I am very lost when expectation values meet DENSITY. I know the following: <X> = sum[X*prob(X)] <(dX)^2>=<X^2>-<X>^2 I am COMPLETELY lost on how to find the "value" X and its "probability" P(X) in situations dealing with DENSITY. This might be best described by an example: Given: (e-1)/(e+2)=Ap e=dielectric constant; A=constant; p=DENSITY Show that the fluctuations in dielectric constant of a small quantity of N moles of matter in a large system are: <(de)^2>= [kT(kappa) *(e-1)^2 * (e+2)^2 ] / 9V where V is avg volume ---------------------- In short....where do I even begin to calculate <e> or <e^2>?? I fail to understand how this problem even relates to the expectation value... An expectation value is found by taking the value at a given point, times its probability right? (Sum of[X*P(X)] where P(X) is the probability that X will happen). How does this relate in any way to the given problem?? Am I mistaken in my understanding or can anyone at least point me in the slightest of directions? 5 hours later and nothing to show for it is incredibly frustrating.