Re: CONSERVATION OF ENERGY

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

Tom ("The Amateur") Wayburn writes:

> Now,  John,  the idea of "internal work" is absolutely out of the
> question in my development of thermophysics.   Work is the transfer of 
> energy (unaccompanied by entropy) through a control surface. ...

1. I don't see any reason to impugn the concept of internal work.  
If one defines work as force dotted into the displacement of the 
associated point of application, then internal forces do perform work 
and, unlike net internal force which is always zero due to pairwise 
cancellation, no such cancellation occurs for internal work.  As I 
mentioned in the post to which you responded, when a gas expands, it 
does so against weakly attractive intermolecular forces.  In the process 
these forces do negative net (internal) work and that work is manifest 
(internally) in the associated decrease in the total kinetic energy of 
the molecules.

2. I don't think you can define work very accurately as "the transfer of 
energy (unaccompanied by entropy) through a control surface."  There are 
lot's of situations (all irreversible) in which work is the only energy 
transfer process going on and in which the entropy changes. 

> I have no problem with internal potential energy.     At least I don't
> think I do.     I suppose  a system composed of M kgs of metal sliding
> horizontally upon a large metal slab with initial velocity V, relative to
> the lab coordinates,  should have 0.5MV^2 added to its "regular"
> (ordinary?) internal energy, U.  Eh?   Likewise if the system is a
> compressed spring that can be released by wishing it to be released  ...
> ?   Or a mass of M kgs 100 meters  above the lab floor  ... ?

Of the three examples you give above, only the compressed spring 
qualifies in my mind as an example of internal potential energy.  The 
first example is bulk translational energy that I don't even like to 
consider "internal," but is, in any event, certainly not internal 
*potential* energy.  The third example does not involve "internal" 
potential energy unless you define the system to be the mass *and* the 
earth.

John
-----------------------------------------------------------------
 A. John Mallinckrodt      http://www.intranet.csupomona.edu/~ajm
 Professor of Physics    mailto:ajmallinckro@csupomona.edu
 Physics Department       voice:909-869-4054
 Cal Poly Pomona            fax:909-869-5090
 Pomona, CA 91768-4031   office:Building 8, Room 223