Work (thermodynamics)

Work (thermodynamics)

In thermodynamics, work performed by a system is the energy transferred by the system to another that is accounted for by changes in the external generalized mechanical constraints on the system. As such, thermodynamic work is a generalization of the concept of mechanical work in physics.

The external generalized mechanical constraints may be chemical, electromagnetic, (including radiative), gravitational or pressure/volume or other simply mechanical constraints, including momental, as in radiative transfer. Thermodynamic work is defined to be measurable solely from knowledge of such external macroscopic constraint variables. These macroscopic variables always occur in conjugate pairs, for example pressure and volume, magnetic flux density and magnetization, mole fraction and chemical potential. In the SI system of measurement, work is measured in joules (symbol: J). The rate at which work is performed is power.

It is customary to calculate amount of energy transferred as work through quantities external to the system of interest, and thus belonging to its surroundings. Nevertheless, for historical reasons, the customary sign convention is to consider work done by the system on its surroundings as positive. Although all real physical processes entail some dissipation of kinetic energy, it is matter of principle that the dissipation that results from transfer of energy as work occurs only inside the system; energy dissipated outside the system, in the process of transfer of energy, is not counted as thermodynamic work. Thermodynamic work does not account for any energy transferred between systems as heat.

Mechanical thermodynamic work is performed by actions such as compression, and including shaft work, stirring, and rubbing. In the simplest case, for example, there are work of change of volume against a resisting pressure, and work without change of volume, known as isochoric work. An example of isochoric work is when an outside agency, in the surrounds of the system, drives a frictional action on the surface of the system. In this case the dissipation is not necessarily actually confined to the system, and the quantity of energy so transferred as work must be estimated through the overall change of state of the system as measured by both its mechanically and externally measurable deformation variables (such as its volume), and its non-deformation variable (usually internal to the system, for example its empirical temperature, regarded not as a temperature but simply as a mechanically measurable variable). In a process of transfer of energy by work, the internal energy of the final state of the system is then measured by the amount of adiabatic work of change of volume that would have been necessary to reach it from the initial state, such adiabatic work being measurable only through the externally measurable mechanical or deformation variables of the system, but including also full information about the forces exerted by the surroundings on the system during the process. In the case of some of Joule's measurements, the process was so arranged that heat produced outside the system by the frictional process was practically entirely transferred into the system during the process, so that the quantity of work done by the surrounds on the system could be calculated as shaft work, an external mechanical variable. For closed systems, internal energy changes in a system other than as work transfer are as heat.