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Re: [Phys-L] Energy & Bonds

John,

A total non-sequitur

I strayed through your weird terminology document

http://www.av8n.com/physics/weird-terminology.htm

And it struck me that, for fun, you should add very famous example, one of my favorites from Voltaire regarding the Holy Roman Empire

http://www.brainyquote.com/quotes/quotes/v/voltaire140970.html

I would venture that since it is such a pithy quote it might make a good poster child as the very first example.

Joel




-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
Sent: Friday, November 15, 2013 5:27 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Energy & Bonds

Yesterday on 11/14/2013 01:59 PM, I wrote:

Note the contrast:
-- The amount of /energy/ we have to put in to move
two previously-bound ions apart approaches a maximum
at large R. Draw the graph of a -1/R potential.
http://www.av8n.com/physics/img48/ion-energy.png
Don't forget the minus sign, and don't forget
that -1/R is not the right answer at small R.
++ Contrast this with the ½ k R^2 potential for a
Hookean spring.
-- The /force/ between the two ions is at a minumum
at large R. Draw a graph of the derivative of -1/R.
http://www.av8n.com/physics/img48/ion-force.png
++ Contrast this with the k R force law for a Hookean
spring.

Just now I put up vastly improved versions of those diagrams.

In particular, they now include a contribution from the atomic core, which is a kinetic energy contribution that comes into play when the ions get close together.
Without this term, the spring model has *zero* range of validity.

With this term, you can see that the spring model is pretty good in a *small* region near the equilibrium point :
separation = 0.1 local units,
force = 0
That is to say, the black dashed line conforms to the blue line over a small range. There is a lot of interesting physics that happens in this small range ... but also a lot of physics that happens waaaaay outside this range.

The scale for "breaking a chemical bond" is outside the range of validity of the spring model, out by orders of magnitude, as you can see in the diagrams.

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