Editing Second law of thermodynamics (section)

=== Carnot's theorem ===
[[Carnot's theorem (thermodynamics)|Carnot's theorem]] (1824) is a principle that limits the maximum efficiency for any possible engine. The efficiency solely depends on the temperature difference between the hot and cold thermal reservoirs. Carnot's theorem states:
* All irreversible heat engines between two heat reservoirs are less efficient than a [[Carnot engine]] operating between the same reservoirs.
* All reversible heat engines between two heat reservoirs are equally efficient with a Carnot engine operating between the same reservoirs.

In his ideal model, the heat of caloric converted into work could be reinstated by reversing the motion of the cycle, a concept subsequently known as [[thermodynamic reversibility]]. Carnot, however, further postulated that some caloric is lost, not being converted to mechanical work. Hence, no real heat engine could realize the [[Carnot cycle]]'s reversibility and was condemned to be less efficient.

Though formulated in terms of caloric (see the obsolete [[caloric theory]]), rather than [[entropy]], this was an early insight into the second law.