Joule


The joule is a derived unit of energy in the International System of Units. It is equal to the energy transferred to an object when a force of one newton acts on that object in the direction of the force's motion through a distance of one metre. 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.
In terms firstly of base SI units and then in terms of other SI units, a joule is defined below :
SymbolMeaning
Jjoule
kgkilogram
mmetre
ssecond
Nnewton
Papascal
Wwatt
Ccoulomb
Vvolt

One joule can also be defined as the following:
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 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, at the time retired but still living, is due to Siemens:
At the second International Electrical Congress, on 31 August 1889, the joule was officially adopted alongside the watt and the quadrant.
Joule died in the same year, on 11 October 1889.
At the fourth congress, 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 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
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. 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.
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, the metre and the kilogram.

Practical examples

One joule represents :
A kilowatt-hour is 3.6 megajoules.

Multiples


; : The yoctojoule is equal to of one joule.
; : The zeptojoule is equal to one sextillionth of one joule. 160 zeptojoules is about one electronvolt. The minimal energy needed to change a bit at around room temperature – approximately 2.75 zJ – is given by the Landauer limit.
; : The attojoule is equal to of one joule.
; : The femtojoule is equal to of one joule.
; : The picojoule is equal to one trillionth of one joule.
; : The nanojoule is equal to one billionth of one joule. 160 nanojoules is about the kinetic energy of a flying mosquito.
; : The microjoule is equal to one millionth of one joule. The Large Hadron Collider produces collisions of the microjoule order per particle.
; : The millijoule is equal to one thousandth of a joule.
; : The kilojoule is equal to one thousand joules. Nutritional food labels in most countries express energy in kilojoules. One square metre of the Earth receives about 1.4 kilojoules of solar radiation every second in full daylight.
; : The megajoule is equal to one million joules, or approximately the kinetic energy of a one megagram vehicle moving at 161 km/h. The energy required to heat 10 liters of liquid water at constant pressure from to is approximately 4.2 MJ. One kilowatt-hour of electricity is 3.6 megajoules.
; : The gigajoule is equal to one billion joules. 6 GJ is about the chemical energy of combusting of crude oil. 2 GJ is about the Planck energy unit.
; : The terajoule is equal to one trillion joules; or about 0.278 GWh. 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 megagrams and orbital velocity of 7.7 km/s, has a kinetic energy of roughly 13 TJ. In 2017 Hurricane Irma was estimated to have a peak wind energy of 112 TJ.
; : The petajoule is equal to one quadrillion joules. 210 PJ is about 50 megatons of TNT which is the amount of energy released by the Tsar Bomba, the largest man-made explosion ever.
; : 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 rating of 9.0 on the moment magnitude scale. Yearly U.S. energy consumption amounts to roughly 94 EJ.
; : The zettajoule is equal to one sextillion joules. The human annual global energy consumption is approximately 0.5 ZJ.
; : The yottajoule is equal to one septillion joules. This is approximately the amount of energy required to heat all the water on Earth by 1 °C. The thermal output of the Sun is approximately 400 YJ per second.

Conversions

1 joule is equal to :
Units defined exactly in terms of the joule include:
In mechanics, the concept of force has a close analogue in the concept of torque :
LinearAngular
ForceTorque
MassMoment 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 CGPM 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 – a compound name derived from its constituent parts. The use of newton metres for torque and joules for energy is helpful to avoid misunderstandings and miscommunications.
The distinction may be seen also in the fact that energy is a scalar – 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
where E is energy, τ is torque, and θ is the angle swept. Since angles are dimensionless, it follows that torque and energy have the same dimensions.

Watt-second

A watt-second 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".

Photography

In photography, the unit for flashes is the watt-second. A flash can be rated in watt-seconds or in joules, but historically, the term "watt-second" has been used and continues to be used. An on-camera flash, using a 1000 microfarad capacitor at 300 volts, would be 45 watt-seconds. Studio flashes, using larger capacitors and higher voltages, are in the 200–2000 watt-second range.
The energy rating a flash is given is not a reliable benchmark for its light output because there are numerous factors that affect the energy conversion efficiency. For example, the construction of the tube will affect the efficiency, and the use of reflectors and filters will change the usable light output towards the subject. Some companies specify their products in "true" watt-seconds, and some specify their products in "nominal" watt-seconds.