The joule, symbol J, is a derived unit of energy, work, or amount of heat in the International
System of Units, equivalent to 0.00094781712 BTU. It is equal to the energy expended in
applying a force of one newton through a distance of one metre, 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. In terms firstly of base SI units and then
in terms of other SI units:
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: The work required to move an electric charge
of one coulomb through an electrical potential difference of one volt, or one '"coulomb volt".
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". This
relationship can be used to define the watt.
Usage This SI unit is named after James Prescott
Joule. As with every International System of Units unit whose name is derived from the
proper name of a person, the first letter of its symbol is upper case. However, when
an SI unit is spelled out in English, it should always begin with a lower case letter, 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
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, 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 - a compound
name derived from its constituent parts. Torque and energy are related to each other using
the equation
where E is the energy, τ is the torque, and θ is the angle moved. 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. 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
One joule in everyday life represents approximately: the energy required to lift a small apple
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. the typical energy released as heat by a person
at rest every 1/60 second. the kinetic energy of a 50 kg human moving
very slowly. the kinetic energy of a 56 g tennis ball
moving at 6 m/s. 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, a kW·h is thus 1000 × 3600 seconds
= 3.6 MJ. Multiples
For additional examples, see: Orders of magnitude Nanojoule
The nanojoule is equal to one billionth of one joule. One nanojoule is about 1/160 of
the kinetic energy of a flying mosquito. Microjoule
The microjoule is equal to one millionth of one joule. The Large Hadron Collider is expected
to produce collisions on the order of 1 microjoule per particle.
Millijoule The millijoule is equal to one thousandth
of a joule. Kilojoule
The kilojoule is equal to one thousand joules. Nutritional food labels in certain countries
express energy in kilojoules. One kilojoule per second is approximately
the amount of solar radiation received by one square metre of the Earth in full daylight.
Megajoule The megajoule is equal to one million joules,
or approximately the kinetic energy of a one-tonne vehicle moving at 160 km/h.
Because 1 watt times 1 second equals 1 joule, 1 kilowatt-hour is 1000 watts times 3600 seconds,
or 3.6 megajoules. Gigajoule
The gigajoule is equal to one billion joules. 6 GJ is about the amount of potential chemical
energy in a barrel of oil, when combusted. Terajoule
The terajoule is equal to one trillion joules. About 63 TJ of energy was released by the
atomic bomb that exploded over Hiroshima. The International Space Station, with a mass
of approximately 450,000 kg and orbital velocity of 7.7 km/s, has a kinetic energy of roughly
13 TJ. Petajoule
The petajoule is equal to one quadrillion 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
The exajoule is equal to one quintillion 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 The zettajoule is equal to one sextillion
joules. Annual global energy consumption is approximately 0.5 ZJ.
Yottajoule The yottajoule is equal to one septillion
joules. This is approximately the amount of energy required to heat the entire volume
of water on Earth by 1 °C. Conversions
1 joule is equal to: 1×107 ergs
6.24150974×1018 eV 0.2390 cal
2.3901×10−4 kcal 9.4782×10−4 BTU
0.7376 ft·lb 23.7 ft·pdl
2.7778×10−7 kilowatt-hour 2.7778×10−4 watt-hour
9.8692×10−3 litre-atmosphere 11.1265 femtograms
1×10−44 foe Units defined exactly in terms of the joule
include: 1 thermochemical calorie = 4.184 J
1 International Table calorie = 4.1868 J 1 watt hour = 3600 J
1 kilowatt hour = 3.6×106 J 1 watt second = 1 J
1 ton TNT = 4.184 GJ See also
Conversion of units of energy Orders of magnitude
Fluence International System of Units
Notes and references