Zeroth Law of Thermodynamics

If a body said to be in equilibrium three conditions must be satisfied.

a)      Mechanical equilibrium   :  There is no external force must be acted on the system

b)      Thermal equilibrium        :  There must haven’t any spontaneous addition of heat.

c)      Chemical equilibrium      :   There must haven’t any chemical reaction.

If these 3 conditions are satisfied the system is said to be in thermodynamic equilibrium.

Consider the system A is an equilibrium with B and B is in the equilibrium with C, then  they are in thermodynamic equilibrium with each other.

Application

One of the familiar application of the Zeroth law is the Thermometer.

When we measure with a Thermometer, we can see only 2 system thermometer and the system whose temperature is to be measured.  But there is a 3^rd system ie. the atmosphere.

So consider a system with temperature (System A) is equilibrium with the atmosphere (System B).  Consider the thermometer as system C which is equilibrium with atmosphere.  So the thermometer and system with temperature is in equilibrium with each other according to Zeroth law.  So the temperature shows on the reading is same as t