Joule Explained

Joule
Standard:	SI
Quantity:	energy
Symbol:	J
Namedafter:	James Prescott Joule
Units1:	SI base units
Inunits1:	kg⋅m^2⋅s^−2
Units2:	CGS units
Inunits2:	erg
Units3:	watt-seconds
Inunits3:	W⋅s
Units4:	kilowatt-hours
Inunits4:	≈
Units5:	kilocalories (thermochemical)
Units6:	BTUs
Units7:	electronvolts
Inunits7:	≈

The joule (pronounced, or ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of one newton displaces a mass through a distance of one metre in the direction of that force. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).^{[1] [2] [3]}

Definition

In terms of SI base units and in terms of SI derived units with special names, the joule is defined as^[4]

Symbol Meaning J joule kg kilogram m metre s second N newton Pa pascal W watt C coulomb V volt

One joule is also equivalent to any of the following:

• 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 produce one watt of power for one second, or one watt-second (W⋅s) (compare kilowatt-hour, which is 3.6 megajoules). This relationship can be used to define the watt.

^[5]

History

The cgs system had been declared official in 1881, at the first International Electrical Congress.The erg was adopted as its unit of energy in 1882. Wilhelm Siemens, in his inauguration speech as chairman of the British Association for the Advancement of Science (23 August 1882) first proposed the Joule as unit of heat, to be derived from the electromagnetic units Ampere and Ohm, in cgs units equivalent to .The naming of the unit in honour of James Prescott Joule (1818–1889), at the time retired but still living (aged 63), is due to Siemens:

"Such a heat unit, if found acceptable, might with great propriety, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat."^[6]

At the second International Electrical Congress, on 31 August 1889, the joule was officially adopted alongside the watt and the quadrant (later renamed to henry).^[7] Joule died in the same year, on 11 October 1889.At the fourth congress (1893), the "international ampere" and "international ohm" were defined, with slight changes in the specifications for their measurement, with the "international joule" being the unit derived from them.^[8]

In 1935, the International Electrotechnical Commission (as the successor organisation of the International Electrical Congress) adopted the "Giorgi system", which by virtue of assuming a defined value for the magnetic constant also implied a redefinition of the Joule. The Giorgi system was approved by the International Committee for Weights and Measures in 1946. The joule was now no longer defined based on electromagnetic unit, but instead as the unit of work performed by one unit of force (at the time not yet named newton)over the distance of 1 metre. The joule was explicitly intended as the unit of energy to be used in both electromagnetic and mechanical contexts.^[9] The ratification of the definition at the ninth General Conference on Weights and Measures, in 1948,added the specification that the joule was also to be preferred as the unit of heat in the context of calorimetry, thereby officially deprecating the use of the calorie.^[10] This definition was the direct precursor of the joule as adopted in the modern International System of Units in 1960.

The definition of the joule as J = kg⋅m²⋅s⁻² has remained unchanged since 1946, but the joule as a derived unit has inherited changes in the definitions of the second (in 1960 and 1967), the metre (in 1983) and the kilogram (in 2019).^[11]

Practical examples

One joule represents (approximately):

• The typical energy released as heat by a person at rest every 1/60 s (~, basal metabolic rate); about 1200kcal / day.
• The amount of electricity required to run a device for .
• The energy required to accelerate a mass at through a distance of .
• The kinetic energy of a mass travelling at, or a mass travelling at .
• The energy required to lift an apple up 1 m, assuming the apple has a mass of 101.97 g.
• The heat required to raise the temperature of 0.239 g of water from 0 °C to 1 °C.^[12]
• The kinetic energy of a human moving very slowly (0.2abbr=onNaNabbr=on).
• The kinetic energy of a tennis ball moving at 6m/s.^[13]
• The food energy (kcal) in slightly more than half of an ordinary-sized sugar crystal (/crystal).

Multiples

: is about one electronvolt. The minimal energy needed to change a bit of data in computation at around room temperature – approximately – is given by the Landauer limit.

: is about the kinetic energy of a flying mosquito.^[14]

: The Large Hadron Collider (LHC) produces collisions of the microjoule order (7 TeV) per particle.

: Nutritional food labels in most countries express energy in kilojoules (kJ).^[15] One square metre of the Earth receives about of solar radiation every second in full daylight.^[16] A human in a sprint has approximately 3 kJ of kinetic energy,^[17] while a cheetah in a (76 mph) sprint has approximately 20 kJ.^[18] One watt-hour of electricity is .

: The megajoule is approximately the kinetic energy of a one megagram (tonne) vehicle moving at (100 mph). The energy required to heat of liquid water at constant pressure from to is approximately . One kilowatt-hour of electricity is .

