When a quantity of heat §Q is supplied to a thermodynamic system, its effect on the system depends on conditions of the variables of temperature, volume and pressure. It also depends on the conditions whether the system being considered is closed or open to the environment and if the system is a reversible or irreversible one. However, for all such cases, we may write from the laws of thermodynamics, dS - (dU + 8W)/T > 0 (3.7.1)
where W represents the external work done by the system and the sign of equality applies to the case where the system is reversible. In the reversible case, §W = p dV, where p is the equilibrium pressure on the system at any instant and dV is the change in volume. Thus, in the reversible case, we may define entropy in terms of the ther-modynamic variables as dS = (dU + p dV)/T (3.7.2)
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