Macroscopic State

I can measure:

• Total energy.
• Volume, energy, pressure of the gas.
• Total magnetization of the spin system.
• etc.

If in two systems these values are equal, they are in the same macroscopic state .

There are many microscopic states, corresponding to a single macroscopic state. Example: 3 spins, $S_i=\pm1$

Macroscopic State Microscopic States Number

M=-3 (-1,-1,-1) 1

M=-1 (-1,-1,+1), (-1,+1,-1), (+1,-1,-1) 3

M=+1 (+1,+1,-1), (+1,-1,+1), (-1,+1,+1) 3

M=+3 (+1,+1,+1) 1