First Postulate and Second postulate (principle of equal priori probabilities) of Statistical Thermodynamics

Statistical thermodynamics used in predicting the macroscopic properties of a system consisting of large number of particles.

Gibbs proposed a method called ensemble method based on two simple postulates

First Postulate:

It states that the time averaged value of thermodynamic property (which is not related to temperature )is equal to the ensemble average value of the same property.

• For this ensemble has to be considered such that it covers all the possible quantum states of the system. i.e. the ensemble should contain all possible stationary states of the system.

Second postulate:(principle of equal priory probabilities)

It states that in an ensemble, formed by all the possible quantum states of an isolated thermodynamic system in equilibrium all the quantum states will be found with equal probability.

• This postulate is known as "principle of equal priori probabilities".

Therefore the postulates put together imply that the system spends equal amount of time in each and every possible quantum state. These two postulates made use of in estimating the thermodynamic properties of the system.

The validity of these postulates depends on the agreement between the experimental values and deductions made from the postulates.