Heat is a form of energy associated with the positions and motion of the molecules of a body.
The total energy that a body contains as a result of the positions and the motions of its molecules is called its internal energy. The first law of thermodynamics states that in any process the change in a system's internal energy is equal to the heat absorbed from the environment minus the WORK done on the environment. The second law of thermodynamics states that in a system the entropy cannot decrease for any spontaneous process. A consequence of this law is that an engine can deliver work only when heat is transferred from a hot reservoir to a cold reservoir or heat sink. The Concise Columbia Encyclopedia is licensed from Columbia University Press.