Work (thermodynamics)

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In thermodynamics, work is the quantity of energy transferred from one system to another without an accompanying transfer of entropy. It is a generalization of the concept of mechanical work in mechanics. In the SI system of measurement, work is measured in joules (symbol: J). The rate at which work is performed is power.

History[change | edit source]

1824[change | edit source]

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. Specifically, according to Carnot:

We use here motive power (work) 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.

1845[change | edit source]

Joule's apparatus for measuring the mechanical equivalent of heat.

In 1845, the English physicist James Joule wrote a paper On the mechanical equivalent of heat for the British Association meeting in Cambridge[1]. In this work, he reported his best-known experiment, in which the work 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.

Overview[change | edit source]

According to the First law of thermodynamics, it is useful to separate changes to the internal energy of a thermodynamic system into two sorts of energy transfers. Work refers to forms of energy transfer which can be accounted for in terms of changes in the macroscopic physical variables of 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. This is in contrast to heat energy, which is carried into or out of the system in the form of transfers in the microscopic thermal motions of particles.

The concept of thermodynamic work is slightly more general than that of mechanical work because it includes other types of energy transfers as well. The electrical work required to move a charge against an external electrical field can be measured, as can the work required to move heat against a temperature gradient. Thermodynamic work need not have any mechanical component to be considered such.

Other pages[change | edit source]

References[change | edit source]

  1. Joule, J.P. (1845) "On the Mechanical Equivalent of Heat", Brit. Assoc. Rep., trans. Chemical Sect, p.31, which was read before the British Association at Cambridge, June