Zeroth Law of Thermodynamics

What is the Zeroth Law of Thermodynamics?

The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

The physical meaning is expressed by Maxwell in the words: "All heat is of the same kind".

As an illustration, suppose we have three objects as shown on the slide. Object A and Object B are in physical contact and in thermal equilibrium. Object B is also in thermal equilibrium with Object C. There is initially no physical contact between Object A and Object C. But, if Object A and Object C are brought into contact, it is observed that they are in thermal equilibrium

Background of the Zeroth Law

The zeroth law was first formulated and labelled by R. H. Fowler in 1931. 

As the name suggests, its value as a fundamental physical principle was recognized more than half a century after the formulation of the first and the second laws of thermodynamics.

It was named the zeroth law since it should have preceded the first and the second laws of thermodynamics.

Application in Thermometers 

The simple observation of the Zeroth Law of Thermodynamics (which cannot be concluded from the other laws of thermodynamics) allows us to create a thermometer. It serves as a basis for the validity of temperature measurement. 

By replacing the third body with a thermometer, the zeroth law can be restated as :         

Two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.

We can calibrate the change in a thermal property, such as the length of a column of mercury, by putting the thermometer in thermal equilibrium with a known physical system at several reference points. 

Celsius thermometers have the reference points fixed at the freezing and boiling point of pure water. If we then bring the thermometer into thermal equilibrium with any other system, we can determine the temperature of the other system by noting the change in the thermal property. Objects in thermodynamic equilibrium have the same temperature.