Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Induced dipole moments



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 is the attractive
Coulomb force increases with the distance (producing a local
potential well around 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