One joule is roughly the energy needed to lift an apple a metre
A joule sounds abstract, but it is surprisingly everyday: about what it takes to raise a small apple one metre, or to run a one-watt gadget for a second. Your resting body gives off that much heat every sixtieth of a second. The unit's namesake, James Prescott Joule, died the year it was adopted.
The joule is the SI unit of energy. Formally it is the work done when a force of one newton pushes something one metre along the direction of the force, which in base units means a kilogram times a metre squared per second squared. It is equally the heat produced when one ampere flows through one ohm of resistance for one second, the work of moving one coulomb of charge across one volt, and one watt sustained for one second. A kilowatt-hour on an electricity bill equals 3.6 million joules.
The name came from a proposal by Wilhelm Siemens. In his inaugural address to the British Association on 23 August 1882, after taking its chair, he suggested a heat unit derived from the ampere and ohm and named in honour of Joule, whose research had advanced the dynamical theory of heat. Joule, then 63 and retired, lived to see delegates formally approve it, alongside the watt, on 31 August 1889, and died on 11 October that year.
Its definition kept evolving. In 1935 the International Electrotechnical Commission adopted the Giorgi system, approved by the international weights and measures committee in 1946, which recast the joule as purely mechanical work, meant to serve electrical and mechanical uses alike. The 1948 General Conference added that it should be the preferred unit of heat in calorimetry, officially pushing aside the calorie. The formula has stayed fixed since 1946, but the joule inherits every change to its ingredients, including the metre in 1983 and the kilogram in 2019.
Some reference points: warming 0.239 grams of water by one degree takes a joule, as does moving a 56-gram tennis ball at 6 metres per second. At the tiny end, 160 zeptojoules make roughly one electronvolt, and the Landauer limit sets about 2.75 zeptojoules as the minimum to flip one bit of data at room temperature. As with every SI unit honouring a person, the symbol is a capital J, but the word stays lowercase.
Source: Joule
