any of three principles variously stated in equivalent forms.
being the principle that the change of energy of a thermodynamic system is equal to the heat transferred minus the work done.
the principle that no cyclic process is possible in which heat is absorbed from a reservoir at a single temperature and converted completely into mechanical work.
a law stating that mechanical work can be derived from a body only when that body interacts with another at a lower temperature; any spontaneous process results in an increase of entropy