What is a joule?

The joule (symbol: J) is the SI unit of energy. It is defined as the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one meter (1 N⋅m). It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

The joule is named after the English physicist James Prescott Joule.

Common uses of the joule include measuring:

• Energy (e.g., kinetic, potential, thermal)
• Work
• Heat

One joule is exactly equal to:

• 1 kg⋅m²/s² (based on the definition in terms of force and distance)
• 1 N⋅m (newton-meter, the SI unit of torque)
• 1 W⋅s (watt-second)

Some practical examples:

• The energy required to lift a small apple (approximately 100 grams) vertically through 1 meter is roughly 1 joule.
• The energy released as heat by a person at rest, about 100 joules per second.

The joule is related to other units of energy, such as the calorie (1 calorie ≈ 4.184 joules). Larger multiples are often used, such as the kilojoule (kJ), which is equal to 1000 joules.