Work (thermodynamics) - Overview

Overview

The first law of thermodynamics relates changes in the internal energy of a closed thermodynamic system to two forms of energy transfer, as heat and as work. Essential to the thermodynamic concept of work is that the energy transfer in fictive principle be able to occur at a finite rate without any of it necessarily being dissipated by friction or chemical degradation, which are necessarily dissipative. A thermodynamic dissipative process is one in which energy, internal, bulk flow kinetic, or system potential, is transduced from some initial form to some final form, the capacity to do mechanical work of the final form being less that that of the initial form. For example, transfer of energy as heat is dissipative because it is a transfer of internal energy from a body at one temperature to a body at a lower temperature. The second law of thermodynamics implies that this reduces the capacity of that internal energy to do mechanical work.

The concept of thermodynamic work is more general than that of simple mechanical work because it includes other types of energy transfers as well. Thermodynamic work is strictly and fully defined by its external generalized mechanical variables. The other form of energy transfer between closed systems is as heat. Heat is measured by change of temperature of a known quantity of calorimetric material substance; it is of the essence of heat transfer that it is not mediated by the external generalized mechanical variables that define work. This distinction between work and heat is essential to thermodynamics.

Work refers to forms of energy transfer between closed systems that can be accounted for in terms of changes in the external macroscopic physical constraints on the system, for example energy which goes into expanding the volume of a system against an external pressure, by driving a piston-head out of a cylinder against an external force. The electrical work required to move a charge against an external electrical field can be measured.

This is in contrast to heat, which is the energy that is transported or transduced as the microscopic thermal motions of particles and their associated inter-molecular potential energies, or by thermal radiation. There are just two forms of macroscopic heat transfer between closed systems: conduction, and thermal radiation. There are several forms of dissipative transduction of energy that can occur internally within a system at a microscopic level, such as friction including bulk and shear viscosity, chemical reaction, unconstrained expansion as in Joule expansion and in diffusion, and phase change; these are not transfers of heat between systems. Convection of internal energy is a form a transport of energy but not, as sometimes mistakenly supposed (a relic of the caloric theory of heat), a form of heat transfer, because convection is not in itself a microscopic motion of microscopic particles or their intermolecular potential energies, or photons, nor is it a form of work.