Work (thermodynamics)

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. Through suitable passive linkages, the whole of the work done by such forces can lift a weight in the surroundings.We use here motive power to express the useful effect that a motor is capable of producing. This effect can always be likened to the elevation of a weight to a certain height. It has, as we know, as a measure, the product of the weight multiplied by the height to which it is raised. 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. Through suitable passive linkages, the whole of the work done by such forces can lift a weight in the surroundings. The externally measured forces and external effects may be electromagnetic, gravitational, or pressure/volume or other macroscopically mechanical variables. For thermodynamic work, these externally measured quantities 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 or magnetic flux density and magnetization. By an external system that lies in the surroundings, not necessarily a thermodynamic system as strictly defined by the usual thermodynamic state variables, work can be said to be done on a thermodynamic system. Part of such externally applied work can change the system's state variables other than its temperature and entropy, while the rest of such externally applied work appears to the thermodynamic system as heat transferred to it. The paddle stirring experiments of Joule provide an example, illustrating the concept of isochoric (or constant volume) mechanical work. Such work is not adiabatic, because it occurs through friction in and on the surface of the thermodynamic system. When such externally applied macroscopic mechanical work, described by macroscopic mechanical variables in the surroundings, is done isochorically, it is not thermodynamic work done on the thermodynamic system, as defined here, because it depends on a non-equilibrium process—namely, kinetic friction—and cannot be described with state variables. An example of a non-mechanical form of externally applied work is Joule heating, which also is not adiabatic, because it occurs through friction as the electric current passes through the thermodynamic system. When it is done isochorically, and no matter is transferred, such an energy transfer is regarded as a heat transfer into the system of interest. Thermodynamic work is a specialized version of the concept of work in physics. In the SI system of measurement, work is measured in joules (symbol: J). The rate at which work is performed is power. Work, i.e. 'weight lifted through a height', was originally defined in 1824 by Sadi Carnot in his famous paper Reflections on the Motive Power of Fire, where he used the term motive power for work. Specifically, according to Carnot: In 1845, the English physicist James Joule wrote a paper On the mechanical equivalent of heat for the British Association meeting in Cambridge. In this paper, he reported his best-known experiment, in which the mechanical power released through the action of a 'weight falling through a height' was used to turn a paddle-wheel in an insulated barrel of water. In this experiment, the friction and agitation of the paddle-wheel on the body of water caused heat to be generated which, in turn, increased the temperature of water. Both the temperature change ∆T of the water and the height of the fall ∆h of the weight mg were recorded. Using these values, Joule was able to determine the mechanical equivalent of heat. Joule estimated a mechanical equivalent of heat to be 819 ft•lbf/Btu (4.41 J/cal). The modern day definitions of heat, work, temperature, and energy all have connection to this experiment. In this arrangement of apparatus, it never happens that the process runs in reverse, with the water driving the paddles so as to raise the weight, not even slightly. The doing of such isochoric mechanical work by the device, which lies external to the water, while the energy supplied by the fall of the weight becomes heat passing into the water, is irreversible.