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Re: CONSERVATION OF ENERGY



Commenting on my remarks on free expansion, brian whatcott says

I want to read this as 'molecules recede from each other impelled by
a repulsive intermolecular force [called electric repulsion]...
...thus a positive "internal work" contribution has been done raising
Pi from its earlier more negative value'.

So would John or anyone else explain why molecules of gas are mutually
attracted and thereby set me straight?

There are two errors lurking here that, taken together, produce the
correct result--i.e., the internal potential energy decreases and,
therefore, the internal kinetic energy increases. (Who said two wrongs
don't make a right?)

First, since internal work is done at the *expense* of internal
potential energy, a positive internal work would have the effect of
*reducing*, not raising, the internal potential energy.

Second, the reason that the internal potential energy *is* negative (and
the reason that it gets less so when the average distance between
molecules increases) is that the intermolecular force in a gas is weakly
attractive, not repulsive, down to distances that are much smaller than
the typical average separation of molecules. At even smaller distances
it does become strongly repulsive. The long range attraction results
from dipole interactions while the short range repulsion is the result
of intermingling electron clouds.

John
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A. John Mallinckrodt http://www.intranet.csupomona.edu/~ajm
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