Impossible are those processes in which nothing happens but a complete transformation of into . [Kelvin]Example: Expansion of a gas against a weighted piston; here we have a complete change of into , but the end state is different from the initial one (larger ).
Impossible are those processes in which nothing happens but a transport of heat from a colder to a warmer reservoir. [Clausius]Example: Heat pump; here we do transport , but in order to do so we have to feed in mechanical energy.
Thermodynamics in a square: Maxwell's mnemonic device