Q.State Zeroth Law of Thermodynamics.

Zeroth Law of Thermodynamics

The zeroth law of thermodynamics states that if two systems $A$ and $B$ are each in thermal equilibrium with a third system $C$, then $A$ and $B$ are in thermal equilibrium with each other. This transitive property allows a consistent definition of temperature: the common physical quantity that is equal for all systems in mutual thermal equilibrium.

Thermal equilibrium is reached when there is no net flow of heat between systems placed in contact through a diathermic (heat‑conducting) wall; an adiabatic wall instead prevents heat exchange and keeps systems thermally isolated. The zeroth law thus underlies the very concept of temperature and the operation of thermometers — a thermometer (system $C$) placed in turn against two bodies reads the same value only when they are at the same temperature. It is called the 'zeroth' law because it is logically prior t …