Joule

The joule (symbol: J) is the SI unit of energy. In base units, one joule equals one kilogram-metre squared per second squared.

Definition

It is the work performed when a one-newton force moves an object one metre in the direction of the force. It also equals the heat dissipated when a one-ampere current flows through a one-ohm resistor for one second. The unit is named after English physicist James Prescott Joule.

The official definition, from the International Bureau of Weights and Measures, states the joule is the work done when a one-newton force moves its point of application one metre in the force’s direction. One joule is also equivalent to the work needed to move one coulomb of charge through a one-volt potential difference (one coulomb-volt), or the work needed to produce one watt of power for one second (one watt-second). As with all SI units named after a person, the symbol is capitalised (J), but the full name is lowercase except at the start of a sentence or in titles.

History

The joule’s history begins with the CGS system, made official in 1881. The erg became its energy unit in 1882. That same year, Wilhelm Siemens proposed the joule as a heat unit, derived from the ampere and ohm, equal to 10⁷ erg.

He suggested naming it after James Prescott Joule, then 63 and retired, for his work on the mechanical theory of heat. The joule was officially adopted at the second International Electrical Congress in 1889, alongside the watt and quadrant (later renamed henry). Joule died that year. In 1893, the “international joule” was defined from the “international ampere” and “international ohm.” In 1935, the International Electrotechnical Commission adopted the Giorgi system, which redefined the joule.

By 1946, the International Committee for Weights and Measures approved the joule as the unit of work from one unit of force (later named newton) over one metre, intended for both electromagnetic and mechanical contexts. The 1948 General Conference on Weights and Measures confirmed this definition and preferred the joule over the calorie for heat. The modern SI definition (J = kg⋅m²⋅s⁻²) has remained unchanged since 1946, though the joule as a derived unit has been affected by later redefinitions of the second (1960, 1967), metre (1983), and kilogram (2019).

Practical examples of one joule include: the heat released by a resting person every 1/60 second (about 5,000 kJ per day); the electricity to run a 1 W device for one second; the energy to accelerate a 1 kg mass at 1 m/s² over one metre; the kinetic energy of a 2 kg mass moving at 1 m/s (or a 1 kg mass at about 1.41 m/s); the energy to lift a 101.97 g apple one metre; the heat to raise 0.239 g of water from 0 °C to 1 °C; the kinetic energy of a 50 kg human moving at 0.2 m/s; the kinetic energy of a 56 g tennis ball at 6 m/s; and the food energy in just over half a sugar crystal (0.102 mg).

Multiples include: the zeptojoule (160 zJ is about one electronvolt; the Landauer limit for changing a data bit at room temperature is about 2.75 zJ); the nanojoule (160 nJ is roughly the kinetic energy of a flying mosquito); the microjoule (the Large Hadron Collider produces collisions of about 7 TeV, or microjoules, per particle); the kilojoule (food labels often use kJ; one square metre of Earth receives about 1.4 kJ of solar radiation per second in full daylight; a sprinting human has about 3 kJ of kinetic energy, a cheetah at 122 km/h about 20 kJ; one watt-hour is 3.6 kJ); and the megajoule (roughly the kinetic energy of a one-tonne vehicle at 161 km/h, or the energy to heat 10 litres of water at constant pressure).