The Perfect Gas

Abstract

In the second part of the book which covers Chaps.7 to 13 we study the properties of various simple substances using the concepts and methods which we have developed earlier. We start with rarefied monatomic gases, which we describe in the perfect gas model as a set of non-interacting mass points following the laws of classical mechanics. We start by justifying this model (§7.1), showing, in particular, that the translational degrees of freedom of the molecules in a gas or liquid can to a very good approximation be treated by classical statistical mechanics, in contrast to the internal degrees of freedom of the atoms, associated with the motion of the electrons, or of the molecules, associated with the rotations and vibrations of the constituents relative to one another. The partition function technique then enables us to explain the macroscopic thermodynamic properties of those gases at equilibrium, which have been known experimentally for a long time (§§ 7.2 and 7.3).

“«C’est pour le gaz!» hurlait un employé dans la porte qu’un enfant lui avait ouverte.”

A. Camus, L’Exil et le Royaume

“La pureté de l’air entre pour beaucoup dans l’innocence des mœurs.”

Balzac, Le Médecin de Campagne

“Front éternel paume parfaite Puits en plein air essieu de vent”

Paul Éluard, Médieuses

“Nobody is perfect.”

Billy Wilder, Some like it Hot (reply of Joe Brown to Jack Lemmon)