At its origins, thermodynamics was the study of heat and engines. Carnot transformed it into a scientific discipline by explaining engine power in terms of transfer of “caloric”. That idea became the second law of thermodynamics when Thomson and Clausius reconciled Carnot’s theory with Joule’s conflicting thesis that power was derived from the consumption of heat, which was determined to be a form of energy. Eventually, Clausius formulated the 2nd-law as the universal entropy growth principle: the synthesis of transfer vs. consumption led to what became known as the mechanical theory of heat (MTH). However, by making universal-interconvertibility the cornerstone of MTH their synthesis-project was a defective one, which precluded MTH from developing the full expression of the second law. This paper reiterates that universal-interconvertibility is demonstrably false—as the case has been made by many others—by clarifying the true meaning of the mechanical equivalent of heat. And, presents a two-part formulation of the second law: universal entropy growth principle as well as a new principle that no change in Nature happens without entropy growth potential. With the new principle as its cornerstone replacing universal-interconvertibility, thermodynamics transcends the defective MTH and becomes a coherent conceptual system.
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