The Second Law of Thermodynamics

Finally, the second law (it is really an axiom--it has never been proved; only never disproved.)

There are quite a few equivalent ways to state the second law. If the statements are correct, they are all equivalent. Some are harder to understand than others; some are included only for historical purposes. I think the following way of stating the second law is useful.

A

Every thermodynamic system possesses an extensive state function called ``the entropy'' which can be calculated by a reversible (that is, a quasi-static limit of a real process) path from an arbitrarily chosen ``Reference State'' by integrating the heat absorbed by the system divided by the absolute temperature of the system.

In other words, ``Entropy, the state function, exists for every system...''

B

During any observable process, the entropy of a system plus its surroundings (together defining an isolated system or universe) never decreases.

And, in other words, ``...and it always increases for the universe''