Second law of thermodynamics

From Glossary of Meteorology

second law of thermodynamics

An inequality that is fundamentally different from the first law because it specifies the direction in which a natural process will evolve rather than merely requiring that certain quantities are conserved.

As formulated by Planck, the second law asserts that a thermodynamic state function, S, known as entropy, exists for all physical systems. For the universe and for a system isolated from its surroundings,
Equality prevails only for reversible processes or when the system is in a steady state. When the universe of a system is a maximum, no further evolution of the system is possible. The second law is often asserted in other forms, including the following.
When two bodies at different temperatures interact, the temperature of the hotter body can only decrease and that of the colder body can only increase unless work is done.
No device can continuously deliver mechanical work and produce no effect other than cooling a reservoir.
In the neighborhood of every state that can be reached reversibly, there exist states that cannot be reached by a reversible, adiabatic process, or, in other words, that can be reached only irreversibly or cannot be reached at all.

Dutton, J. A. 1995. Dynamics of Atmospheric Motion. Dover Press, . 45–51, 406–410.

Sommerfeld, A. 1964. Thermodynamics and Statistical Mechanics. Academic Press, . 26–36, 39.