the power ratings of common appliances and explains what the rating means
- Explains why a higher-powered device does the same job faster but uses more energy
The lesson
Power tells you how fast energy moves from one place to another. You already know energy is never created or destroyed, it only changes form or location. Power measures the speed of that change: how many joules transfer every second. The unit for power is the watt, named after the engineer James Watt. One watt means one joule of energy transferred each second.
A 100 W bulb runs for 5 seconds. Power is energy divided by time, P = E ÷ t. Flip that around to find energy: E = P × t. So this bulb transfers 100 × 5 = 500 joules in those 5 seconds.
A bigger wattage number does not mean a device is stronger, it means it moves energy faster. That is why a 2000 W kettle boils water quicker than a 500 W kettle, but it also uses energy at a much faster rate while it runs.
Power, measured in watts, is how many joules of energy move each second: P = E ÷ t.
Watch it
Where it sits
Learn first
This opens up
Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.