Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

"Dissipation" (was Re: momentum conservation(2))



I want to second Jahn Barrer's eloquent defense of the word "dissipation"
which I have come to embrace for precisely the same reasons. I find that
its pointed use helps students to begin laying a sturdy foundation for
their eventual study of thermal physics while studying mechanics.

I want my students to understand 1) that energy seems *always* to be
conserved and 2) that, in real processes, it tends to change from readily
apparent and useful forms to less readily apparent and useful forms
mediated by rubbing, sanding, scouring, eroding, scratching, scraping,
banging, jolting, slapping, crashing, smashing, jerking, jostling,
etc... well ,you get the idea.

I just can't think of a better single word than "dissipation" to describe
and unite these ubiquitous processes and to serve as a mental place holder
of sorts for the associated critically important idea that will eventually
find more precise expression in the the second law of thermodynamics.

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm

On Tue, 14 Mar 2000, John Barrer wrote:

... There was a clear need to account for the
inefficiencies present when shifting energy from one
storage mode ("bin") to another. The word chosen was
"dissipated" (Ediss). When this topic is presented, it
is made VERY clear that this energy is not lost, but
is distributed throughout the system, typically
showing up as a temperature increase. It is energy
that still exists but is difficult to recover and put
to "use" in the form of Ek, Eg, or Eel. Perhaps
"irrecoverable" or "dilute" would be a better word?
But I think the word is less important than the
concept. I have used a physical analogy of Eg, Ek, and
Eel "buckets" between which energy may be "poured",
but each transfer is always accompanied by a little
"spillage" of energy into another (lower) container
from which it is very difficult to recover the energy
(the Ediss). Maybe I'm a bit like the newly converted
religious zealot, but I think that this approach
provides much more clarity and is still scientifically
correct. It completely avoids the pseudowork issues
(eg, "work done" by friction) that Sherwood and Chabay
have addressed. Instead we speak of "energy dissipated
by friction". I suppose we could say "energy converted
to increased internal kinetic energy which shows up as
a temperature rise", but I think Ediss is simpler and
creates no difficulties within the context it is
presented. This approach also bypasses the classical
work-energy theorem which is often presented as a
special case that only "works" when there is no
friction. Pie charts are employed as a visual way to
keep track of energy changes in a system during a
sequence of events. If no energy enters or leaves
(equivalent to the more traditional statement "no work
done by or on the system"), then the size of the pies
stays constant although the Ediss slice of the pie
always gets larger. If the process involves an object
coming to rest at the reference elevation, the entire
"final" pie is Ediss - not lost energy, but energy
that is no longer available (at least not easily). ...