Laws of thermodynamics

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:

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 always increases.

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 (see Bose–Einstein condensate and negative temperature). This law is more clearly stated as "the entropy of a perfectly crystalline body at absolute zero temperature is zero".

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", that established the criteria for engine efficiency and James Joule, who demonstrated the equivalence of heat and work in the early 1840s. They opened the way to the formal development of the subject by William Thomson (Lord Kelvin) in 1849 and Rudolph Clausius in 1850.

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. The techniques of statistical thermodynamics were established largely by Ludwig Boltzmann.

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.

Quotes & humor

• A common scientific joke, as stated by C.P. Snow, expresses the four laws simply and surprisingly accurately as:

Zeroth: "You must play the game."

First: "You can't win."

Second: "You can't break even."

Third: "You can't quit the game."

• Attributed to Arnold Sommerfeld:

"Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you any more."

• The British duo Flanders and Swann wrote a comic song (possibly the comic song) about the First and Second laws, called simply "First and Second Law"