# Laws of thermodynamics

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The laws of thermodynamics form a basis for the study of thermodynamics. There are four laws of very general validity, and as such they do not depend on the details of the interactions or the systems being studied. Hence, they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer. Examples of this include Einstein's prediction of spontaneous emission around the turn of the 20th century and current research into the thermodynamics of black holes.

## The four laws of thermodynamics

There are several ways to define the laws of thermodynamics. These are practical definitions:[1][2]

Zeroth law of thermodynamics, stating that thermodynamic equilibrium is an equivalence relation:
If two thermodynamic systems are in thermal equilibrium with a third, they are also in thermal equilibrium with each other.

The reason why this is the 'zeroth' law is that it was added after the others, although it was felt that it should come before them. Hence the unusual name. This law is often used to define the concept of temperature.

First law of thermodynamics, about the conservation of energy:
The increase in the energy of a closed system is equal to the amount of energy added to the system by heating, minus the amount lost in the form of work done by the system on its surroundings.

In other words, the energy of a closed system is constant. Considering the universe to be a large closed system leads to another version of the first law, that energy can neither be created nor destroyed or the total energy in the Universe is constant.

Second law of thermodynamics, about entropy:
The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value.

Using the concept of entropy the second law can be stated as the entropy of the universe increases as a result of all thermodynamic processes. This law forbids the existence of perpetual motion machines, since such a machine would need to perform work without releasing any heat to the environment, and such perfect machines are impossible by this law.

Third law of thermodynamics, about absolute zero temperature:
As a system asymptotically approaches absolute zero of temperature all processes virtually cease and the entropy of the system asymptotically approaches a minimum value

## History

The principal participants in the historical development of the laws of thermodynamics include Sadi Carnot, who in 1824 published "Reflections on the Motive Power of Fire",[3] that established the criteria for engine efficiency and James Joule, who demonstrated the equivalence of heat and work in the early 1840s[4]. They opened the way to the formal development of the subject by William Thomson (Lord Kelvin) in 1849 [1] and Rudolph Clausius in 1850.[5]

Thermodynamics was an esoteric subject until Willard Gibbs in the late 1800s almost single-handedly developed the concepts enabling it to be applied to chemical phenomena.[6] The techniques of statistical thermodynamics were established largely by Ludwig Boltzmann.[7]

The work of these pioneers led to the formulation of four laws. The very fundamental zeroth law was added after the first three were developed and hence its strange and unusual name.

## References

1. Kelvin, William Thomson (1849). "An Account of Carnot's Theory of the Motive Power of Heat - with Numerical Results Deduced from Regnault's Experiments on Steam." Transactions of the Edinburgh Royal Society, XVI. January 2. Scanned Copy
2. Cengel, Yunus A. and Boles, Michael A. (2005). Thermodynamics - An Engineering Approach. McGraw-Hill. ISBN 0-07-310768-9.
3. Nicolas Léonard Sadi Carnot (1824). Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance. Paris: Chez Bachelier, Librarie.  full text in French.
4. J.P. Joule (1845) "On the Mechanical Equivalent of Heat", Brit. Assoc. Rep., Trans. Chemical Sect, p.31, read before the British Association at Cambridge, June
5. Rudolph Clausius (1850), "Über die bewegende Kraft der Wärme, Part I, Part II", Annalen der Physik 79: 368–397 and 500–524. See English translation at On the Moving Force of Heat, and the Laws regarding the Nature of Heat itself which are deducible therefrom. Phil. Mag. (1851), 2, 1–21, 102–119.
6. J. Willard Gibbs and Henry Bumstead (1906). The Scientific Papers of J. Willard Gibbs. Longmans, Green and Co.  Full text at Google Books
7. Ludwig Boltzmann (1896). Vorlesungen über Gastheorie (Lectures on Gas Theory), 1st Edition. Johann Ambrosius Barth, Leipzig, Germany.