Microstate and Macrostate
Keywords:microstate, Illustration, molecules
This article is about the basic concept of microstate and macrostate in statistical mechanics.
A complete specification of the set of six coordinates(x,y,z,px,py,pz) of each particle of a system (within the limits of the dimensions of the cell accommodating its representative point) is said to define the microstate of the system.
In such a microstate specification what is stated is where each molecule is situated and with what momentum (necessarily, therefore the direction) each particle is moving within of course the limits of δx's and δpx's.
Such a detailed specification however is quite unnecessary, even if possible to make, to describe the observable (macro) properties like pressure, temperature, etc. of the system. For instance, the ordinary density (i.e., mass per unit volume) of a gas is the same if the same number of molecules lie in each cell of ordinary space, no matter which are the molecules concerned. This means that the observable macroproperties just depends entirely on how many phase points lie in each cell of the phase space.
A specification of the number of phase points in each cell of the phase space, that is, the number Ni and for all i's defines what is termed the macrostate of the system.
Thermal physics —A.B. Gupta , H.P. Roy