Joule

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This article is about the unit of energy. For other uses, see Joule (disambiguation).
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
   kilowatt hours    2.78×10−7 kW⋅h
   kilocalories    2.39×10−4 kcal
   BTUs    9.48×10−4 BTU
   electronvolts    6.24×1018 eV

The joule (/ˈl/ or sometimes /ˈl/), symbol J, is a derived unit of energy, work, or amount of heat in the International System of Units, equivalent to 0.00094781712 BTU.[1] It is equal to the energy expended (or work done) in applying a force of one newton through a distance of one metre (1 newton metre or N·m), or in passing an electric current of one ampere through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).[2][3][4]

In terms firstly of base SI units and then in terms of other SI units:

\rm J  = {}\rm \frac{kg \cdot m^2}{s^2} = N \cdot m = \rm Pa \cdot m^3={}\rm W \cdot s = C \cdot V

where kg is the kilogram, m is the metre, s is the second, N is the newton, Pa is the pascal, W is the watt, C is the coulomb, and V is the volt.

One joule can also be defined as:

Usage[edit]

This SI unit is named after James Prescott Joule. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (J). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (joule), except in a situation where any word in that position 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[edit]

Main article: newton metre

In angular mechanics, torque is analogous to the linear Newtonian mechanics parameter of force, moment of inertia to mass, and angle to distance. Energy is the same in both systems. Thus, although the joule has the same dimensions as the newton-metre (1 J = 1 N·m = 1 kg·m2·s−2), these units are not interchangeable: the CGPM has given the unit of energy the name "joule", but has not given the unit of torque any special name, hence the unit of torque is known as the newton-metre (N·m) - a compound name derived from its constituent parts.[5] Torque and energy are related to each other using the equation

E= \tau \theta\

where E is the energy, τ is the torque, and θ is the angle moved (in radians). Since radians are dimensionless, it follows that torque and energy have the same dimensions.

The use of newton-metres for torque and joules for energy is useful in helping avoid misunderstandings and miscommunications.[5]

An additional solution is to realize that joules are scalars – they are the dot product of a vector force and a vector displacement whereas torque is a vector. Torque is the cross product of a distance vector and a force vector. Drawing a traditional vector arrow over "newton-metre" in a torque resolves the ambiguity.

Practical examples[edit]

One joule in everyday life represents approximately:

  • the energy required to lift a small apple (with a mass of approximately 100 g) vertically through one metre of air.
  • the energy released when that same apple falls one metre to the ground.
  • the heat required to raise the temperature of 1 g of water by 0.24 K.[6]
  • the typical energy released as heat by a person at rest every 1/60 second (approximately 17 ms).[7]
  • the kinetic energy of a 50 kg human moving very slowly (0.2 m/s or 0.72 km/h).
  • the kinetic energy of a 56 g tennis ball moving at 6 m/s (22 km/h).[8]
  • the kinetic energy of an object with mass 1 kg moving at √2 ≈ 1.4 m/s.

Since the joule is also a watt-second and the common unit for electricity sales to homes is the kW·h (kilowatt-hour), a kW·h is thus 1000 (kilo) × 3600 seconds = 3.6 MJ (megajoules).

Multiples[edit]

For additional examples, see: Orders of magnitude (energy)
SI multiples for joule (J)
Submultiples Multiples
Value Symbol Name Value 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
Common multiples are in bold face

Nanojoule[edit]

The nanojoule (nJ) is equal to one billionth (10-9) of one joule. One nanojoule is about 1/160 of the kinetic energy of a flying mosquito.[9]

Microjoule[edit]

The microjoule (μJ) is equal to one millionth (10-6) of one joule. The Large Hadron Collider (LHC) is expected to produce collisions on the order of 1 microjoule (7 TeV) per particle.

Millijoule[edit]

The millijoule (mJ) is equal to one thousandth (10-3) of a joule.

Kilojoule[edit]

The kilojoule (kJ) is equal to one thousand (103) joules. Nutritional food labels in certain countries express energy in kilojoules (kJ).

One kilojoule per second (1 kilowatt) is approximately the amount of solar radiation received by one square metre of the Earth in full daylight.[10]

Megajoule[edit]

The megajoule (MJ) is equal to one million (106) joules, or approximately the kinetic energy of a one-tonne vehicle moving at 160 km/h (100 mph).

Because 1 watt times 1 second equals 1 joule, 1 kilowatt-hour is 1000 watts times 3600 seconds, or 3.6 megajoules.

Gigajoule[edit]

The gigajoule (GJ) is equal to one billion (109) joules. 6 GJ is about the amount of potential chemical energy in a barrel of oil, when combusted.[11]

Terajoule[edit]

The terajoule (TJ) is equal to one trillion (1012) joules. About 63 TJ of energy was released by the atomic bomb that exploded over Hiroshima.[12] The International Space Station, with a mass of approximately 450,000 kg and orbital velocity of 7.7 km/s,[13] has a kinetic energy of roughly 13 TJ.

Petajoule[edit]

The petajoule (PJ) is equal to one quadrillion (1015) joules. 210 PJ is equivalent to about 50 megatons of TNT. This is the amount of energy released by the Tsar Bomba, the largest man-made nuclear explosion ever.

Exajoule[edit]

The exajoule (EJ) is equal to one quintillion (1018) joules. The 2011 Tōhoku earthquake and tsunami in Japan had 1.41 EJ of energy according to its 9.0 on the moment magnitude scale. Energy in the United States used per year is roughly 94 EJ.

Zettajoule[edit]

The zettajoule (ZJ) is equal to one sextillion (1021) joules. Annual global energy consumption is approximately 0.5 ZJ.

Yottajoule[edit]

The yottajoule (YJ) is equal to one septillion (1024) joules. This is approximately the amount of energy required to heat the entire volume of water on Earth by 1 °C.

Conversions[edit]

1 joule is equal to:

Units defined exactly in terms of the joule include:

  • 1 thermochemical calorie = 4.184 J[14]
  • 1 International Table calorie = 4.1868 J[15]
  • 1 watt hour = 3600 J
  • 1 kilowatt hour = 3.6×106 J (or 3.6 MJ)
  • 1 watt second = 1 J
  • 1 ton TNT = 4.184 GJ

See also[edit]

Notes and references[edit]

  1. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 120, ISBN 92-822-2213-6 
  2. ^ American Heritage Dictionary of the English Language, Online Edition (2009). Houghton Mifflin Co., hosted by Yahoo! Education.
  3. ^ The American Heritage Dictionary, Second College Edition (1985). Boston: Houghton Mifflin Co., p. 691.
  4. ^ McGraw-Hill Dictionary of Physics, Fifth Edition (1997). McGraw-Hill, Inc., p. 224.
  5. ^ a b 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."
  6. ^ "Units of Heat - BTU, Calorie and Joule". Engineeringtoolbox.com. Retrieved 2013-09-16. 
  7. ^ This is called the basal metabolic rate. It corresponds to about 5,000 kJ (1,200 kcal) per day. The kilocalorie (symbol kcal) is also known as the dietary calorie. "At rest" means awake but inactive.
  8. ^ Ristinen, Robert A.; Kraushaar, Jack J. (2006). Energy and the Environment (2nd ed.). Hoboken, NJ: John Wiley & Sons. ISBN 0-471-73989-8. 
  9. ^ CERN - Glossary
  10. ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". Retrieved 2005-10-05. 
  11. ^ IRS publication
  12. ^ 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
  13. ^ International Space Station Fact Sheet
  14. ^ 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.
  15. ^ Feynman, Richard (1963). "Physical Units". Feynman's Lectures on Physics. Retrieved 2014-03-07.