two bodies are said to be in thermal equilibrium if no transfer of heat takes place when they are placed in contact. We can now state the zeroth law of thermodynamics as follows :
If two bodies A and B are in thermal equilibrium and A and C are also in thermal equilibrium then B and C are also in thermal equilibrium.
It is a matter of observation and experience that is described in the zeroth law. It should not be taken as obvious. For example, if two persons A and B know each other and A and C know each other, it is not necessary that B and C know each other.
the zeroth law allows us to introduce the concept of temperature to measure the hotness or coldness of a body. All bodies in thermal equilibrium are assigned equal temperature. A hotter body is assigned higher temperatures than a colder body. Thus, the temperature of two bodies decide the direction of heat flow when the two bodies are put in contact. heat flows from the body at higher temperature to the body at lower temperature.
