Perpetual motion

Perpetual-motion machines come in two kinds. A machine of the first kind would produce work with no energy input at all, violating the first law (conservation of energy). A machine of the second kind would extract heat from a single reservoir and convert it entirely into work, violating the second law (the Kelvin–Planck statement).

The second kind is the subtler and more instructive: it breaks no conservation law. A ship that propelled itself by cooling the sea by a fraction of a degree would conserve energy exactly. It is impossible only because it would require heat to flow with no compensating increase in entropy elsewhere — which the second law forbids.

The repeated failure of such machines is one of the strongest experimental supports for the laws of thermodynamics; patent offices in several countries refuse to consider perpetual-motion claims without a working model precisely because the laws make them impossible.

Inventors pursued perpetual motion for centuries; by the mid-nineteenth century the first and second laws had explained exactly why each scheme failed, turning a history of frustration into a proof of physical principle.