Re: Fields etc

["Rauber, Joel Phys" <RAUBERJ@MG.SDSTATE.EDU>]

Jim,
A very provocative and interesting post.  But to stir the pot a little,
couldn't one turn your discussion up-side-down and say that electrons aren't
real, but just a handy device to do calculations concerning what is real,
E&B fields? (It being merely historical accident that this handy fictitious
device was thought of first before the reality of the fields were
discovered.)

Joel (eagerly awaiting comments)
 ----------
From: Jim Green
To: PHYS-L
Subject: Re: Fields etc
Date: Monday, February 15, 1999 12:19PM

I admit that I probably should not respond to William Beaty's post but it
is Monday morning and I have had a weekend and a Sabbath to rest so I will
plunge headlong into this -- The post will likely be long, but I will not
say further after this.  Nevertheless, if you are not up to it, please
delete now:

FEYNMAN

Let me start with William's Feynman quote:

|  A reference: FIELD ENERGY AND FIELD MOMENTUM, The Feynman Lectures on
|  Physics, ch 27, vol. II
|
|Same with grass-seeds which align when placed between the plates of a
|capacitor with high P.D. between its plates: Something is there in empty
|space, although it does not refect light and hence is not visible.  We
|might mentally-model this "Something" as a population of (virtual?)
|photons, or as e-fields and/or b-fields.  But just because the "Something"
|is invisible, or because there are more than one model used to describe
|it, this does not force us to declare that it is nothing but an abstract
|concept.

This seems to be offered sort of like a scriptural quote -- just because
the Great Feynman said something, it must be true.  Many mis-conceptions
were started by very great people.  Most notably by Aristotle who said all
sorts of weird stuff -- some of which we are living with to this day.  For
example he promulgated the idea that all things are made of fire, water,
earth, and air (or some such) from which caloric ideas still haven't died a
proper death -- even after Rutherford's clear experiments to the contrary,
here we are 150 yr later still talking about "heat" as a flowing substance.

Even Feynman joins in this deception in his famous parable of the two
windows W and Q (I don't remember the details - Feynman aficionados can
look it up in the Lectures)   He has some stuff -- presumably "energy" --
being thrown out of one or the other of these windows to describe the First
Law -- as if energy and heat and work were some sort of substance(s) which
can move.

This is patently silly.  Nevertheless, he has been trapped by the ancient
myths and language usage.     And so too have many others.

If I ever have the opportunity to talk personally with Prof Feynman, I will
pin him down on these points.  I have great faith that he will say
something like "Oh yes, of course you are right - I just use this imagery
in my freshman lectures to entertain the kids and clarify things for them.
I call this bit of subterfuge "mental modeling".  I got the parable from
one of my old professors  when I was a student."   And I think that he got
it from one of his professors.

But Dick, I will respond, this _doesn't clarify things; it just perpetuates
bad understanding of physics.  I see why you do it.  Aristotle so taught
his students and they so taught theirs and they so taught theirs and ....
But Aristotle didn't know any better; you do  (or should I say did?) so
repent.

Look, Dick, you have grass seeds being pushed about by a "field" of some
sort, but you know full well that what is happening is that the various
electrons in the grass seed are interacting according to the Coulomb's
"Law" with the electrons in the plates.  You have chosen to INVENT the idea
of a field and to anthropomorphize this so called E-field (and/or B-field
if the plate's electrons are moving) because you think this is an easier
way to describe the situation.  But often this is counter-productive unless
you make clear to your students the fact that this is just a handy
invention -- otherwise they might actually believe that this invention of
yours is actually "real" (in the sense that _they_ use this word.) -- and
some day propagate mis-understandings to _their_ students.

And, yes Dick, I agree that when the situation gets really complicated, as
when the charges are distant or moving very fast, these new fangled ideas
do help.  But we should always remind ourselves and our students that we
just INVENTED the idea of fields.

At this point the following will flash through both his mind and mine
because we both have it emblazoned there from HS or before:  If two
charges, q1 and q2,  are separated by a distance, r, then the force of one
on the other is kq1q2/r^2 -- and if I want to variously put q3 then q4 then
q5 etc near q1, it helps if I calculate kq1/r^2 just once and call that E,
say, and then I only have to calculate q2E, q3E, q4E, q5E etc etc.  And
because this procedure is so handy I will (arbitrarily) CALL kq1/r^2 the
E-field due to q1. But I should always remember that I just conjured up
this little method -- it is not really "real" in any customary sense.

