It's a universal tendency for people to simplify things in order to
understand them. That's fine, as long as they realize that their
understanding is based on a simplification, and they don't try to apply
it to areas where the simplification is no longer valid. While the idea
of charge flow as electron flow works just fine in a vacuum tube, it
isn't at all true in general.

Current is the rate of flow of charge, which as I'll explain isn't the
same as the flow of electrons. Charge can be positive or negative. A
shortage of electrons in an atom's valence shell results in a positively
charged atom (a positive ion), and an excess of electrons in a
negatively charged one (a negative ion). In a conductor, electrons are
quite free to move about. In a semiconductor, they're not, and the
crystal lattice can contain either an excess of electrons (N type
material), a deficiency of them (P type material), or a normal number
(intrinsic material). In a vacuum tube, the flow of (negative) charge is
simply the physical flow of electrons, and the flow of positive charge
becomes a mathematical concept, moving the opposite direction. But this
isn't necessarily so in other media. In a wire, for example, charge
flows much faster (near the speed of light) than electrons (which flow
at a rate on the order of a few miles per hour). If you jam a bunch of
electrons into one end of a wire, an equal number very quickly pops out
the other -- but these aren't the same ones that went into the other end
-- those will slowly drift along the wire at a few miles per hour. The
rate of charge flow is dictated by how long it took electrons to pop out
of the other end of the wire after jamming some in the input end, not
how long it takes the added electrons to drift their way along. So in a
wire, for example, charge isn't the same as movement of electrons. If
you try to envision physical current (charge flow) in a wire as being
the same as physical current in a vacuum tube, you'll be misleading
yourself.

Now imagine sucking a bunch of electrons out of one end of the wire.
There'll be an electron-poor region at the wire end. A "wave" of
electron-poor region will propagate to the other end of the wire at
nearly the speed of light, and a bunch of electrons will be sucked into
the other end of the wire. The propagation of this wave of an
electron-poor region is the physical flow of positive charge. Envision,
if you must, sucking water through a drinking straw that's already
filled with water. Bear in mind, though, that this isn't an exact model
of what's happening, so be careful in using it.

It's important to be able to separate the concepts of moving charges and
moving electrons, if you're going to have the versatility of
understanding things other than vacuum tubes, like positive ion
generators, lightning, charge flow in a semiconductor, or even a wire.
Once you do, it becomes just as easy to envision positive charge flow as
negative charge flow. If you can't do this without imagining physical
marble-like particles carrying the charge, you have no hope of
understanding an electromagnetic field, or other more abstract and
mathematical concepts.

Roy Lewallen, W7EL

-- A quick web search brought this brief explanation of how electrons
behave in a conductor:
http://hyperphysics.phy-astr.gsu.edu...ic/ohmmic.html. I'm
sure it would be easy to find a lot more good information (as well as
some pretty bad stuff) if anyone is interested enough to look.

Bill Turner wrote:

On Fri, 08 Oct 2004 16:01:04 -0700, Roy Lewallen wrote:


It's a common mistake to equate "current" or "charge" with "electrons",


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What other kind of current is there besides the flow of electrons? Even
the flow of "holes" in a semiconductor is propagated by the absence of
electrons.

And isn't charge merely the presence or absence of electrons? I'm not
talking mathematical concepts, just the actual physical happening?

--
Bill W6WRT