Work (thermodynamics)



Thermodynamic potentials
Internal energy U(S,V)
Helmholtz free energy A(T,V) = U − TS
Enthalpy H(S,p) = U + PV
Gibbs free energy G(T,p) = U + PV − TS
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In joules (symbol: J). The rate at which work is performed is power.

History

1824

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

  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 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 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. An extremely important fact to understand is that thermodynamic work need not have any mechanical component to be considered such.

Mathematical definition

According to the First Law of Thermodynamics, any net increase in the internal energy U of a thermodynamic system must be fully accounted for, in terms of heat δQ entering the system minus work δW done by the system:

dU = \delta Q - \delta W\;

The letter d indicates that internal energy U is a property of the state of the system, so changes in the internal energy are state functions of the system.

Pressure-volume work

Chemical thermodynamics studies PV work, which occurs when the volume of a fluid changes. PV work is represented by the following differential equation:

dW = -P dV \,

where:

  • W = work done on the system
  • P = external pressure
  • V = volume
W=-\int_{V_i}^{V_f} P\,dV

Like all work functions, PV work is path-dependent. (The path in question is a curve in the Euclidean space specified by the fluid's inexact differential; to be more rigorous, it should be written đW (with a line through the d).

In other words, from a mathematical point of view, đW is not an exact one-form. The line-through is merely a flag to warn us there is actually no function (0-form) W which is the potential of đW. If there were, indeed, this function W, we should be able to just use Stokes Theorem to evaluate this putative function, the potential of đW, at the boundary of the path, that is, the initial and final points, and therefore the work would be a state function. This impossibility is consistent with the fact that it does not make sense to refer to the work on a point in the PV diagram; work presupposes a path.

PV work is often measured in the (non-SI) units of litre-atmospheres, where 1 L·atm = 101.3 J.

Free energy and exergy

The amount of useful work which can be extracted from a thermodynamic system is discussed in the article Gibbs free energy.

See also

References

  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
 
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