Zeroth Law of Thermodynamics

There exists for every thermodynamic system in equilibrium a property called temperature. Equality of temperature is a necessary and sufficient condition for thermal equilibrium.




Consider three bodies, X, Y and Z.

  Z is in thermal equilibrium with Y.

  X is in thermal equilibrium with Y.

  Then Z is in thermal equilibrium with X



To try to visualize this further, consider a hot cup of tea.

After about twelve hours, the saucer, the cup and the tea will all be at the same temperature.

The saucer is in equilibrium with the cup.
The cup is in equilibrium with the tea.
Therefore the tea is in equilibrium with the saucer.
They are each in thermal equilibrium with each other.