03 Transformations and their representations

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L.D.D Advanced Building Physics - Transformations and their representation According to the Gibbsian thermodynamics, the equilibrium states of a simple system are completely determined by the values U, V, Nj as previously described. We have seen that it is possible to define a function entropy (S) for all the equilibrium states of a system. The variables S, U, V, Nj identify the space of thermodynamic configuration. We have defined the quasi static transformations those which are obtained with an orderly succession of equilibrium states. If we consider a simple system, we can represent any of its quasi static transformations with a line in the three dimensional space [S, U, V] (--> N fixed) If instead we consider a non quasi static transformation, then the non equilibrium states can not be represented. 1 - REVERSIBLE AND IRREVERSIBLE TRANSFORMATIONS Consider an Isolated composed system. For this system the second principle of thermodynamics is valid under the form of the 2nd postulate and its consequence. If we start from an A equilibrium and remove some constraints, the system in general will no longer be in equilibrium and will have the tendency to move into a new equilibrium state identified by the fact that it's the one with maximum entropy compatible with the new constraints. Since the space of virtual states has expanded, it is possible that state B exists with higher entropy than ste starting one (A). In this case, according to the second principle, the system will transform into state B. If on the contrary there's not any other available state with higher entropy, the composed isolated system will remain in the inital state A. ---> so, in general, we have: SICSB ≥ SICSA 1) Suppose that: SICSB > SICSA Starting from the state B, we want to return to the state A…