entropy
Summary A measure of the disorder of a system is its entropy S , a state function whose value increases with an increase in the number of available microstates. Areversible process is one for which all intermediate states between extremes are equilibrium states; it can change direction at any time. In contrast, anirreversible process occurs in one direction only. The change in entropy of the system or the surroundings is the quantity of heat transferred divided by the temperature. The second law of thermodynamics states that in a reversible process, the entropy of the universe is constant, whereas in an irreversible process, such as the transfer of heat from a hot object to a cold object, the entropy of the universe increases.