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

["S.Goelzer" <sgoelzer@COEBROWNACADEMY.COM>]

Jim Green wrote:

> > Mr Chairman, I rise in dismay at the use of the word "dissipation" in
> > connection with the concept of energy.  To say that energy (in any guise)
> > dissipates is to imply that it moves or flows.  I join with those who
> > proclaim that this is a dangerous image to put into the minds of
> > students.  Energy should be thought of as a "property" of a system: it can
> > be increassed or decreased by doing work (or being work as in the case of
> > PE) but it does not move -- unless of course the system moves -- It does
> > not flow from one system to another -- It does not dissipate,
> >
> > Of course one is quite free to use any language which communicates
> > _correct_ principles, but I think that it would be very difficult --
> > although common place -- to talk about energy as dissipating and teach
> > correct principles.
Seems like there is much arguing over simple semantics.

Students taking the physics already have to deal with the sacred physics usage of
the words work and conservation. Neither of these words are used in their physics
context in the 'real' world. Students must _learn_ the 'proper' physics meaning.
Words like flow, dissipation, transfer that are giving offense can only do so if
they are being used in an undefined manner. There is no reason we cannot define
or redefine these words, as we have done with the words work and conservation.
Redefining the meaning words based on context is a long standing convention in
the English language.

Dissipating is a dual context word. In one sense it means to disappear gradually,
as in "With each success my fears dissipated." Not appropriate for discussing
energy.

or

It can be used meaning to spread out or grow less concentrated as in " After the
fight, the crowd dissipated" - no one disappeared, the organization of the crowd
changed. In this context this word can be appropriate for making an analogy for
energy transferring (would the less dangerous word(s) be working or worked
into?)  to a less useful form.

Example:

I have a wooden frame on wheels (approx 1cu ft) strung on the inside with a
lattice of large steel nuts connected by rubber bands. When this demo is stopped
suddenly the nuts vibrate violently and randomly. This allows students to "see"
KE dissipating to heat* (or thermal or internal or ____ ). It shows that I cannot
use the random motion of the nuts to make the car go forward again. That energy
has been _lost_* and can no longer be used to do useful work, but the energy did
not disappear. Discussion of why the nuts themselves come to rest on their own
must follow.

*as in money that falls out of your pocket into a sewer grating: you can no
longer do anything with it, but it still exists and any effort to recover it will
be expensive.
* for my students, heat means it got warmer, I'll tackle changing their vocab. a
few words at a time. None of us learned all of the tech. vocab. at once either.

--

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Scott Goelzer
Coe - Brown Northwood Academy
Northwood NH 03261
603-942-5531 ext43
sgoelzer@coebrownacademy.com
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