Zeroth Law of Thermodynamics



Zeroth law is the fundamental law of thermodynamics, but  it was discovered after the discovery of first and second law of thermodynamics that is why it is called "zeroth law of thermodynamics".



Statements


It has two statements:


Statement#1:  If two objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other". 




Statement#2: "Two systems are in thermal equilibrium if and only if they have same temperature".





Explanation: Let us consider two objects  A and B which are not in thermal contact. Consider another object C. We wish to determine whether A and B are are in thermal equilibrium with each other. We surround them with an ideal insulating box so that they cannot interact with surrounding. We separate system A and B with an insulating wall and let system C interacts with both systems  A and B. The interaction is due to thermal conductor placed mutually between A, B and C.

We wait until thermal equilibrium is attained.

To find out that system A and B are in thermal equilibrium, we replace insulating wall by a conducting wall that lets A and B interact. What happens?

Experiment shows that nothing happens; there are no additional to A or B.


We can say: If C is initially in thermal equilibrium with both A and B, then A and B are also in thermal equilibrium with each other.