Editing Second law of thermodynamics (section)

=== Principle of Carathéodory ===
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[[Constantin Carathéodory]] formulated thermodynamics on a purely mathematical axiomatic foundation. His statement of the second law is known as the Principle of Carathéodory, which may be formulated as follows:<ref>[[Constantin Carathéodory|Carathéodory, C.]] (1909).</ref>
<blockquote>In every neighborhood of any state S of an adiabatically enclosed system there are states inaccessible from S.<ref>Buchdahl, H.A. (1966), p. 68.</ref></blockquote>
With this formulation, he described the concept of [[adiabatic accessibility]] for the first time and provided the foundation for a new subfield of classical thermodynamics, often called [[Ruppeiner geometry|geometrical thermodynamics]]. It follows from Carathéodory's principle that quantity of energy quasi-statically transferred as heat is a holonomic [[process function]], in other words, <math>\delta Q=TdS</math>.<ref name="Sychev1991">{{cite book |last=Sychev |first=V. V. |title=The Differential Equations of Thermodynamics |year=1991 |publisher=Taylor & Francis |isbn=978-1-56032-121-7}}</ref>

Though it is almost customary in textbooks to say that Carathéodory's principle expresses the second law and to treat it as equivalent to the Clausius or to the Kelvin-Planck statements, such is not the case. To get all the content of the second law, Carathéodory's principle needs to be supplemented by Planck's principle, that isochoric work always increases the internal energy of a closed system that was initially in its own internal thermodynamic equilibrium.<ref name="Munster 45">Münster, A. (1970), p. 45.</ref>{{sfnp|Lieb|Yngvason|1999|p=49}}<ref name="Planck 1926">[[Max Planck|Planck, M.]] (1926).</ref><ref>Buchdahl, H.A. (1966), p. 69.</ref>{{clarify|date=February 2014}}