In: Physics
Calculate ln Omega (k) for N large and k = fN where f is a fraction. Assume k and N-k are also large and use Stirling's approximation.
Entropy & 2 nd Law of Thermodynamics
Exam Study. Extremely Urgent.
Configurational entropy refers to the entropy resulting from imperfect mixing of different atoms in the same site in a crystal.
When you roll a die, there are six possible outcomes, with the following probability
P(X=1) + P(X=2) + ... + P(X=6) = 1
The probability that any particular number will come up is
The uncertainty associated with that probability is 0 if one outcome has a probability of 1, and a maximum when the outcomes are equally likely.
Imagine a material with M atoms that can occupy N sites. If the
mixing is ideal, the atoms have no interaction with each other, so
more than one can occupy the same site. The total number of
configurations, 
, in this system is
 = NM
Copyright c 2009 by C.H.
Mak
But, this is slightly incorrect (in what is known as Gibbs' paradox) because there is overcounting of identical configurations like these

With the overcounting removed we have
 = N! / (Np1! * Np2!
...Npn!)
 is the probability that a given number of atoms n in
a given number of sites N will have a particular configuration.
Or
 = ln N! - 
ln(Npi)!
 = -N 
pilnpi
 = 
 is the probability that a given number of atoms M in
a given number of sites N will have a particular configuration.
Boltzmann defined configurational entropy as
 (This is engraved on Boltzmann's tomb in
Vienna!)