c) From reality to idealization: the introduction of the model of reversible transformations as those quasi-static
transformations that can occur in both directions, given that no irreversible processes, like the emblematic ones, are
supposed to occur. A seeming paradox: how can reversible transformations explain the irreversibility of natural
phenomena?
d) The specific and crucial role of reversible transformations when applied to ideal thermal engines: i) pointing
out an “intrinsic” (not removable) asymmetry between heat and work (the engine’s efficiency is less than 1, even in
absence of irreversible processes); ii) highlighting the need of introducing a new state variable - entropy - able to
differentiate between work and heat in ideal cases (in ideal transformations interaction via work transfers only
energy whereas interaction via heat transfers both energy and entropy).
f) From idealization to reality: the production of entropy due to the presence of unavoidable spontaneous
processes and the meaning of entropy as the entity able to quantify the irreversible change in real transformations.
g) A new insight on the First and Second Principle of thermodynamics in order to identify the two possible faces
of the cryptic concept of heat: Q= ΔU+L (the energetic face), Q=TΔS (the entropic face).