What is measured in joules?
Quick answer
The joule (J) is the standard unit of energy in the International System of Units (SI). It’s also used for work and heat, which are forms of energy transfer. One joule is the energy used when a force of one newton moves something one meter, roughly the energy needed to lift a small apple 1 meter.
What one joule looks like
1 joule = 1 newton × 1 meter = 1 kg·m²/s²
| Example | Approximate energy |
|---|---|
| Lifting a 100 g apple by 1 m | 1 J |
| A single heartbeat’s work | About 1 J |
| A 60-watt bulb for one second | 60 J |
| One food Calorie (kcal) | 4,184 J |
| One kilowatt-hour | 3,600,000 J (3.6 MJ) |
| One tonne of TNT | About 4.2 billion J |
Newtons, the unit of force, are named after Isaac Newton; see Newton’s three laws of motion.
Joules vs watts
People often mix these up:
- Joule measures an amount of energy.
- Watt measures power: how fast energy is used, in joules per second.
A 1,000-watt microwave uses 1,000 joules every second.
Joules vs calories
Food labels in the US and UK usually show Calories (with a capital C), which are really kilocalories. Many other countries list energy in kilojoules (kJ) as well. One food Calorie equals about 4.2 kJ.
Kinds of energy measured in joules
| Type | Example |
|---|---|
| Kinetic | A moving car |
| Potential | A stretched spring; see elastic potential energy |
| Thermal | Heat in a cup of tea |
| Electrical | Energy from a battery |
| Chemical | Energy stored in food and fuel |
Who was James Joule?
James Prescott Joule (1818–1889) was an English brewer and scientist. In a famous experiment, he used a falling weight to spin paddles in water and showed that the water warmed up, proving that mechanical work and heat are forms of the same thing. This helped establish the law of conservation of energy. The unit was named after him in 1889.
For how sunlight becomes electrical energy, see how solar panels work.
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