5 - Thermodynamic processes

Abstract

The lifting of a constraint in a composite system in an equilibrium state that is compatible with the constraint leads to a new state of equilibrium. The transition from one state to another state is called a “process.” This chapter looks at these thermodynamic processes and begins with quasistatic processes. Any real physical process is a temporal evolution of both equilibrium and nonequilibrium states and proceeds at a finite rate. In the limit that the process evolves infinitely slowly, it becomes an infinitely dense succession of equilibrium states—that is, it becomes quasistatic. In reversible process the system and its surroundings are at equilibrium with each other at all times, and a reversal of the direction of the process does not require any external agency. The process thus proceeds at constant total entropy. A reversible—that is, isentropic quasistatic process is essentially a useful artifice representing an ideal limiting case. Many real physical processes do, however, approach reversibility surprisingly closely. Any real physical process is irreversible. The very fact that the process is assumed to proceed implies that the entropy in the final state is greater than that in the initial state. It is impossible to reverse the process by manipulating constraints within the same isolated system because a manipulation resulting in a decrease in entropy in the isolated system would be a violation of the entropy maximum principle.