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Joule

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The joule (symbol: J) is the SI unit of energy. It is named in honor of the physicist James Prescott Joule (1818 to 1889).

Definition

The joule is a derived unit defined as the work done to exert a force of one newton for a distance of one metre, so the same quantity may be referred to as a newton metre or newton-metre (also with meter spelling), with the symbol N·m or N m. It can also be written as kg·m2·s−2. However, the newton metre is usually used as a measure of torque, not energy.

One joule is also:

  • the work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V.
  • the work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt-hour), with the symbol W·s

Another way of visualizing the joule is the work required to lift a mass of about 102 g (e.g. a small apple) for one metre under the Earth's gravity.

Conversions

1 joule is equal to:



SI multiples

Multiple Name Symbol Multiple Name Symbol
100 joule J      
101 decajoule daJ 10–1 decijoule dJ
102 hectojoule hJ 10–2 centijoule cJ
103 kilojoule kJ 10–3 millijoule mJ
106 megajoule MJ 10–6 microjoule µJ
109 gigajoule GJ 10–9 nanojoule nJ
1012 terajoule TJ 10–12 picojoule pJ
1015 petajoule PJ 10–15 femtojoule fJ
1018 exajoule EJ 10–18 attojoule aJ
1021 zettajoule ZJ 10–21 zeptojoule zJ
1024 yottajoule YJ 10–24 yoctojoule yJ

Kilojoule

A kilojoule (abbreviation: kJ) is a unit of energy equal to 1000 joules.

  • One kJ is the amount of work done by a one kilowatt device in one second.
  • One kJ is sufficient to melt 3 grams of ice at 0 °C
  • Approximately one kJ of work is done when 100 kilograms is lifted through one meter at Earth's surface (or, if one kilogram is lifted through 100 meters.)
  • One kJ is equal to 5/18 (approx. 0.2778) of a watt hour.

Megajoule

A megajoule (abbreviation: MJ) is a unit of energy equal to 1,000,000 joules.

  • One MJ is the approximate amount of work done by a one kilowatt device in fifteen minutes.
  • One MJ is sufficient to melt 3 kilograms of ice at 0 °C.
  • One MJ is equal to 5/18 (approx. 0.2778) of a kilowatt-hour.
  • One MJ is the approximate food energy of a half-liter of orange juice.
  • One MJ is the approximate food energy of 3 bananas.
  • One MJ is the approximate food energy of 50 g (a small bar - just under 2 oz ) of chocolate.
  • One MJ is the approximate energy output of a 100 bhp motor running, at full power and load, for 15 seconds.
  • One MJ is a 100 kg person lifted up 1000 m in Earth's gravity. Note this is 240 food calories, but walking up a 1000 m (approx 3000 ft) slope/mountain would burn far more. See Potential energy.
  • One MJ is an average-sized man swimming breast-stroke for 18 minutes, or an average-sized woman swimming breast-stroke for 26 minutes.

See also

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