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Re: "electric current" is not electron flow



On Sat, 6 Oct 2001, John Clement wrote:
Electrostatic charge transfer is not well understood, and I've seen
frequent mention that, for some types of materials, "frictional charging"
is probably caused by the transfer of ions both neg and pos types, not of
bare electrons.

The term some types of materials, also implies that for many other materials
the one way model is probably operative.

Of course. But that's no excuse for saying that triboelectrification is
caused by electron transfer. It's caused by *charge* transfer. We could
simplify it by assuming a Conventional Charge (which is positive charge),
or we could tell students what's really going on. I think the ongoing
controversy is caused by breaking the barrier between these two, and by
maintaining two kinds of "conventional current."


While frictional charging may
involve some positive transfer, the two way idea is then generalized by the
students to mean that the positive charges then move around a conductor as
well as the negative charges moving.

And they'd be right. In a totally general conductor, positive charges
move one way and negative charges move the other. In the natural world
this is the norm, and metallic conduction is an exotic thing created by
humans. Do you define "conductor" as meaning "conductive material?" I
think the word "conductor" might figure into the controversy somewhat,
because the folks who insist that electric current in a "conductor" is an
electron flow, are defining the word "conductor" as meaning "plastic
coated metal wire."

Side notes: In my case, this subject gets me riled NOT because I'm a
nitpicker, or because I can't stand to be "wrong" under any circumstances.
It's because "electron flow" is an invalid mental model which is a source
of numerous misconceptions. Long ago I identified this "electron flow"
thing as being a central cause of many of my own misconceptions about the
physics of circuitry. I was taught that electric currents were "really"
electron flows. I was also taught that "conductor" means "metal wire." I
was even taught that batteries are not part of a complete circuit ( I
learned that one plate is a charge source, the other plate is a charge
sink.) After years of talking to other technical people, I found that
many many others have the same misconceptions, and these misconceptions
come from the textbooks, not from the students.

After I finally learned that electric currents were NOT made of flowing
electrons, the scales fell from my eyes regarding several important
topics: the nervous system, batteries, and many facets of biochemistry.
Before this occurred I could not make head nor tail out of explanations of
nerve operation. Also, even though I was an electrical engineer, I had no
real idea how batteries worked. The electrochem stuff from chemistry
class didn't make intuitive sense (although I certainly could memorize it
for passing the tests!) The villian in all these cases was: "current is
electron flow." It sat there in my mind while distorting or rejecting any
incoming information that conflicted with it.


Sticking to the case of electron movement creates fewer problems,
and is much better pedagogy.

No, it is just simplified. But it is bad pedagogy because it creates
serious problems, although these problems might only arise after students
have left that classroom.


That being
said, one must always admit to students that the real world is more complex
and messier than the models we construct to try to understand it. In either
case the currently accepted standard simplified model of electrostatic
charging is a one way model.

Yes, and (heh!) ...the one-way charge is positive! I realize I'm bucking
the flow here. I've never seen a textbook which uses Conventional Current
in explaning tribo-charging. However, my point is that anyone who says
that electric current *IS* a flow of electrons is causing their students
some serious greif. Yes, that means that many textbooks are causing harm.
But many textbooks also say that electrons in wires flow at nearly the
speed of light, that battery plates are charge sources/sinks, or teach
other blatently wrong concepts. Fortunately these sorts of issues are not
determined by voting, and as Galileo said, in questions of science, the
authority of a thousand is not worth the humble reasoning of a single
individual.


One can certainly say that current is the flow of both positive and negative
particles in the case of a solution, such as NaCL in water, however in the
case of a copper wire, the copper generally stays in place and the electrons
flow. The case of a semiconductor is quite subtle.

This is just my point! Real conductivity is quite complicated. ( In
semiconductors we can avoid the "hole" controversy by saying that we can
have Valance-band conductivity and Conduction-band conductivity in the
same material.)

If we want to explain a "simplified" conductor, shouldn't we use the
standard which physics has always used? Conventional positive charges
flowing in a conventional current? I think it's misguided to change the
"conventional current" into a negative flow. It's actually an attempt to
un-simplify the simplified explanation while denying that this is the
issue. As I said in another message, we're fortunate that the currents in
wires are a flow of negative charges. It automatically sets up a
"discrepant event" which waits for any student who is first taught
Conventional Current, and who later attempts to learn how conductivity
really works. This controversy generated by "conventional current" is
partly caused by tenacious misconceptions which don't want to let go of
educators' minds without a fight.


The always and forever model is clearly wrong with respect
to the always, but is a very good model when dealing with conductors.

No, it is a very good model when dealing with *metals.* (Again, does
"conductor" mean "metal wire" in the above?) Repeating myself: in the
simplified physics explanation, electric current is a flow of positive
charges. In the non-simplified explanation, electric current is a flow of
all sorts of things. If we say that electric current is usually an
electron flow, then we're prejudiced towards human-made devices, since
conductors in the natural world are very rarely metallic. Also, it makes
us prejudiced against electronic components, since electron conduction
only reliably applies to the connecting wires (the "conductors.")



When you charge objects by rubbing what fraction of the electrons in the
positively charged object are transferred to the negatively charged
substance: a. 100%, b. 50% c. 10%, d. 1% e. an extremely small fraction
A few students will answer e, and a fair number will answer a.

The correct answer is generally only given by some students who have tested
as formal thinkers.

Errors in many K-6 textbooks seem to have originated in attempts to
explain complex ideas simply. I'm a big fan of making an explanation
"wrong" in order to simplify it. Right and wrong is not the issue. The
issue is whether the simplified explanation creates a barrier to further
learning. In the above situation you're forced to teach kids that
tribo-charging is caused by electron transfer. Perhaps this means that
the concept being taught should be saved for later grades?

Another side issue: tribo-charging is little studied and not well
understood. It is not impossible that nearly *all* tribo-charging is
caused by ion transfer. If this turned out to be true, would you re-think
your above test question?


Once students have accommodated the simpler models, then more complex models
can be tackled. The pedagogy for doing that is not as clearly defined, and
will be researched. I will also admit that I do not know of any research
which compares using an electron flow model vs. a conventional current
model. It should be possible to compare these as to the ability of the
students to form a realistic stable useful model. Is anyone doing this
research???

Good question. I haven't followed this research since 1990. A great
collection of papers (from 1985) is:

ASPECTS OF UNDERSTANDING ELECTRICITY: PROCEEDINGS OF AN INTERNATIONAL
WORKSHOP, R. Duit, W. Jung, C. von Roneck (Eds), Verlag Schmidt &
Klaunig, 1985 (QC530.A78 1985)

Does anyone know of similar collections?


Do your students say they have no idea why plants have flowers, or think
that the water on the outside of a cold glass diffused through the glass?
Mine do even after having bio. and chem. I would bet that many of yours
have these problems. Let us tackle the basics first!

I'm just a lowly engineer, with no students and no research papers, so
please see everything I say in that light: http://amasci.com/miscon/nitpik.html


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