On the Proper Use of Equilibrium Thermodynamics

Abstract

Like the other two traditional branches of macroscopic physics, mechanics and electromagnetism, thermodynamics was constructed progressively by induction. To start with, experimental observations were synthesized by laws, such as Newton’s, Coulomb’s or Gay-Lussac’s laws; this process, which started very early for mechanics (Archimedes), accelerated from the seventeenth to the nineteenth century for the whole of macroscopic physics. During the nineteenth century one realized through a new induction that these laws could be derived from a few unifying principles which were more abstract, but very general: Lagrangian or Hamiltonian analytical mechanics, the Maxwell equations, and the Laws of thermodynamics. Finally, the first third of the twentieth century has brought a new understanding and unification by basing all these principles upon the new, microscopic, quantum and statistical physics. Apart from its philosophical interest, each stage of the unification has extended the possibilities for predictions by allowing one to proceed henceforth by deduction. From fundamental principles one derives new laws, on which one can confidently build even before experimental checks; in particular we have seen and we shall see how statistical physics allows us to calculate equations of state which thermodynamics introduces as empirical data. One can thus consider, at the present level of knowledge, that thermodynamics is incomplete: statistical mechanics must intercede when one tries either to get a better understanding of the significance of thermodynamic quantities, or to calculate them for some substance or other starting from its microscopic structure. The preceding chapters have shown us in a general way how to realize this programme; the following chapters will present us with many applications to simple substances.

“Abandonnant les théories ambitieuses d’ilya quarante ans, encombrées d’hypothèses moléculaires, nous cherchons aujourd’hui à élever sur la Thermodynamique seule l’édifice tout entier de la physique mathématique. Les deux principles de Meyer et de Clausius lui assureront-ils des fondations assez solides pour qu’il dure quelque temps? Personne n’en doute; mais d’où nous vient cette confiance?”

H. Poincaré, La Science et l’Hypothèse, 1906

“Je ne sais ce que c’est des principes, sinon des règles qu’on prescrit aux autres pour soi.“

D. Diderot, Jacques le Fataliste

“Quelque loi qu’il vous dicte, il faut vous y soumettre.“ Racine, Phèdre

“Voulez-vous de bonnes lois; brûlez les vôtres, et faites-en de nouvelles.“

Voltaire, Dictionnaire Philosophique