The second law of thermodynamics states that the entropy of closed systems never decreases, because closed systems spontaneously evolve towards thermal equilibrium -- the state of maximum entropy. Equivalently, perpetual motion machines of the second kind are impossible.
The second law is a postulate of thermodynamics, but it can be understood and proven using the underlying quantum statistical mechanics. It is an expression of the fact that over time, differences in temperature, pressure, and chemical potential decrease in an isolated physical system, leading eventually to a state of thermodynamic equilibrium. In the language of statistical mechanics, entropy is a measure of the number of microscopic configurations corresponding to a macroscopic state. Because equilibrium corresponds to a vastly greater number of microscopic configurations than any non-equilibrium state, it has the maximum entropy, and the second law follows because random chance alone almost guarantees that the system will evolve towards equilibrium.
The second law may be expressed in many specific ways, but the first formulation is credited to the French scientist Sadi Carnot in 1824 (see Timeline of thermodynamics).
Read more about Second Law Of Thermodynamics: Description, History, Derivation From Statistical Mechanics, Non-equilibrium States, Quotations
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