- The Kelvin-Planck statement: "It is impossible to construct a device that operates in a cycle and produces no other effect than the transfer of heat from a cooler body to a hotter body."
- The Clausius statement: "Heat cannot spontaneously flow from a colder object to a hotter object without an external source of energy."
- The Carnot theorem: "The efficiency of a heat engine operating between two fixed temperatures is independent of the working substance used."
- The entropy principle: "The total entropy of an isolated system can never decrease."
All of these formulations are equivalent to each other, and they all express the fundamental principle that heat flows from hot to cold and that useful work can only be extracted from a system by adding energy or reducing entropy.
