# Entropy - Second Law of Thermodynamics

Second Law of Thermodynamics

The second law of thermodynamics states that in general the total entropy of any system will not decrease other than by increasing the entropy of some other system. Hence, in a system isolated from its environment, the entropy of that system will tend not to decrease. It follows that heat will not flow from a colder body to a hotter body without the application of work (the imposition of order) to the colder body. Secondly, it is impossible for any device operating on a cycle to produce net work from a single temperature reservoir; the production of net work requires flow of heat from a hotter reservoir to a colder reservoir, or a single expanding reservoir undergoing adiabatic cooling, which performs adiabatic work. As a result, there is no possibility of a perpetual motion system. It follows that a reduction in the increase of entropy in a specified process, such as a chemical reaction, means that it is energetically more efficient.

It follows from the second law of thermodynamics that the entropy of a system that is not isolated may decrease. An air conditioner, for example, may cool the air in a room, thus reducing the entropy of the air of that system. The heat expelled from the room (the system), which the air conditioner transports and discharges to the outside air, will always make a bigger contribution to the entropy of the environment than will the decrease of the entropy of the air of that system. Thus, the total of entropy of the room plus the entropy of the environment increases, in agreement with the second law of thermodynamics.

In mechanics, the second law in conjunction with the fundamental thermodynamic relation places limits on a system's ability to do useful work. The entropy change of a system at temperature T absorbing an infinitesimal amount of heat in a reversible way, is given by . More explicitly, an energy TRS is not available to do useful work, where TR is the temperature of the coldest accessible reservoir or heat sink external to the system. For further discussion, see Exergy.

Statistical mechanics demonstrates that entropy is governed by probability, thus allowing for a decrease in disorder even in an isolated system. Although this is possible, such an event has a small probability of occurring, making it unlikely.

### Other articles related to "second law of thermodynamics, law, laws, thermodynamics":

Henry Adams - Second Law of Thermodynamics
... American Teachers of History proposing a "theory of history" based on the second law of thermodynamics and the principle of entropy ...
Second Law Of Thermodynamics - Quotations
... The law that entropy always increases holds, I think, the supreme position among the laws of Nature ... But if your theory is found to be against the second law of thermodynamics I can give you no hope there is nothing for it but to collapse in deepest humiliation ... for entropy to increase in isolated systems is expressed in the second law of thermodynamics — perhaps the most pessimistic and amoral formulation in all human thought ...
Relational Order Theories - Related Areas of Current Interest - Second Law of Thermodynamics
... The development of non equilibrium thermodynamics and the observations of cosmological generation of ordered systems, identified above, have engendered proposed modifications in ...

### Famous quotes containing the word law:

According to the law of nature it is only fair that no one should become richer through damages and injuries suffered by another.
Marcus Tullius Cicero (106–43 B.C.)