Analyzing a process in terms of entropy production is shown to provide a quantitative approach to the second law of thermodynamics. The second law, ΔS≥0, is applied to the work–entropy relation obtained by rewriting the first law of thermodynamics in terms of the total entropy increase. The significance of entropy in macroscopic thermodynamics is established, and the limitations imposed by the second law are made evident. Irreversibility, entropy production, and the degradation of energy are seen as manifestations of the second law. The work–entropy relation indicates that entropy‐producing irreversibilities are always accompanied by an amount of energy Wlost=TΔS that becomes unavailable to do work. The Kelvin–Planck and Clausius forms of the second law, as well as Carnot’s principle and the inequality of Clausius, are obtained from the work–entropy relation. It is shown that the increase in entropy for a system is due both to heat flowing into it, and to internal irreversibilities. Availability and the second‐law efficiency are discussed.

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