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Re: electric charge



On Fri, 21 Dec 2001, Larry Cartwright wrote:

Well, I've always assumed that a substance is a collection of elementary
particles such as quarks, leptons and bosons.

Hmmm. To further muddy the water, I could point out that many
characteristics of a macroscopic "substance" must vanish when we examine
them at the microscopic level. Because there are scale effects, water is
not just a bunch of elementary particles. All sorts of emergent phenomena
become significant depending on relations between the particles.
Creatures much larger or much smaller than human beings would perceive
water to be several very different "substances". It appears that
"substance" is in the eye of the beholder. When Mr. Tompkins has shrunk
to the size of an electron, he can no longer see the regular array of
water molecules, to say nothing of the wet slippery ice cubes.

"Charge" has a similar problem: if we zoom in on a *neutral* molecule,
we'll find that it's not neutral at all. At the micro-level there is
significant charge, but at the macro level the charge is zero. This might
not seem to have direct bearing, but consider the following.

If I have a neutral metal sphere suspended in space, and I place a tiny,
insulating, positively-charged object on its surface, what happens? The
positive charge from that object will spread uniformly across the entire
surface of the sphere. This is only true at the macro level. At the
micro level we'd find that the charged object has not lost any charge, but
instead the electron sea of the metal is attracted to our small positive
object, and partly un-cancels the charge of many positive metal nucleii
at many places on the entire metal sphere.

At the macro level (considering the entire sphere), I'd say that charge is
like a self-repelling substance which spreads across conductive objects,
which can flow through conductors, accumulate at surfaces, etc. Down at
the level of a single particle, I'd say that charge is just a property of
that particle, and can never leave it. But I can never say both things at
the same time, since I perceive two very different worlds depending on the
setting of my "zoom lens."

If I wanted to insist that "charge" is just a property, I could focus my
attention on subatomic particles. If instead I wanted to insist that
"charge" has the characteristics of a substance, I could look at charge
flows in electric circuits, and at classroom experiments in
electrostatics. Is one scale of observation more real than the other?


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