Re: intermolecular forces

[Richard Bowman <rbowman@BRIDGEWATER.EDU>]

On Tue, 20 Feb 2001, Tucker Hiatt wrote quoting John Denker:

> He wrote, in part: "The repulsive [intermolecular] force has to do with
> kinetic energy, and it's not electrostatic.  Think of an uncharged particle
> in a box.  As you decrease the size of the box, the wavefunctions get more
> wiggles per unit length, so their kinetic energy goes up.  This causes a
> repulsive force, i.e. a pressure on the piston."

In fact at the most fundamental level, this is as a result of an
electrostatic force. What John is referring to might be called the "hard
sphere approximation" to gas behavior.  But we also should ask what is
causing one gas molecule to recoil from another one? In fact the
electrostatic repulsion of the electrons of the gas molecules is the
source of the recoil. To a good measure at one atmosphere and near room
temperature, we can model this behavior using the concepts of conservation
of linear momentum and conservation of kinetic energy.

> Should we really think of (certain) quantum mechanical rules as forces?
> For example, does the Pauli exclusion principle describe a true "force"?
> [Astronomers say that such a force is able to counteract gravity and
> temporarily prevent the collapse of white dwarf stars, no?]  Accordingly,
> when we speak of the "four forces of nature," are we speaking only of the
> non-quantum world?

At the quantum mechanical level (and I am guessing that you are lumping
nuclear interactions here, too), we are often modeling electrostatic,
strong and weak forces. Others working in the nuclear field may have more
to add.

Richard
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