Ensembles

As already mentioned, statistical mechanics deals with the averaging processes. This averaging is conveniently carried out by ‘introducing the concept of ensembles’ **, as briefly described below:

The need for introducing this concept arises form the fact that the calculation of thermodynamic properties (E, S, H, P etc.) is based upon the states of the individual molecules, which however, are changing with time (because of intermolecular collisions) and the calculation of average over time for such a large number of molecules is impossible. Moreover, it is not possible to know the state of each and every individual molecule. To overcome this problem, instead of considering a single system , a large number of systems are considered which are exactly identical with the system under consideration in a number of aspects such as total volume, total number of molecules, total energy etc.

A collection of such a large number of systems which are identical with the system under consideration in a number of as pacts such as total volume, total number of molecules, etc. is called an ensemble of systems. They may, however, differ in some thermodynamic prosperity such as energy etc.

Depending upon the thermodynamic variables kept constant, the ensembles are of many different kinds.  The two most common are as follows:

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