Work (thermodynamics)

In thermodynamics, work transfer is an energy transfer in which temperature is not considered during energy transfer. Unit is always in Joules. It is also a result of force acting through the distance. 

where  is the work done,  is the force acting, and  is the distance.

History
1824
Work, i.e. "weight lifted through a height", was originally defined in 1824 by Nicolas Léonard Sadi Carnot in his famous paper Reflections on the Motive Power of Fire. Specifically, according to Carnot:

1845
350px|thumb|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.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 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 
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.

Related pages
Thermodynamics
Chemistry

References

Category:Thermodynamics