Gas in an External Electric Field

We consider here a system consisting of N molecules in a volume V at temperature T placed in a uniform electric field D. D is actually the electric displacement field, which is controllable by external charges (say on the surfaces of a condenser). 

In the present treatment we assume that interactions among the molecules are negligible therefore we put (7(X ) = 0 as in the case of an ideal gas. The remaining integral may now be factored into a product of N factors 

We denoted by qo the contribution of a single molecule to the total PF of the system due to the external field. This is sometimes included in the rotational PF of a single molecule. Here, the rotational partition function is included in q. Thus we rewrite (2.5.1) as 

We now assume that the molecules interact with the external field in two ways. First, if it has a permanent dipole moment jio, its interaction with the field is 

The second interaction is between an induced dipole and the external field. We assume for simplicity that the induced dipole ju nd is in the direction of the field and is linear in D, i.e.. 

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