On determining absolute entropy without quantum theory or the third law of thermodynamics

Andrew M Steane

doi:10.1088/1367-2630/18/4/043022

Abstract

We employ classical thermodynamics to gain information about absolute entropy, without recourse to statistical methods, quantum mechanics or the third law of thermodynamics. The Gibbs–Duhem equation yields various simple methods to determine the absolute entropy of a fluid. We also study the entropy of an ideal gas and the ionization of a plasma in thermal equilibrium. A single measurement of the degree of ionization can be used to determine an unknown constant in the entropy equation, and thus determine the absolute entropy of a gas. It follows from all these examples that the value of entropy at absolute zero temperature does not need to be assigned by postulate, but can be deduced empirically.

Highlights

On determining absolute entropy without quantum theory or the third law of thermodynamics This content has been downloaded from IOPscience
Equations (22) and (11) give our second example of the main point asserted in this paper. They show that, in order to find out how much entropy a monatomic gas has got, under ordinary conditions where it behaves to good approximation like an ideal gas, one does not need quantum theory and one does not need to carefully track the heat supplied as the system is warmed reversibly from absolute zero
This is because the law of mass action can be converted into a statement about entropy, instead of the usual form in terms of chemical potential

Summary

15 April 2016

Original content from this Abstract work may be used under We employ classical thermodynamics to gain information about absolute entropy, without recourse the terms of the Creative. Volume change dV , not absolute volume, V, is involved in the expression for work done on a simple compressible fluid, but it does not follow from this that there is no ready definition of absolute volume, or empirical method to determine absolute volume, without appealing to a postulate concerning, for example, the value of V in the limit of infinite pressure. We will present several simple empirical methods that allow the absolute entropy of a fluid to be deduced without any use of quantum theory, and without calorimetry or cooling to near absolute zero This will clarify what classical thermodynamics can and cannot tell us about entropy.

Thermal radiation

The entropy of an ideal gas

A more general system

The third law