Il teorema di Carnot e il rendimento delle macchine termiche

Carnot’s theorem, which plays a central role in thermodynamics, is usually exposed in textbooks in an unsatisfactory way, which does not stress adequately the unique status of Carnot’s cycle. Moreover, the core of this theorem, that is the concept of thermal efficiency, becomes intrinsically ambiguous when applied to machines which exchange heat in both senses with the same thermal reservoir, as usually happens in reversible cycles. This ambiguity is not recognised, which leads to considerable confusion in didactics. In this paper, two different definitions of thermal efficiency, called η high and η low, are proposed for this case, and results are stated for both.