ABSTRACT
The grand canonical ensemble is built upon the canonical ensemble by relaxing the restriction to a definite number of particles. The relative probability of finding the system with N particles at temperature T , in a volume V , is taken to be
ρ(N , V , T ) = zN QN (V , T ), (13.1) where z = eβµ is the fugacity. The chemical potential µ is a given external parameter, in addition to the temperature. The system exchanges energy with a heat reservoir at temperature T , which determines the average energy, and it exchanges particles with a “particle reservoir” of chemical potential µ, which determines the average number of particles. A schematic representation of the ensemble is shown in Fig. 13.1.
