# When does equilibrium mean accessible microstates are represented with equal probability in an ensemble of systems?

I'm confused to when a system in equilibrium is to be found in any one of its acessible states with equal probability (accoriding to the postulate of equal a priori probabilities). Reif in his book Funndamentals of Statistical and Thermal Physics states this principle and soon after gives examples of ensembles of systems he calles in "equilbrium" but withdifferent energy states occupied by greater numbers of systems in the ensemble than others. How does this reconcile with the posulate?

• The formulation of you question is unclear. But you may be referring to different statistical ensembles. Equal probability refers to the microcanonical ensemble, a system which is CLOSED (think of a gas in a box that does not exchange anything witht the exterior). You can also imagine a gas in a box, but this time, the box has wall with a given temperature, in this case the system is not closed but energy exchange (between the wall and the gas) happens. We would say that this system is described by a canonical ensemble where every microstate has a different weigth. But closed or not, both .. Commented Jul 13 at 23:03
• systems are in equilibrium, in the sense that their macroscopic quantities (energy, volume, pressure ....) do not evolve anymore. It is just that the equilibrium state is described by different probability distributions according to the constraints (is the system closed, open, ...) Commented Jul 13 at 23:05