Joule

Joule
Unit system:	SI derived unit
Unit of...	Energy
Symbol:	J
Named after:	James Prescott Joule
Unit conversions
1 J in...	is equal to...
SI base units	1 kg·m²/s²
CGS units	1×10⁷ erg

The joule (pronounced /ˈdʒuːl/ or /ˈdʒaʊl/; symbol J), named after James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy expended in applying a force of one Newton through a distance of one metre (1 Newton·metre or N·m). In terms of dimensions:

$\rm 1\ J = 1\ N \cdot m = \left ( \frac{kg \cdot m}{s^2} \right ) \cdot m = \frac{kg \cdot m^2}{s^2}=Pa \cdot m^3= 1\ W \cdot s$

One joule is defined as the amount of work done by a force of one newton moving an object through a distance of one metre. Other relationships are:

The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt (C·V). This relationship can be used to define the volt;
The work required to continuously produce one watt of power for one second; or one watt second (W·s) (compare kilowatt hour). This relationship can be used to define the watt.

1 Usage
2 Confusion with newton metre
3 Practical examples
4 Multiples
5 Conversions
6 See also
7 References
8 External links

Usage

This SI unit is named after James Prescott Joule. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (J). When an SI unit is spelled out in English, it should always begin with a lowercase letter (joule), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

—Based on The International System of Units, section 5.2.

Confusion with newton metre

Main article: Newton metre

While it is dimensionally correct to express joules as newton metres or N·m, such use is discouraged^[1] by the SI authority to avoid confusion with torque. Torque and energy are fundamentally different physical quantities. For example, adding 1 N·m of torque to 1 N·m of energy gives a dimensionally consistent result of 2 N·m, but this quantity is physically meaningless.

Practical examples

One joule in everyday life is approximately:

the energy required to lift a small apple one meter straight up.
the energy released when that same apple falls one meter to the ground.
the energy released as heat by a person at rest, every hundredth of a second.
one hundredth of the energy a person can receive by drinking a drop of beer.
the kinetic energy of a 50 kg human moving very slowly (0.2 m/s or 720 m/h).
the kinetic energy of a tennis ball moving at 23 km/h (14 mph).^[2]

Multiples

For additional examples, see: Orders of magnitude (energy)

SI multiples for joule (J)
Value	Symbol	Name	Value	Symbol	Name
Submultiples			Multiples
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
Common multiples are in bold face

Nanojoule

The nanojoule (nJ) is equal to one billionth of one joule. One nanojoule is about 1/160 of the kinetic energy of a flying mosquito.^[3]

Microjoule

The microjoule (μJ) is equal to one millionth of one joule. The Large Hadron Collider (LHC) is expected to produce collisions on the order of 1 microjoule (7 TeV) per particle.

Millijoule

The millijoule (mJ) is equal to one thousandth of one joule.

Kilojoule

The kilojoule (kJ) is equal to one thousand joules. Food labels in some countries express food energy in kilojoules. One kilojoule is about the maximum amount of solar radiation received by one square metre of the Earth in one second.^[4]

Megajoule

The megajoule (MJ) is equal to one million joules, or approximately the kinetic energy of a one-ton vehicle moving at 160 km/h (100 mph).

Gigajoule

The gigajoule (GJ) is equal to one billion joules. Six gigajoules is about the amount of chemical energy in a barrel of oil.^[5]

Terajoule

The terajoule (TJ) is equal to one trillion joules. About 60 terajoules were released by the nuclear bomb that exploded over Hiroshima.^[6]

Conversions

1 joule is equal to:

1×10⁷ ergs (exactly)
6.24150974×10¹⁸ eV (electronvolts)
0.2390 cal (thermochemical gram calories or small calories)
2.3901×10⁻⁴ kcal (thermochemical kilocalories, kilogram calories, large calories or food calories)
9.4782×10⁻⁴ BTU (British thermal unit)
0.7376 ft·lbf (foot-pounds 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
1×10⁻⁴⁴ foe (exactly)

Units defined exactly in terms of the joule include:

1 thermochemical calorie = 4.184 J^[7]
1 International Table calorie = 4.1868 J^[7]
1 watt hour = 3600 J
1 kilowatt hour = 3.6×10⁶ J (or 3.6 MJ)
1 ton TNT = 4.184 GJ

References

↑ From the official SI website: "A derived unit can often be expressed in different ways by combining base units with derived units having special names. Joule, for example, may formally be written newton metre, or kilogram metre squared per second squared. This, however, is an algebraic freedom to be governed by common sense physical considerations; in a given situation some forms may be more helpful than others. In practice, with certain quantities, preference is given to the use of certain special unit names, or combinations of unit names, to facilitate the distinction between different quantities having the same dimension."
↑ Ristinen, Robert A., and Jack J. Kraushaar. Energy and the Environment. 2nd ed. Hoboken, NJ: John Wiley & Sons, Inc., 2006.
↑ CERN - Glossary
↑ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant. Retrieved 2005-10-05.
↑ IRS publication
↑ Los Alamos National Laboratory report LA-8819, The yields of the Hiroshima and Nagasaki nuclear explosions by John Malik, September 1985. Available online at http://www.mbe.doe.gov/me70/manhattan/publications/LANLHiroshimaNagasakiYields.pdf
↑ ^7.0 ^7.1 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.

SI units

Base units	Ampere · Candela · Kelvin · Kilogram · Metre · Mole · Second

Derived units	Becquerel · Coulomb · degree Celsius · Farad · Gray · Henry · Hertz · Joule · Katal · Lumen · Lux · Newton · Ohm · Pascal · Radian · Siemens · Sievert · Steradian · Tesla · Volt · Watt · Weber

Accepted for use with SI	Atomic mass unit · Astronomical unit · Day · Decibel · Degree of arc · Electronvolt · Hectare · Hour · Litre · Minute · Minute of arc · Neper · Second of arc · Tonne Atomic units · Natural units

See also	SI prefixes · Systems of measurement · Conversion of units

Book:International System of Units · Category:SI base units