: is about the chemical energy of combusting 1oilbbl of petroleum.^[19] 2 GJ is about the Planck energy unit. One megawatt-hour of electricity is .

: The terajoule is about (which is often used in energy tables). About of energy was released by Little Boy.^[20] The International Space Station, with a mass of approximately and orbital velocity of,^[21] has a kinetic energy of roughly . In 2017, Hurricane Irma was estimated to have a peak wind energy of .^{[22] [23]} One gigawatt-hour of electricity is .

: is about of TNT, which is the amount of energy released by the Tsar Bomba, the largest man-made explosion ever. One terawatt-hour of electricity is .

: The 2011 Tōhoku earthquake and tsunami in Japan had of energy according to its rating of 9.0 on the moment magnitude scale. Yearly U.S. energy consumption amounts to roughly, and the world final energy consumption was in 2021.^[24] One petawatt-hour of electricity is .

: The zettajoule is somewhat more than the amount of energy required to heat the Baltic Sea by 1 °C, assuming properties similar to those of pure water.^[25] Human annual world energy consumption is approximately . The energy to raise the temperature of Earth's atmosphere 1 °C is approximately .

: The yottajoule is a little less than the amount of energy required to heat the Indian Ocean by 1 °C, assuming properties similar to those of pure water. The thermal output of the Sun is approximately per second.

Conversions

1 joule is equal to (approximately unless otherwise stated):

• (exactly)
  • (gram calories)
• (food calories)
  • (foot-pound)
• (foot-poundal)
• (kilowatt-hour)
• (watt-hour)
• (litre-atmosphere)
• (by way of mass–energy equivalence)

Units defined exactly in terms of the joule include:

• 1 thermochemical calorie = 4.184J^[26]
• 1 International Table calorie = 4.1868J^[27]
• 1W⋅h = 3600J (or 3.6kJ)
• 1kW⋅h = (or 3.6MJ)
• 1W⋅s =
• 1ton TNT =
• 1foe =

Newton-metre and torque

See main article: Newton-metre. In mechanics, the concept of force (in some direction) has a close analogue in the concept of torque (about some angle):

Linear	Angular
Force	Torque
	Moment of inertia
Displacement	Angle

A result of this similarity is that the SI unit for torque is the newton-metre, which works out algebraically to have the same dimensions as the joule, but they are not interchangeable. The General Conference on Weights and Measures has given the unit of energy the name joule, but has not given the unit of torque any special name, hence it is simply the newton-metre (N⋅m) – a compound name derived from its constituent parts. The use of newton-metres for torque but joules for energy is helpful to avoid misunderstandings and miscommunication.^[28]

The distinction may be seen also in the fact that energy is a scalar quantity – the dot product of a force vector and a displacement vector. By contrast, torque is a vector – the cross product of a force vector and a distance vector. Torque and energy are related to one another by the equation$$E\; =\; \backslash tau\; \backslash theta\backslash ,,$$

where E is energy, τ is (the vector magnitude of) torque, and θ is the angle swept (in radians). Since plane angles are dimensionless, it follows that torque and energy have the same dimensions.

Watt-second

A watt-second (symbol W s or W⋅s) is a derived unit of energy equivalent to the joule. The watt-second is the energy equivalent to the power of one watt sustained for one second. While the watt-second is equivalent to the joule in both units and meaning, there are some contexts in which the term "watt-second" is used instead of "joule", such as in the rating of photographic electronic flash units. ^[29]

