Units of work and energy

Beginner

The joule is one newton-metre.

Key relation1 J = 1 N·m

The joule equals 1 N·m = 1 kg·m²/s². Other units: calorie (4.184 J), kilowatt-hour (3.6 MJ), electron volt (1.6×10⁻¹⁹ J).

Worked example

Lifting a 1 kg book by 1 m takes about 9.8 J.

Common mistake

Torque also has units N·m but is not energy.

In more detail

Work and energy share the joule as their SI unit, since work transfers energy. One joule is the work done by a force of one newton over one metre.

Food labels quote energy in kilojoules or kilocalories.
Electricity bills charge for energy in kilowatt hours, which are joules multiplied by 3.6 million.
A 60 W bulb uses 60 joules of energy each second, which is why its energy use is quoted in joules per second.

How many joules are in a kilowatt hour?

3.6 million joules, or 3.6 megajoules.

Is a calorie the same as a joule?

No. A food calorie is about 4184 joules, so the units are different.

Is a joule a big or small amount of energy?

A joule is small. Lifting a small apple by one metre takes roughly one joule of energy.