The application of thermodynamics to some irreversible processes

Abstract

Each equilibrium state of a system is characterised by a set of values of the thermodynamic coordinates which do not depend on the way in which the equilibrium state was reached. Processes that are, effectively, a succession of equilibrium states can also be described in terms of the coordinates, but this is not true of processes in which there are finite gradients in one or more of the intensive coordinates. Such processes are, of course, irreversible. However, provided that a given process takes place between an initial equilibrium state and a final equilibrium state, a thermodynamical treatment can be used to relate the values of the coordinates in those equilibrium states, even though it can give no information about the process. The technique is to find a notional reversible process linking the required initial and final equilibrium states and then use that process to calculate relations between the thermodynamic coordinates (or state functions). This procedure will now be examined in detail as it is applied to two irreversible processes.