Re: "electric current" is not electron flow

[John Clement <clement@HAL-PC.ORG>]

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


> In K-6 textbooks we often find the misconception that electric current is
> always and forever a flow of negative charges.  Since this error appears
> in the textbooks, I assume that many educators believe this misconception
> as well, and must be teaching it to their students.  I don't know if this
> is still true in grades 7-12, but I've met many technical people who still
> believe that electric current is defined as a flow of electrons, and that
> postive charges cannot flow at all.

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.  While one speaks of N
and P carriers, the reality is that the electrons are rearranged, and the
nuclei are not moving appreciably in the opposite direction.  The N and P
model is wonderful for studying semiconductor electronics, which my students
do not get to.  The always and forever model is clearly wrong with respect
to the always, but is a very good model when dealing with conductors.
Indeed any text that says always and forever about any physical process will
usually be wrong.

> Or perhaps they should talk about CHARGE FLOW, with just a nod at
> "electric current."  In other words, teach about all the situations where
> positive charges flow.  In the end this will demonstrate why the "electric
> current" concept is so useful:  it greatly simplifies things by
> emphasizing the flow rate while hiding all the stuff about charge velocity
> and direction.

Unfortunately one must have the students learn about a simplified model
before trying to learn about the complex cases, while acknowledging that the
complex cases do exist.  The means that first they must learn the one way
model, and the electron motion model.  Indeed they must also come to a
better understanding of atoms at the same time they are learning.  One
example of this latter problem can be clearly seen by asking the following
question of students:

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.  After discussion, and asking things like "What holds
atoms together" or "What happens when you remove all of the electrons from
and atom" .... they will generally come to the more realistic answer.  The
students who understand the correct answer will sometimes look at the other
students with amazement, and imply they are stupid.  In a sense they are
right, but only because they are able to think formally, and have thought it
through.  The lower level students can come up to this higher level of
thinking, just as young children progress from preoperational up to concrete
thinking.  This particular question is one I use in class every year, so I
know how students react to it.

One of the reasons why they must first learn the simple cases is that to
progress in thinking ability a student must be challenged by a slightly
surprising result that can be accommodate by changed thinking.  A complex
situation involving a very abstract idea can not be accommodated.  The low
level student will not form any sort of mental model.  Since most of my
students have low thinking scores, and little understanding of the simple
models, I can not hope to push them to understand the extremely complex
cases.  Some of my higher level students will actually do outside reading,
and make progress on their own.

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???

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 would also like to comment on the word concrete.  It is often used with
several meanings.  In Piagetian psychology it is used as a label for a
certain level of thinking.  This level can not be exactly defined, but is
often applied to students who score below a certain level, say 1/3 of the
maximum score on the Lawson test.  This does not mean that they always have
to have concrete examples.  Lawson calls this lower level of thinking
Empirical-inductive, and the higher level Hypothetico-deductive.  Lower
level thinkers do have great difficulty handling situations with multiple
variables, and they do not understand proportional reasoning well, but they
should be able to handle simple control of variables, and simple
conservation (mass or volume).  Concrete thinkers have difficulty asking
what if questions, and I have observed often do not understand proof by
contradiction.  When presented with a contradiction to their arguments they
often do not change it, but just restate it.  A model that involves too many
variables will not be accessible to them.

Lawson has evidence for an even higher level of thinking which could be
labeled "theoretical".  As I understand it, he claims that this level allows
individuals to understand models where the constituents are not physical.
It may be that this higher level is necessary to truly understand things
like N and P semiconductors.  In my experience one does not have to
understand the physics completely to be able to use it in engineering
problems, but one must be a formal thinker to be a good engineer.

John M. Clement
Houston, TX

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