At this point Dick and I might be interrupted by a list member (as I was
recently) with the thought that by then the idea of a "field" is nearly
universal.  "Hey, everybody uses this concept  and it is obviously useful
and clearly universal, so in that sense couldn't he think of the "field" as
"real".   We would both respond that the philosophy of what "exists" and
what is "real" has been debated for centuries. (Maybe by this time, we
would say millennia.)   Or maybe one of us would remind him that many many
people believed in  UFOs, but we don't usually say that they were "real" --
even though several movies were made featuring them.

LIGHT

For my part, I would never think of a light beam as traveling in/on/through
anything "real".  I thought we had overcome those thoughts long ago.  I do
confess that I do sometimes think of a photon as a real particle (but I
know that I am wrong) or worse as a "real" "quantum" of a "real" "e_field".
 Ugh!

I also confess to slipping and saying "straight line" from time to time,
but I am getting better.  If I slip in class I always correct myself -- or,
because the students have been taught well, and because students will be
students,  one of them will jump at the chance to correct me. (:-)  I think
that we all show signs of having been taught by those who were taught by
those who were taught by those who were taught by someone who didn't quite
understand correctly.  Alas, our schools and colleges are full of such
people.   It might be nice to put an end to some of these circles.

RADIO

Electrons in the broadcaster's antenna oscillate and due to the Coulomb
force cause the electrons in the antenna on my car to oscillate.  I can
talk about "radio waves", but I know there  is nothing "real" there.  No,
William, I don't think of radio waves as being real.  In some circles, even
the electrons are thought not to be "real" in this sense.


ENERGY

I could talk about energy flow -- and sometimes if I am very careful this
imagery might be helpful in a class, but I can't now think when I might do
it without a very long prelude speech --- the temptation should be
subordinated to an opportunity to reinforce the work/energy theorem.  By
doing work, energy can be increased or decreased, but it doesn't really
flow in the sense that _I_ use the word or that _I_ think would be helpful
to students.  And I just don't see that value of the idea that "energy" is
"in" an e-field.  I just can bring myself to think of "energy" as a
substance.  And neither did Young, who first used the concept.  He thought
of it as a "property" of an object -- like color or density.  We wouldn't
say that "blue" flows from one object to another, would we.  We could say
that "paint" -- ie a substance -- flows.  Just as we should not say that
"heat" rises, but that "hot air" -- a substance - rises.  -- and that the
_molecules_ have energy.  There is no such substance as "heat".

Hey, this is just me over here in my little corner.  You all should fell
free to talk the way you think you otter.


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen
 ------------------------- Original message header:
> MAIL FROM:<owner-phys-l@LISTS.NAU.EDU>
> RCPT TO:<RAUBERJ@mg.sdstate.edu>
> RCPT TO:<QuistO@mg.sdstate.edu>
> DATA
> Received: from mailgate.nau.edu (mailgate.nau.edu [134.114.96.19]) by
fafhrd.sdstate.edu (AIX4.3/UCB 8.8.8/8.7) with ESMTP id MAA32724; Mon, 15
Feb
1999 12:25:17 -0600
> Received: from mailgate ([134.114.96.19]) by mailgate.nau.edu (PMDF V5.2-29
#31141) with ESMTP id <0F7700MT4HSKVA@mailgate.nau.edu>; Mon, 15 Feb 1999
11:20:23 -0700 (MST)
> Date: Mon, 15 Feb 1999 11:19:27 -0700
> From: Jim Green <JMGreen@SISNA.COM>
> Subject: Re: Fields etc
> Sender: "phys-l@lists.nau.edu: Forum for Physics Educators"
<PHYS-L@LISTS.NAU.EDU>
> To: PHYS-L@LISTS.NAU.EDU
> Reply-to: "phys-l@lists.nau.edu: Forum for Physics Educators"
<PHYS-L@LISTS.NAU.EDU>
> Message-id:<0F7700MF1K9FVA@mailgate.nau.edu>
 ------------------------- End of message header.