The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI).[ American Heritage Dictionary of the English Language, Online Edition (2009). Houghton Mifflin Co., hosted by Yahoo! Education.][ The American Heritage Dictionary, Second College Edition (1985). Boston: Houghton Mifflin Co., p. 691.][ McGraw-Hill Dictionary of Physics, Fifth Edition (1997). McGraw-Hill, Inc., p. 224.]
Definition
According to the International Bureau of Weights and Measures the joule is defined as "the work done when the point of application of 1
MKS units of force newton moves a distance of 1 metre in the direction of the force."
In terms of SI base units and in terms of SI derived units with special names, the joule is defined as
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|style="padding-left: 2em;" | { class="wikitable"
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| 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 Voltage 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.
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 and aged 63, followed the recommendation of Siemens:
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).[Pat Naughtin: A chronological history of the modern metric system, metricationmatters.com, 2009.]
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.
In 1935, the International Electrotechnical Commission (as the successor organisation of the International Electrical Congress) adopted the "Giovanni 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.[ CIPM, 1946, Resolution 2, Definitions of electric units. bipm.org.] 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.[ 9th CGPM, Resolution 3: Triple point of water; thermodynamic scale with a single fixed point; unit of quantity of heat (joule)., bipm.org.] This is the definition declared 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).
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 / 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.
-
The kinetic energy of a human moving very slowly ().
-
The kinetic energy of a tennis ball moving at .
-
The food energy (kcal) in slightly more than half of an ordinary-sized sugar crystal (/crystal).
Multiples
- . 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.
- 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).
[ × 70 kg × (10 m/s)2 = 3500 J] while a cheetah in a (76 mph) sprint has approximately 20 kJ.[ × 35 kg × (35 m/s)2 = ] .
- 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 . .
- is about the chemical energy of combusting of petroleum.
- The terajoule is about (which is often used in energy tables). About of energy was released by Little Boy.
- is about of TNT, which is the amount of energy released by the Tsar Bomba, the largest man-made explosion ever. .
- 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.
- 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.
- 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.
Conversions
1 joule is equal to (approximately unless otherwise stated):
-
(exactly)
-
(foot-pound)
-
(foot-poundal)
Units with exact equivalents in joules include:
-
1 thermochemical calorie = 4.184J
[ 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.]
-
1 International Table calorie = 4.1868J
-
1W⋅h =
-
1kW⋅h =
-
1W⋅s =
-
1ton TNT =
-
1foe =
Newton-metre and torque
In
mechanics, the concept of
force (in some direction) has a close analogue in the concept of
torque (about some angle):
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| 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 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 distinction may be seen also in the fact that energy is a scalar quantity – the dot product of a force Euclidean 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
where E is energy, τ is (the vector magnitude of) torque, and θ is the angle swept (in ). 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.
External links
