Joule
The SI unit of energy — one newton acting through one metre — named for James Prescott Joule.
Definition
The joule (symbol J) is the SI unit of energy, work, and heat, defined as the work done when a force of one newton acts through a distance of one metre: 1 J = 1 N·m = 1 kg·m²/s². Because heat is a form of energy, heat too is measured in joules, a unification made possible by Joule's own demonstration of the mechanical equivalent of heat.
One joule is a modest amount on the human scale — roughly the energy to lift a small apple one metre. Related quantities include the watt (one joule per second of power), the calorie (4.186 J), and the kilowatt-hour (3.6 million J). The thermochemical calorie used in nutrition is actually a kilocalorie, 4186 J.
The unit's adoption as the universal measure of energy reflects the nineteenth-century insight that mechanical, thermal, electrical, and chemical energy are interconvertible and should share a single unit.
History
Named in honour of James Prescott Joule and adopted by the international scientific community; it became the SI unit of energy in the twentieth century.