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

The four laws are:

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. 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.

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".

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"