1 - Systems, States, and Processes

Abstract

The state of a system is the numerical specification of the circumstance in which it is found. There are two important kinds of states of macroscopic systems: the macroscopic state, which concerns only variables pertaining to the system as a whole, and the microscopic state, which pertains to the mechanical variables of individual molecules. The equilibrium macroscopic state of a one-phase fluid system of one component is specified by the values of three independent state variables. All other macroscopic state variables are dependent variables, with values given by mathematical functions of the independent variables. If the microscopic state of a macroscopic system is specified, the macroscopic state is completely determined. Many microscopic states can correspond to a single macroscopic state, and a macroscopic state can be represented as an average over very many microscopic states of the system. Processes are the means by which the state of the system changes. Microscopic processes are governed by Newton's second law if classical mechanics is an adequate approximation. The two principal kinds of macroscopic processes are reversible processes and irreversible processes. The direction of a reversible process can be changed by an infinitesimal change in the surroundings. The system passes through equilibrium states during a reversible process.