Re: example of alternate form of 1st law

[Gene Mosca <mosca@USNA.EDU>]

Carl Mungan wrote:

> 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), there's no way to
> include this term on the LHS of the 1st law as stated above.
>
> 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.
The situation presented seems a dilemma if you require that W = Int F dot
ds, where ds is the displacement of the point of application of F.  However,
in thermodynamics Q is defined as the energy transfer across a boundary
associated with a temperature gradient at the boundary, and W is defined as
the energy transfer across a boundary not associated with a temperature
gradient at the boundary.  Consequently, not all W needs to be categorized
as Int F dot ds.
--
Gene