The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium with a separate third body, then the first two bodies are also in thermal equilibrium with each other.
This means that if system A is in thermal equilibrium with system C and system B is also in equilibrium with system C, then system A and B are also in thermal equilibrium.
An example demonstrating the Zeroth Law
Consider two cups A and B, with boiling water. When a thermometer is placed in cup A, it gets warmed up by the water until it reads 100 °C. When it reads 100 °C, we say that the thermometer is in equilibrium with cup A. When we move the thermometer to cup B to read the temperature, it continues to read 100 °C. The thermometer is also in equilibrium with cup B. By keeping in mind the zeroth law of thermodynamics, we can conclude that cup A and cup B are in equilibrium with each other.
The zeroth law of thermodynamics enables us to use thermometers to compare the temperature of any two objects that we like.
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