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Re: contact electrification



As usual, I always learn interesting things when I pursue these
threads, and this has been no exception. It seems to me that, if
there is no reliable model to explain triboelectricity, and if there
is no reliable way to create a repeatable series, then it might have
even more value as a student exercise. Just about everything we do in
an introductory class "has an answer." But in real science, there is
no guaranteed answer, and it should be a good exercise for the
students to see at least one good example of a phenomenon, and an
easily demonstrable one at that, for which we don't know the answer
and don't even have a good model. Friction also falls in this area,
but there we have at least a sort of reasonable phenomenological rule
to calculate frictional forces, even if the actual physics of the
process is much more complicated than the concept of coefficient of
friction would imply.

But with triboelectricity, we don't get numbers, just a more or less
reproducible list and lots more questions than we had to start with.
This should teach the students a valuable lesson about science. As
Eddington is reputed to have said, "When an investigator has
developed a formula which gives a complete representation of the
phenomena within a certain range, he may be prone to satisfaction.
Would it not be wiser if he would say, 'Foiled again! I can find out
no more about nature along this line.'"

Hugh

To answer various questions that have popped up in the "triboelectric
series" thread:

1) "Triboelectric" is a perfectly good word that refers to electrification
by rubbing. Since rubbing is now seen to be a somewhat peripheral issue,
the more "in" term (and the more physical term) is "contact electrification".

2) It has been known for about 200 years that, strictly speaking, there is
no such thing as a triboelectric series. Depending on the chemical and
physical state of the surfaces, I can create objects A, B, and C that obey
the nontransitive relationships
x(A,B) > 0
x(B,C) > 0
x(C,A) > 0
where x(,) designates the charge of the first object when
contact-electrified against the second. It is even possible to have two
objects D and E such that
x(D,E) > 0 if you rub gently, while
x(D,E) < 0 if you rub harder.

A related but easier-to-visualize piece of physics is this: given fixed
terminals (A1 and A2) made of absolutely identical material (A) plus a
moving part of type B, I can create a generator which uses contact
electrification to move charge from A1 to A2 at a steady rate. Does that
imply A > B > A????

3) There _is_ such a thing as a work function, which is one of several
physical properties that contribute to contact electrification. The work
function itself is hard to pin down, since it depends on the chemical and
physical history of the surface. But it can be measured, e.g. by a Kelvin
bridge (which I am told was not invented by Kelvin).

4) I don't know of an accessible pedagogical discussion of contact
electrification. It's a complicated phenomenon, but reasonably well
understood. Back when I was a grad student the kind folks at Xerox let me
pick their brains (and their library) on the subject. Mostly it's just a
combination of everyday things:
-- work function
-- capacitance
-- tunneling and/or corona
-- simple fluid flow
-- etc.

--

Hugh Haskell
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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