Joule

The joule (symbol: J) is the SI unit of energy, which is defined as the potential to do work. The joule has base units of kg·m²/s² = N·m. The base unit conversion can be remembered using the equation E = mc², where E is in joules, m is in kilograms, and c is the speed of light in meters per second.

Definition

The joule is a derived unit defined as Joules the work done or energy required 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 with the symbol N·m. However, the newton metre is usually used as a measure of work.

As a rough guide, 1 joule is the absolute minimum amount of energy required to lift a one kilogram object up by a height of 10 centimetres on the surface of Earth.

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

Conversions

1 joule is exactly 10⁷ ergs.

1 joule is approximately equal to:

• 6.24150636309 ×10¹⁸ eV (electron-volts)
• 0.238845896628 cal (calorie) (small calories)
• 2.390 ×10⁻⁴ Calorie or kilocalorie (food)
• 9.47817120313 ×10⁻⁴ BTU (British thermal unit)
• 0.737562149277 ft·lbf (Foot-pound force)
• 23.7 ft·pdl (foot poundals)
• 2.7778 ×10⁻⁷ kilowatt-hour
• 2.7778 ×10⁻⁴ watt-hour
• 9.8692 ×10⁻³ litre-atmosphere

Also, some very rough approximations of a joule from the real world are:

• the energy required to lift a small apple (102 g) one metre against Earth's gravity
• the amount of energy, as heat, that a quiet person produces every hundredth of a second.
• 1/100th of the energy a person can get by drinking a single droplet of beer.

Units defined in terms of the joule include:

• 1 thermochemical calorie = 4.184 J (exact)
• 1 International Table calorie = 4.1868 J (exact)
• 1 watt-hour = 3600 J (exact)
  • 1 kWh = 1 kilowatt-hour = 3.6×10⁶ J = 3.6 MJ

History

A joule is the mechanical equivalent of heat meaning the number of units of work in which the unit of heat can perform. Its value was found by James Prescott Joule in experiments that showed the mechanical energy Joule's equivalent, and represented by the symbol J. The term was first introduced by Dr. Mayer of Heilbronn.

SI multiples

SI multiples of joule (J)

Submultiples			Multiples
Value	SI symbol	Name	Value	SI symbol	Name
10−1 J	dJ	decijoule	101 J	daJ	decajoule
10−2 J	cJ	centijoule	102 J	hJ	hectojoule
10−3 J	mJ	millijoule	103 J	kJ	kilojoule
10−6 J	μJ	microjoule	106 J	MJ	megajoule
10−9 J	nJ	nanojoule	109 J	GJ	gigajoule
10−12 J	pJ	picojoule	1012 J	TJ	terajoule
10−15 J	fJ	femtojoule	1015 J	PJ	petajoule
10−18 J	aJ	attojoule	1018 J	EJ	exajoule
10−21 J	zJ	zeptojoule	1021 J	ZJ	zettajoule
10−24 J	yJ	yoctojoule	1024 J	YJ	yottajoule
10−27 J	rJ	rontojoule	1027 J	RJ	ronnajoule
10−30 J	qJ	quectojoule	1030 J	QJ	quettajoule

See also

The joule is named after James Prescott Joule. As with every SI unit named for a person, its symbol starts with an upper case letter (J), but when written in full, it follows the rules for capitalisation of a common noun; i.e., joule becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

• Conversion of units
• SI prefixes
• Orders of magnitude
• Orders of magnitude (energy)
• Electronvolt
• Watt-hour

References

• The adoption of joules as units of energy, FAO/WHO Ad Hoc Committee of Experts on Energy and Protein, 1971. A report on the changeover from calories to joules in nutrition.

External links

• Conversion of J in other units