Re: CONSERVATION OF ENERGY

[twayburn@juno.com (Thomas L Wayburn)]

On Fri, 25 Jul 97 14:05:30 EDT Al Bachman <BACHMAN@rfmh.org> writes:
>   Let me attempt to present corrected versions of Ludwik's A & B.
>
> A. The puck sliding across the floor with initial ke=K is brought to 
> rest
> by the frictional force R.
> 1. The Work-Energy Principle tells us that:  delta(K) = Work done by 
> forces,
> assuming no other forms of energy present.
> 2. Work done by R = -Rx
> 3. Therefor:   0 - K = -Rx   and   x = K/R
>
> B. The puck and floor form an isolated system ( no external forces, 
> etc.)
>    so:  delta(U) = 0
> 1. Therefor if we can enumerate all the categories of energy that 
> might change,
> Ui, and since energy is additive:
>      delta(U1) + delta(U2) +.....   = 0
> where delta(Ui) = Ui(when ke=0) - Ui(when ke=K)
>
> 2. Among the forms of energy to be considered are:
>   a. bulk Kinetic Energy
>   b. potential energies that can change due to deformations (eg. 
> scratching
>      of surfaces, crazing, transfer of material from one surface to 
> another)  ***CAN WE NEGLECT?****
>   c. energy internal to the bodies associated with change of 
> temperature
>      (used to be called "Sensible Heat")  *****IT SURE DID.****

> 3. If you wait for the entire system to come to a uniform temperature 
> (still
> isolated) AND the change of energy forms b. are NEGLIGIBLE, then
>         0-K +c*(m+M)*dT = 0
> { I like to represent delta(K) by (0 - K) to remind students that it 
> is a
> CHANGE.)
>
>    Have the terms "Sensible Heat" and "Latent Heat" disappeared? 
***No, not at all,  and I think I want to revise my plan to ditch the
verb to heat.   In the forms listed by you they are harmless (I now
think).****
>
C.  You could have analyzed the floor too.  After all, you seem to know
its mass and other physical properties.

While I agree that work crossed the boundary of each subsystem, I  need
to account for the way in which that work was "lost" by friction, which
is the prototypical form of  *lost work*.    The hard part is deciding
what to divide it by in the entropy balance.  If we integrate over the
rising T of both subsystems over the course of the experiment, we get
mechanical lost work, which might be what we want.  But, this work could
have been used to pump "thermal energy" from the environment up to the
temperature of the system, which is varying as the work is being trashed.
 Substitute a Carnot heat pump to compute LW and divide it by 300 K, say,
in the entropy balance(s).

>    As far as "transfer" vs. "transform", I believe we talk of 
> transferring
> energy between modes (eg ov vibration), as well as across system 
> boundaries.
But,  unless the modes are taken to be subsystems, these transfers are
not heat or work.  Perhaps, the analyst needs to chop up his experiment
into more subsystems.  I think Clifford Truesdell complained about that
in his lecture (in Italy) on rational thermophysics.  I think we might
need to look at eigenmodes of some systems.  I never solved a problem
like that though.   I sure am having a devil of a time with radiative
heat transfer.   Classical methods just flat out bomb.

(I think that the equilibrium temperature that Jim Green gave me for a
spinning object just outside the earth's atmosphere might  permit a
solution for thermodynamic lost work from the entropy balance! - *not*
the combined first and second laws.  On the other hand, I can't use a
temperature that is affected by the sun or the earth (as a radiator,
emissivity = ~0.8.  Anyone else compute that number?))

Regards / The amateur