Recalling that k=R/Na (Na is Avogadro's number), show that the density of fluctuations of an ideal gas are given by :
<(dp)^2> / p^2 = 1 / (N*Na) where p is the density (mass/V)
That is, the relative mean square density deviation is the reciprocal of the number of molecules in the subsystem.
<(dN)^2> = <N^2> - <N>^2 from my book
The Attempt at a Solution
I have no idea where to even begin with this..I have a whole series of problems that ask me to find the "mean square deviation of ____". I understand what an expectation value is, and that i have the given equation for the value.
But HOW do I find <P> or <P^2> ?? Normally I would take the value*probability (sum of X*P(X) right?) but in this case....what is my probability? what is my value??
Where do I even start!?!?
Plz help I have 4 problems like this and am stuck in the exact same place on all 4.