| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 421.1 | Hope this helps... | BSS::GINDHART | | Wed Apr 02 1997 14:10 | 13 |
| Van der Waals force is attractive. It results from a weak attraction
of one atom's electrons by another atom's nucleus. The electrons, being
mobile will find equilibrium at a position which effectively avoids the
other atoms electrons, thus resulting in an overall attractive force.
A better description lies in considering the quantum mechanical orbits
created by two nearby atom, but this is somewhat messy.
As far as the quarks and gluons, I have never been fully up on
that theory.
chas.roc
|
| 421.2 | It does... | WARFUT::CHEETHAMD | | Thu Apr 03 1997 00:24 | 7 |
| re .1 Yes thanks, that makes sense. I was assuming an equal probability
distribution for the electrons, which would give a repulsive force with
an inverse square law (I think). The distribution of electrons in a crystal
structure, where each atom has multiple neighbours, would take a little
figuring though :-)
Dennis
|
| 421.3 | | AUSS::GARSON | DECcharity Program Office | Thu Apr 03 1997 14:56 | 4 |
| re .2
I would have thought that a uniform (spherical) distribution of electrons
about the nucleus with an inverse square law would give no net force.
|
| 421.4 | | DRAGNS::WALLACE | | Fri Apr 04 1997 11:20 | 13 |
| If you had 2 atoms with spherical electron shells you would
indeed get repulsion. The trouble is that once you start
bringing the atoms close together the electrical field of each
atom distorts the electrical field of the other, and the charge
distibution is no longer spherical. What you end up with is 2
dipoles, arranged like:
- + - +
In which case it's possible to understand why they attract.
Vince
|
| 421.5 | | REGENT::POWERS | | Tue Apr 08 1997 11:33 | 21 |
| > distibution is no longer spherical. What you end up with is 2
> dipoles, arranged like:
>
> - + - +
>
> In which case it's possible to understand why they attract.
But that arrangement is not symmetrical.
If we're talking similar atoms, as we would be in a homogeneous gas,
then the charge distribution should be symmetrical about the center point
of the two atoms.
Maybe like:
-3 +4 -1 -1 +4 -3
Where some of the negative charge gets pushed to outside edge while
other moves to the inside.
- tom]
|
| 421.6 | | AUSS::GARSON | DECcharity Program Office | Tue Apr 08 1997 16:18 | 13 |
| re .5
>But that arrangement is not symmetrical.
>If we're talking similar atoms, as we would be in a homogeneous gas,
>then the charge distribution should be symmetrical about the center point
>of the two atoms.
Even so, the arrangement shown in .4 is of lower energy than a
symmetrical one (as in .5). I doubt that you can arrange those charges
symmetrically and get a net attractive force.
I believe that spontaneous symmetry breaking can occur in transitioning
to a lower energy state.
|
| 421.7 | My brane hertz(again) | WARFUT::CHEETHAMD | | Wed Apr 09 1997 07:38 | 7 |
| re .5 Do Van der Waals forces occur in a gas ?
re .6
>> I believe that spontaneous symmetry breaking can occur in transitioning
>> to a lower energy state.
deeeerrrrrrrrrrr ????
|
| 421.8 | | DRAGNS::WALLACE | | Mon May 05 1997 11:36 | 30 |
| >> If you had 2 atoms with spherical electron shells you would
>> indeed get repulsion.
I wrote this in .4, and it is of course incorrect (shouldn't
write notes when tired). A spherical distribution of charge
looks like a point charge of the same magnitude.
What's important is that when we're talking about electrons
having a spherical distribution around a nucleus, its the
quantum mechanical wave function that's spherical, not the
electrical charge distribution. Averaged over time the
charge should also be spherically distributed, but at any
given instant it will not be.
Think in terms of a hydogen atom, with 1 proton and 1 electron.
At any particular moment you have 2 point charges creating a
dipole. When two hydrogen atoms start to approach each other
there is a tendency for the orbits of the two electrons to
"synchronize" with each other, so that you get the -+-+ charge
distribution, which creates a net attraction. Crude picture
of synchronized electrons :-)
- -
-+-+ => + + => +-+- => + +
- -
This tendency for the electrons to "synchronize" provides an
intuitive explanation for Van der Waals forces. For the
real explanation of course, do the QM math :-))))
|
| 421.9 | | AUSS::GARSON | DECcharity Program Office | Mon May 05 1997 15:39 | 15 |
| re .8
> its the quantum mechanical wave function that's spherical
I think only for s orbitals.
> At any particular moment you have 2 point charges creating a
> dipole.
I don't think that it is reasonable from a QM point of view to say
this. Instead I think QM would say that you have a superposition of
many states each of which is a bit like a dipole.
How one works that into an explanation of the van der Waals force I
don't know.
|