The joule (pronounced /ˈdʒuːl/, JOOL or /ˈdʒaʊl/ JOWL; symbol: J) is the unit of energy in the International System of Units (SI).^{[1]} It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied. 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).^{[2]}^{[3]}^{[4]}
joule  

Unit system  SI 
Unit of  energy 
Symbol  J 
Named after  James Prescott Joule 
Conversions  
1 J in ...  ... is equal to ... 
SI base units  kg⋅m^{2}⋅s^{−2} 
CGS units  1×10^{7} erg 
wattseconds  1 W⋅s 
kilowatthours  ≈2.78×10^{−7} kW⋅h 
kilocalories (thermochemical)  2.390×10^{−4} kcal_{th} 
BTUs  9.48×10^{−4} BTU 
electronvolts  ≈6.24×10^{18} eV 
In terms of SI base units and in terms of SI derived units with special names, the joule is defined as


One joule can also be defined by any of the following:
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.
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 10^{7} erg. 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:
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).^{[6]} 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.^{[7]}
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.^{[8]} 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.^{[9]} 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⋅m^{2}⋅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).
One joule represents (approximately):

1 joule is equal to (approximately unless otherwise stated):
Units defined exactly in terms of the joule include:
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 
Mass  Moment of inertia 
Displacement  Angle 
A result of this similarity is that the SI unit for torque is the newtonmetre, 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 newtonmetre (N⋅m) – a compound name derived from its constituent parts.^{[26]} The use of newtonmetres for torque but joules for energy is helpful to avoid misunderstandings and miscommunication.^{[26]}
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
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.
A wattsecond (symbol W s or W⋅s) is a derived unit of energy equivalent to the joule.^{[27]} The wattsecond is the energy equivalent to the power of one watt sustained for one second. While the wattsecond is equivalent to the joule in both units and meaning, there are some contexts in which the term "wattsecond" is used instead of "joule", such as in the rating of photographic electronic flash units. ^{[28]}
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.