Could some one tell me ... ?

[7923509%TWNCTU01.BITNET@BITNET.CC.CMU.EDU]

Hi:
Could anyone tell me the following question ?
 
1.What is the reason that the states of the k th unit Sk = 1 with probability
               1
  Pk = -------------------
        1+exp(-delta Ek/T)
 
  where delta Ek is the energy gap between the 1 and 0 states of the k th unit,
        T        is a parameter which acts like the temperature
 
2.Why this local decision rule ensures that in thermal equilibrium the relative
  probability of two global states is determined by their energy difference,and
  follows a Boltzmann distribution:
 
   Pa
  ---- = exp(-(Ea - Eb))/T
   Pb
 
  where Pa is the probability of being in the a th global state and
        Ea is the energy of that state.
 
  and how do we know that whether the system reaches thermal equilibrium or not?
 
reply should be send to 7923509 at twnctu01 thank's a lot!