Re: Induced dipole moments

[Jack Uretsky <jlu@HEP.ANL.GOV>]

        My problem with your model is quantitative and arises from
Slater's hint that a conducting sphere is a good model for an atom.
The induced field is proportional to the external static field.  I
doubt that you can find a rigid electronic charge distribution with
this property.  The essence of the matter is that the electronic
charge distribution, like the charge distribution inside the conducting
sphere is not rigid.  I have no clue, in any event, as to why you
visualize the electronic charge distribution as rigid.
        Put another way, the field of a spherical negative charge
distribution and an off-center point positive charge is not a simple
dipole.
                                regards,
                                        Jack


On Sun, 31 Dec 2000, Ludwik Kowalski wrote:

> WITH ONE CORRECTION IN MY FIRST PARAGRAPH.
> Jack Uretsky wrote:
>
> > There are three principal parts of dielectric behavior...
> > any individual atom can be polarized by the field, ...the
> > displacement of ions with positive charge, with respect
> > to ions with negative charge ... [and] molecules with
> > permanent dipole moments. You have excluded the third
> > case.  Which of the first two cases are you trying to understand?
>
> It is the first one. And, after the hint from David, I am no
> longer puzzled. The issue was to explain why does the attractive
> Coulomb force increases with the distance (producing a local
> potential well near a critical distance D>0). No QM is needed
> to understand this. Here how it goes:
>
> Of the two rigid clouds one (electrons) is much larger than
> another (nucleus). Suppose the proton is in the center of the
> negative cloud. The F=0 because it has as many electrons on
> the left as on the right. Start increasing d, the distance between
> the proton and the center of the negative cloud. Now you have
> more electrons on one side than on another and F is not zero.
> The asymmetry increases with d and F increases. But only up
> to a critical distance which depends on the shape of the negative
> cloud. Because of this one would need QM to predict the shape
> of the potential. This would be ncessary only if one wanted to
> calculate the induced dipole moment for a particular molecule.
> It is not needed to explain the phenomenon of polarization.
>
> This model is sufficient to explain why any not-too-large E will
> polarize atoms and molecules. An excessive E will break them.
> I wish I could produce an equally satisfactory classical model
> of attractive electric forces in solids, as listed by Brian. Only
> NaCL-like structures are "crystal-clear" to me in this respect.
> Simple explanations, appropriate for the first physics course
> would be appreciated. Happy New Year to everybody!
> Ludwik Kowalski

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