Re: Induced dipole moments

[brian whatcott <inet@INTELLISYS.NET>]

> 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?

At 22:21 12/30/00 -0500, David Bowman responded:

> 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

David is essentially describing a model of the force which bonds
atoms in molecules and crystals. There is general understanding that
a progressive stress is required to develop a progressive strain,
and that the cohesive force is electrical in nature.

 The kind of electron sharing involved between atoms is classified
in several groups: ionic bond,covalent bond, metallic bond, and
 molecular bond depending on shell filling, electron sharing, and
charge asymmetry.
In the case of a covalent (shared electron) bond in a material
which is not asymmetric - and this is the situation of interest
to Ludwik, a binding force is still observed, and it is attributed
at this level to continued random movement of the valence electrons
with a resultant temporary dipole moment fluctuating with time.

  Though the dipole moment averages to zero over significant time
the resultant force does not. Crystals using this weak structural
force have low melting points:

Ar   -186 degC
Kr   -153
Xe   -108
CH4  -161
Cl2   -34 degC

[This is the general approach taken by
Oldenberg & Rasmussen, Modern Physics for Engineers,
 McGraw-Hill Para 12.1]


brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!