Zeroth Law of Thermodynamics:
“If two bodies are separately in thermal equilibrium with a third body, then the first two bodies are also in thermal equilibrium with each other“.
Example: If A and B are separately in thermal equilibrium with C, then according to the Zeroth law. A and B are also in thermal equilibrium with each other.
This may appear to be an obvious truth and we are tempered to believe that the statement need not be considered a separate law. But it is not true, our experiences dictate that this type of argument doesn’t always hold.
Significance of the Zeroth Law:
From the analogies, it can be concluded that there must exist a characteristic property of everybody, whose equality dictates thermal equilibrium among the bodies. This characteristic property is the temperature of a body.
Temperature is a physical property of everybody, whose equality is the necessary and sufficient condition for thermal equilibrium among the bodies.
According to this definition,
1. All substances kept in a room for a long time are at the same temperature.
2. Temperature of hot water is different from that of cold water.
3. A hot body and a cold body are kept in thermal contact, attain the same temperature after some time, etc.