Editing Second law of thermodynamics (section)

=== Clausius inequality ===
The [[Clausius theorem]] (1854) states that in a cyclic process
: <math>\oint \frac{\delta Q}{T_\text{surr}} \leq 0.</math>

The equality holds in the reversible case<ref>[http://scienceworld.wolfram.com/physics/ClausiusTheorem.html ''Clausius theorem''] at [[Wolfram Research]]</ref> and the strict inequality holds in the irreversible case, with ''T''<sub>surr</sub> as the temperature of the heat bath (surroundings) here. The reversible case is used to introduce the state function [[entropy]]. This is because in cyclic processes the variation of a state function is zero from state functionality.