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Induced dipole moments



I am having a conceptual difficulty with a non-polar
molecule in an electric field, as described in Tipler
(Physics for Scientists and Engineers, 1991, section
18.7, page 616). A non-polar molecule has no dipole
moment when E is zero.

1) A uniform electric field produces a separation of
+ and -. The molecule is oriented with its dipole
moment along a line of E. So far all is clear; a polar
molecule (in which + and - are separated even when E
is zero) behaves in the same way.

2) Tipler's explanation of the induced dipole moment
is not clear to me. Referring to Figures 18.22 or 18.23,
he writes:

"The positive and negative charges separate until the
attractive force they exert on each other balances the
forces due to the external electric field."

In this very first chapter on electricity the "force they
exert on each other" is the Coulomb force between
the center of + and the center of - (which are initially
at the same point). In an atom + would be the nucleus
while - would be the cloud of electrons.

3) It is true that the external field tends to separate +
and -; the separation forces are +qE an -qE. The above
statement of Tipler would make sense if the magnitude
of the force of mutual attraction (between + and -) were
increasing with the distance between them. For example,
if + and - were connected with a tiny plastic spring.

4) But Tipler states that the attractive force is due to
Coulomb's interaction. Coulomb force decreases with
the distance. Therefore this force does not become large
enough to stop the process of separation of + from - by
the external electric field E. The Coulomb force is infinite
when separation is zero and no finite field should be able
to induce a dipole moment in a molecule or atom. Right?

5) Am I missing something important? If my objection is
valid then how to explain dipole moments? Is this another
case (like hydrogen atom) in which an explanation is not
possible in classical physics?
Ludwik Kowalski