Physical Significance of Entropy:
1. Entropy as a measure of the Disorder of the system:
We know that all the spontaneous process in which heat is transferred through a finite temperature. Spontaneous process is also known as irreversible process because these processes take place at very fast rate. All the natural processes are spontaneous processes.
All these processes are accompanied by increase in the disorder of the system.
For example:
When we heat a metallic rod from one end the initially all the molecules are present in some order. But when, heat transfer take place from hot end to cold end disorder of the molecules take place. This is due to attainment of the equilibrium which will make uniform distribution of the energy in the system. Hence the state of the order of the molecules gets disturbed and gives rise to greater disorder. Hence spontaneous processes are accompanied by increase in entropy as well as increase in the disorder of the system.
2. Entropy as a measure of Probability:
From above we concluded that spontaneous processes are accompanied by increase in entropy as well as increase in the disorder of the system. A little consideration will show that when a spontaneous process takes place it moves from less probable state to a more probable state. Hence, there is a close relationship between entropy S and the thermodynamic probability W of the state of the system. This relationship is expressed by Boltzmann as:
S = k ln W + constant
Where k denotes the Boltzmann constant which is equal to gas constant per single molecule i.e. R/NA.
As constant is equal to zero, therefore:
S = k ln W
This is called the Boltzmann entropy equation
