Re: Induced dipole moments

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Ludwik's "conceptual difficulty with a non-polar molecule":
> ...
> 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?

No, wrong. The Coulomb force is only of the 1/r^2 form for the force
between 2 *point* charges.  In the case of an atom or molecule the
negative charge distribution is continuously spread out over a few
Angstrom's even though the positive charge distribution is concentrated
into one or a few more (effectively) point charges.  Not only is the
negative charge spread out, it *surrounds* the positive charge.  The
Coulomb force between the negative and the positive charge distributions
would only begin to approximate the 1/r^2 form if they both were widely
separated from each other so only the monopole contribution from the
negative charge distribution interacted dominantly with a point positive
charge.

> 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?

I think the important thing you are missing is that the negative
charge distribution for a given atomic configuration is a continuous
distribution, not a point distribution.

David Bowman
David_Bowman@georgetowncollege.edu