text: In a general sense, the<unk>  law says that temperature differences between systems in contact with each other tend to even out and that work can be obtained from these non-equilibrium differences, but that loss of heat occurs, in the form of entropy, when work is done. Pressure differences, density differences, and particularly temperature differences, all tend to equalize if given the opportunity. This means that an isolated system will eventually come to have a uniform temperature.  A heat engine is a mechanical device that provides useful work from the difference in temperature of two bodies:
  During the 19th century, the<unk>  law was synthesized, essentially, by studying the dynamics of the Carnot heat engine in coordination with James Joule's Mechanical equivalent of heat experiments. Since any<unk> engine requires such a temperature difference, it follows that no useful work can be derived from an isolated system in equilibrium; there must always be an external energy source and a cold sink. By definition, perpetual motion machines of the<unk>  kind would have to
  violate the<unk>  law to function.
option_0: Second law of thermodynamics
option_1: Franck-Condon principle
option_2: Third law of thermodynamics
option_3: Planck's law
option_4: Equation of state
option_5: Chemical thermodynamics
option_6: Weber–Fechner law
option_7: Hy's law