EVOLUTION OF BASIC AND APPLIED THERMODYNA MICS

Classical thermodynamics cannot accurately describe natural processes. Investigation of the latter mush resort to a nonequilibrium theory. I rrespective of reversibility or irreversibility, thermodynamics deals with material systems which consist of a very large number of elementa ry entities possessing internal energy. According to the second law of thermodynamics, as expressed by Clausius, any isolated macroscopic sy stem ceases to undergo change when the entropy reaches its maximum. Th e system has then reached an equilibrium state corresponding to the st ate of maximum particulate disorder. However, real systems are not iso lated systems; their boundaries let through energy (closed systems) or energy and matter (open systems). When the physical characteristics o f the system and those of the environment are closely related, the cha nges are reversible and are amenable to classical thermodynamics, but when these characteristics are very different the exchanges which take place are abrupt and irreversible; classical thermodynamics no longer applies. The system may, however be assumed to consist of a very larg e number of small subsystems in internal thermodynamic equilibrium, bu t not in equilibrium with each other. Entropy production (energy dissi pation) depends on the transfer of heat, matter, quantity of motion an d on the transfer due to chemical reactions. Near equilibrium, in the linear domain, the progress of change depends on a potential whose min imum value acts as an attractor. The system has no historical ''dimens ion''. Away from equilibrium on the other hand, the progress of charge no longer depends on a potential. The history of the system has to be taken into account. Nonequilibrium stationary states can be stable or unstable, the limiting case being that of marginal stability (e.g. Po incare's boundary cycles). The study of the stability of solutions mak es use of Lyapunov's functions. Internal fluctuations are of great imp ortance in the vicinity of the instability regions. A distinction is m ade between the phase transitions at equilibrium which end in microstr uctures, and nonequilibrium phase transitions. In the latter the cross ing of thresholds constitutes an abrupt transition, giving rise to the formation of heterogeneities which break the symmetries to form dissi pative structures (Prigogine) which are macrostructures.