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In thermodynamics, work performed by a system is energy transferred by the system to its surroundings, due solely to macroscopic forces exerted by the system on its surroundings, where those forces, and their external effects, can be measured. Such work is the only kind by which a thermodynamic system can be made to lift a weight.
The externally measured forces and external effects may be electromagnetic,[1][2][3] gravitational,[4] or pressure/volume or other macroscopically mechanical variables.[5] Thermodynamic work is defined to be measurable solely from knowledge of such external macroscopic factors. For thermodynamic work, these external factors are exactly matched by values of or contributions to changes in macroscopic internal state variables of the system, which always occur in conjugate pairs, for example pressure and volume[5] or magnetic flux density and magnetization.[2] In the SI system of measurement, work is measured in joules (symbol: J). The rate at which work is performed is power.
Work sometimes is said to be done on a system of interest by an external system that lies in the surroundings, not necessarily a thermodynamic system as strictly defined by the usual thermodynamic state variables. The paddle stirring experiments of Joule provide an example, illustrating the concept of isochoric (or constant volume) mechanical work. Though the external system does macroscopic mechanical work, described by its own macroscopic mechanical variables, the thermodynamic work, as defined here, such as pressure-volume work, done by the system of interest, is zero. The external system mixes and generates friction on and in the system of interest. Another form of isochoric work is Joule heating. Because no thermodynamic work is done by the system of interest, and no matter is transferred, such an energy transfer is regarded as a heat transfer into the system of interest. A system of interest cannot raise a weight by such work. It is a consequence of the second law of thermodynamics that a thermodynamic system in its own state of internal thermodynamic equilibrium cannot do isochoric mechanical work on an external system; it can do isochoric work on an external system only through long range non-mechanical forces such as electromagnetic or gravitational.
Thermodynamic work is a version of the concept of work in physics.