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.
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.[7]
"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."[8]
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).[9]
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.[10]
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.[11] 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.[12]
This definition was the direct precursor of the joule as adopted in the modern
International System of Units in 1960.[citation needed]
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).[13]
Practical examples
One joule represents (approximately):
The amount of electricity required to run a 1
W device for 1
s.
The energy required to accelerate a 1
kg mass at 1
m/s2 through a distance of 1
m.
160 zeptojoule is about one
electronvolt.[citation needed] The minimal energy needed to change a bit of data in computation at around room temperature – approximately 2.75 zJ – is given by the
Landauer limit.[citation needed]
Nutritional food labels in most countries express energy in kilojoules (kJ).[17] One square metre of the
Earth receives about 1.4 kilojoules of
solar radiation every second in full daylight.[18] A human in a sprint has approximately 3 kJ of kinetic energy,[19] while a cheetah in a 122
km/h (76 mph) sprint has approximately 20 kJ.[20] One
watt-hour of electricity is 3.6 kilojoules.[citation needed]
Megajoule
The megajoule is approximately the kinetic energy of a one megagram (tonne) vehicle moving at 161
km/h (100 mph).[citation needed] The energy required to heat 10 L of liquid water at constant pressure from 0 °C (32 °F) to 100 °C (212 °F) is approximately 4.2
MJ.[citation needed] One
kilowatt-hour of electricity is 3.6 megajoules.[citation needed]
210 petajoule is about 50
megatons 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 3.6 petajoules.[citation needed]
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.[27] Human annual
world energy consumption is approximately 0.5 ZJ. The energy to raise the temperature of Earth's atmosphere 1 °C is approximately 2.2 ZJ.[citation needed]
Yottajoule
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.[27] The thermal output of the
Sun is approximately 400 YJ per second.[citation needed]
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.[30] The use of newton-metres for torque but joules for energy is helpful to avoid misunderstandings and miscommunication.[30]
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[citation needed]
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.[citation needed]
Watt-second
A watt-second (symbol W s or W⋅s) is a
derived unit of
energy equivalent to the joule.[31] 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. [32]
Notes
^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.
^Siemens, Cal Wilhelm (August 1882).
Report of the Fifty-Second Meeting of the British Association for the Advancement of Science. Southhampton. pp. 1–33. pp. 6–7: 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 1⁄1000th part of the arbitrary unit of a pound of water raised 1° Fahrenheit and the 1⁄4000th 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.
^Bonnie Berkowitz; Laris Karklis; Reuben Fischer-Baum; Chiqui Esteban (11 September 2017).
"Analysis – How Big Is Hurricane Irma?". Washington Post. Retrieved 2 November 2017.
^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.
^
ab"Units with special names and symbols; units that incorporate special names and symbols".
International Bureau of Weights and Measures. Archived from
the original on 28 June 2009. Retrieved 18 March 2015. 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.