Statistical Mechanics - Grand Canonical Ensemble : Why Grand Canonical Ensemble? 

This video is just an introduction to grand canonical ensemble. There is an obvious question that we have already two ensembles-Micro canonical and Canonical,then why we need a third ensemble. Here you will find the detail answer of this very question. The canonical ensemble model could not be applied to the processes where the number of particles varies e.g chemical processes and quantum processes where the particles i.e photons are created and annihilated. So,there must be another ensemble which allows the subsystem to exchange energy as well as particles with the remainder of the system i.e reservoir. Such an ensemble which allows the subsystem to exchange energy as well as particles with the reservoir is called grand canonical ensemble.