Problems and Solutions on Thermodynamics and Statistical Mechanics

226

Then

Problems d Solutiom on Thermodynamics d Statiaticd Mechanics

The condition for classical approximation to be valid is that the quantiza-

h'

tion of the rotational energy can be neglected, that is, kT >> -

md2.

2060

The response of polar substances (e.g., HC1, H20, etc) to applied elec-

tric fields can be described in terms of a classical model which attributes

to each molecule a permanent electric dipole moment of magnitude p.

(a) Write down a general expression for the average macroscopic polar-

ization jj (dipole moment per unit volume) for a dilute system of n molecules

per unit volume at temperature T in a uniform electric field E.

(b) Calculate explicitly an approximate result for the average macro-

scopic polarization jj at high temperatures (KT > pE).

(MITI