Notes and References

1. https://web.archive.org/web/20060413141420/http://education.yahoo.com/reference/dictionary/entry/joule American Heritage Dictionary of the English Language
2. The American Heritage Dictionary, Second College Edition (1985). Boston: Houghton Mifflin Co., p. 691.
3. McGraw-Hill Dictionary of Physics, Fifth Edition (1997). McGraw-Hill, Inc., p. 224.
4. 2016-01-28 . NIST Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI Prefixes . NIST . en.
5. Web site: What Is a Joule? - Chemistry Definition . 2024-04-07 . ThoughtCo . en.
6. The unit of heat has hitherto been taken variously as the heat required to raise a pound of water at the freezing-point through 1° Fahrenheit or Centigrade, or, again, the heat necessary to raise a kilogramme of water 1° Centigrade. The inconvenience of a unit so entirely arbitrary is sufficiently apparent to justify the introduction of one based on the electro-magnetic system, viz. the heat generated in one second by the current of an Ampère flowing through the resistance of an Ohm. In absolute measure its value is 10⁷ C.G.S. units, and, assuming Joule's equivalent as 42,000,000, it is the heat necessary to raise 0.238 grammes of water 1° Centigrade, or, approximately, the th part of the arbitrary unit of a pound of water raised 1° Fahrenheit and the th of the kilogramme of water raised 1° Centigrade. Such a heat unit, if found acceptable, might with great propriety, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat. . Cal Wilhelm . Siemens . Carl Wilhelm Siemens . Report of the Fifty-Second Meeting of the British Association for the Advancement of Science . 1–33 . 6-7 . August 1882 . Southhampton.
7. Pat Naughtin: A chronological history of the modern metric system, metricationmatters.com, 2009.
8. Book: . Proceedings of the International Electrical Congress. New York . American Institute of Electrical Engineers . 1894 .
9. http://www.bipm.org/en/CIPM/db/1946/2/ CIPM, 1946, Resolution 2, Definitions of electric units
10. http://www.bipm.org/en/CGPM/db/9/3/ 9th CGPM, Resolution 3: Triple point of water; thermodynamic scale with a single fixed point; unit of quantity of heat (joule).
11. 2018-05-11 . SI Redefinition . NIST . en.
12. Web site: Units of Heat – BTU, Calorie and Joule . Engineering Toolbox . 2021-06-14 .
13. Book: Ristinen . Robert A. . Jack J. . Kraushaar . Energy and the Environment . 2nd . Hoboken, NJ . John Wiley & Sons . 2006 . 0-471-73989-8 . registration .
14. Web site: Physics – CERN. public.web.cern.ch. dead. https://web.archive.org/web/20121213173112/https://public.web.cern.ch/public/en/Science/Glossary-en.php. 2012-12-13.
15. Web site: You Say Calorie, We Say Kilojoule: Who's Right? . https://web.archive.org/web/20230515172626/https://www.coca-colacompany.com/au/news/you-say-calorie--we-say-kilojoule-who-s-right- . 2 May 2017. 2023-05-15 .
16. Web site: Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present . 2005-10-05 . https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant . 2011-08-30 .
17. \tfrac{ 1 }{ 2 } ⋅ 70 ~ kg ⋅ \left( 10 ~ m/s \right) ^{2=3500~J}
18. \tfrac{ 1 }{ 2 } ⋅ 35 ~ kg ⋅ \left( 35 ~ m/s \right) ^{2=21,400~J}
19. Web site: Energy Units – Energy Explained, Your Guide To Understanding Energy – Energy Information Administration. www.eia.gov.
20. Web site: Report LA-8819: The yields of the Hiroshima and Nagasaki nuclear explosions . Malik . John . September 1985 . Los Alamos National Laboratory. 18 March 2015 . https://web.archive.org/web/20091011030043/http://www.mbe.doe.gov/me70/manhattan/publications/LANLHiroshimaNagasakiYields.pdf . 11 October 2009 .
21. Web site: International Space Station Final Configuration . . 18 March 2015 . https://web.archive.org/web/20110721012349/http://www.spaceflight.esa.int/users/downloads/factsheets/fs001_12_iss.pdf . 21 July 2011 .
22. News: Analysis – How Big Is Hurricane Irma?. Washington Post. Bonnie Berkowitz. Laris Karklis. Reuben Fischer-Baum. Chiqui Esteban. 11 September 2017. 2 November 2017.
23. News: Rathbone . John-Paul . Fontanella-Khan . James . Rovnick . Naomi . 11 September 2017 . A weakened Irma unleashes more damage on Florida coast . live . https://archive.today/20240804212211/https://webcache.googleusercontent.com/search?q=cache:https://www.ft.com/content/2c58ce3e-9621-11e7-b83c-9588e51488a0 . 4 August 2024 . 11 September 2017 . . New York (Rathbone), Miami (Fontanella-Khan), London (Rovnick) . 0307-1766.
24. 2022 . World Energy Outlook 2022 . International Energy Agency . 239 . 7 September 2023 .
25. Web site: Volumes of the World's Oceans from ETOPO1 . noaa.gov . 19 August 2020 . National Oceanic and Atmospheric Administration . 8 March 2022.
26. http://www.fao.org/docrep/meeting/009/ae906e/ae906e17.htm The adoption of joules as units of energy
27. Web site: Feynman. Richard. Richard Feynman. Physical Units. Feynman's Lectures on Physics. 1963. 2014-03-07.
28. Web site: Units with special names and symbols; units that incorporate special names and symbols . . 18 March 2015 . https://web.archive.org/web/20090628084157/http://www.bipm.org/en/si/si_brochure/chapter2/2-2/2-2-2.html . 28 June 2009 . 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..
29. Web site: What Is A Watt Second?.