24.11 Zeroth law of thermodynamics

I have written about the first, second and third laws of thermodynamics; these three laws are numbered in chronological order and, I believe, in logical order. But, in 1932, the English physicist Sir Ralph Fowler (1889-1944) proposed that there should be a further law that logically belonged before the other three. So it is called the Zeroth law of themodynamics.

The zeroth law considers three objects a, b and c. It states that if a and c are in thermal equilibrium (meaning there is no spontaneous flow of heat between them – see post 16.34) and b and c are in thermal equilibrium, then a and b are in thermal equilibrium.

Although the zeroth law appears in textbooks and is widely considered to be fundamental to the concept of temperature, I believe it is logically unnecessary.

We can consider temperature as a property of objects that defines whether heat flows between them spontaneously, as described in post 16.34. If no spontaneous flow of heat occurs between two objects, we say that they have the same temperature. Now the zeroth law becomes if a and c are at the same temperature and b and c are at the same temperature, then a and b are at the same temperature. It now seems that the zeroth law is a consequence of simple logic, as explained in the next paragraph.

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