Re: COLLISION 2

[Jim Green <JMGreen@SISNA.COM>]

At 09:11 AM 29-11-99 -0500, Mike Monce wonders as follows:
> On Sun, 28 Nov 1999, Jim Green wrote:
>
> > Energy can be increased or decreased but it can't be "transferred".
>
>         Despite you insisting on this, I still don't see how you can make
> such a statement.

Look I have said many times that one is quite welcome to define any word
any way one wants -- but surely we don't intend to learn physics from a
dictionary -- However, once one defines a word one must then be able to use
it in a conversation to communicate.

Now, in a discussion amongst physics instructors who are trying to
internalize the fine points of a concept, precise language is very important.

If by "transfer" one is connoting "motion", this is inappropriate for the
concept of "energy" -- in such a technical discussion.  If one wants to use
imprecise language in a classroom AND is willing to spent the effort to
guard that students don't get confused, ok do it at your own risk.

But here in a precise technical illumination of the subject, you will hear
from people who feel the need to be more precise in order to learn anything.

As to photons:  If I believed that there is really such a real particle  as
a photon, I might be able to discuss it, but I currently assume that a
photon is one of those vagaries of physics language that we use because we
really don't understand what we are talking about.

Now in your suggested example, IF you mean something like a bank "transfer"
where I ask my bank to "transfer" $1^6 to your account AND no dollar bills
move from my bank to yours AND in the conversation we agree that this is
the meaning ascribed to the word "transfer" , the OK.  BUT why go through
al this effort when it is so much easier to explain thermo without the
confusion.

A few specific responses follow:

>         A photon leaves the sun with h-nu of energy, travels through
> space, strikes the surface of the earth and is absorbed by an atom.
>
> Questions:
>         1) Didn't the internal energy of the sun decrease (change) by
>            h-nu?

I don't know how helpful it is to think of a "photon" as a real particle,
but let's do for the nonce -- yes, something in the sun did work on this
assumed particle and increased its energy and at the same time work was
done on the sun to decrease the energy there.

>         2) Didn't the internal energy of the earth increase (change) by
>            h-nu?
 As the "photon" collides with the Earth it does indeed do work on the
Earth and the internal energy does indeed increase.  NOT because "energy"
moves, but because "work" is done on the Earth. Indeed KE is never changed
unless work is done on something.

>         3) How did this happen if the photon did not transfer the
> energy(see a) and b) and c) above).

The word "transfer " is minimally acceptable here IF one takes the time to
explain that what is meant is the likes of the bank analogy.  But why go to
all that effort; just don't use the word "transfer"; it is so much easier
AND you don't risk confusing the hell out of your students.

Of course if your students are imbued with hellish things, if might be a
good idea to scare them a bit.

At least you didn't say "flows".  (:-)

What might be a good idea as well is to make it a list rule for those who
must say "transfer" is to say "transfer (bank)" instead.  Then we would all
understand the same thing. These thoughts are valid for all "properties" of
something -- like charge, mass, blue, etc  -- none of which move
independently without the "something".

I am sorry I said that; now we will have to decide what "something" is --
and, for that matter, what "is" is.

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen