Re: example of alternate form of 1st law But the thread is still: "ENERGY with Q"

[Tom Wayburn <twayburn@WT.NET>]

----- Original Message -----
From: "Carl Mungan" <mungan@USNA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, November 28, 2001 5:35 PM
Subject: example of alternate form of 1st law


> Dadgum it, it turned out to be easier than I expected to come up with
> a problem that doesn't lend itself to Q+W=delta(E). Gene Mosca
> provided me with it.
>
> Drop a block. The earth does work on the block. But the block does no
> work on the earth (because the earth's motion is negligible). Oops!
> The resolution of this situation is of course to include the energy
> of the gravitational field. But unless one is willing to talk about
> work done on a field (and I don't think I am), **** [But, in van Wylen
and Sonntag, when we write the enthalpy term for matter entering the
control volume and ditto for matter leaving, we write Internal energy
per unit mass + Injection work, where Injection work is PV for a "Simple
Compressible Fluid, but might have a Surface tension term, a Magnetic
field term [del W = - (mu sub 0) * H * d(VM), where H is the intensity
of the magnetic field and M is the magnetization, an Elastic solid term,
an Electrical work term, etc.]  **** There is no way to
> include this term on the LHS of the 1st law as stated above.
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******* I will try to write down an equation to handle your particular
problem.   I wish I could send Fig. 8.2.  Draw it?  Don't draw it?  Cut
the page out of the book and scan on my HP Oficejet 710?  Fish or cut
bait?   Whoops, time's up!

Goodnight / T
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*****
> The only way out, I must concede, is to admit that Q+W=delta(E) is
> not useful anytime field forces (which are external to the system)
> are involved. Unfortunately this includes large numbers of problems.
>
> ps: If you want to try balancing energies in the above problem, I
> suggest the following geometry. Take the earth to be spherical and
> uniform in density. Instead of a block, consider a shell of mass dm.
> Let this shell fall inwards spherically starting from infinity. It
> works out nicely: the gain in field energy between the surface of the
> earth and infinity equals the loss in potential energy of the mass
> shell. You may be more comfortable doing this problem using electric
> rather than gravitational fields.
> --
> Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
>        U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
> mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/
What is the best equation writing e-mail maker to use for the people on
this thread?  I have Acrobat, which might "do" equations, Word 2000,
which produces a large variety of encodings including RTF, Netscape,
which I can no longer get to work, but I could download another copy and
set it up and I think it handles equations but I would prefer to be told
if it does, Microsoft HTML, which I don't know anything about,
Frontpage, and Webpublisher, oh, and I could write out my equations and
my answer to the (various) challenges on my 486 in Word 2.0c for Windows
3.1 and produce a postscript file that will print just like I want my
"product" to look on any Postscript printer.  Well, since my Postscript
printer is two large boxes filled with tiny parts, I can't print
Postscript files just now, and some others may be in the same mess,
although anyone associated with a teaching or research institution can
easily find a Postscript printer close by.

Dang ding it.   That was a lot of words, but the problem of sending
equations to Phys-L has not been solved and maybe not even approached.
(A few years ago I tried to work out a Latin alphabet equation
convention with moderate success, but I wasn't getting into subscripts
on subscripts on the limits of integration of multiple integrals.   We
should all be using differential forms by now.   They are easier to
write  --- probably.)

Regards and profuse regrets for the prolixity / T
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