Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: contact electrification



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.