entropy

A thermodynamic state variable denoted by S (s denotes specific entropy, entropy per unit mass). The rate of change of entropy of a thermodynamic system is defined as

where Q is the heating rate in a reversible process and T is absolute temperature. Integration of this equation yields the entropy difference between two states. The entropy of an isolated system cannot decrease in any real physical process, which is one statement of the second law of thermodynamics. The specific entropy of an ideal gas, s_g, may be expressed as

where c_pg is the specific heat at constant pressure of that gas, R_g is its gas constant, and T and p_g are its temperature and pressure. The entropy of a liquid s_l is

where c_l is the specific heat of the liquid.