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Re: copper rods



On Wed, 30 Jul 1997, David Bowman wrote:

Yep! I recently found yet another way to express this. If we could move
electrons at a few cm per second, the wire would heat up a rate of
kilowatts per cubic mm, it would glow white hot and melt, if not vaporize.
Or conversely, if we could grab the electron-stuff of a wire and shove it
along, it would take kilo-newtons force to make it move at cm per second
rate (and of course it would smoke and melt.) The "electric fluid" within
wires is not like water. It behaves more like cold tar being pumped
through a sponge! Amazing that electrical devices work at all.

I think that there *might* be a misconception afoot here. It is *not* true
that the conduction electrons in a current-carrying wire move at speeds
anywhere near the mm/s order of magnitude.

Hi Dave! I see your point. I think my mistake was in using the word
"electrons" to refer to the substance-like entity which should more
accurately be called "electron-stuff", or "charge", or "electric fluid".
It's analogous to the difference between the terms "air" and "air
molecules". When "air" flows at 1cm/second, its molecules do not. If
"charge" is a thing like "air", then I could correctly say that charge
flows slowly within wires. Electric current acts like wind, and in both
materials the particles move very differently than the macroscopic stuff.
The motion of the individual electrons could be seen as being like thermal
motion, or like diffusion in air. The electrons' thermal/diffusion/
quantum motion exists whether there is "charge flow" taking place or not,
so as far as circuitry is concerned, I can ignore it. Just like I ignore
the molecular velocity when I speak of "wind."

Because of the large mean free path length and mostly single independent
particle-like nature of electron conduction, the image of tar as an analogous
fluid may be somewhat misleading. But, because the responding fluid very
quickly reaches its terminal velocity due to interactions (i.e. collisions)
with defects, impurities and phonons, the picture of any friction-dominated
fluid such as cold tar flowing through a sponge *does* have something
going for it.

Yes, the "tar" concept goes better with an image of empty pipes, where the
fluid itself (and adhesion to pipe walls) must provide the resistance. If
wires are like hollow pipes, then we can imagine the electric fluid as
being like tar. Also, since the electron-stuff has so little mass, we
would imagine it as being like "weightless tar". And since the
electon-stuff gives the silvery metallic coloring to metals, an electric
current is like a flow of weightless silver tar. ;)

My picture is one of air flowing through a porous open-spongy
or fluffy insulating material such as attic insulation. In this case the
typical speeds of the molecules in the air are orders of magnitude faster
than the wind speed of the light breeze flowing through the fluffy medium.
This picture also has its faults, as well. Because electrons are *much* less
massive than the molecules of the gases found in air, their speeds would be
much faster, at a given temperature, than the speeds of the air molecules--
even if they (the electrons) were not, additionally, effectively sped up much
further (and reduced in concentration) by the indirect effects of the Fermi-
Dirac statistics.

Or if my "electron flow" terminology is replaced by "electric current",
then we are free to imagine a water electrolyte or a fused salt as the
conductor, where the charged particles are just as massive as those in a
physical fluid. If electrons start causing trouble, just remove them.

The air-thru-fiberglas concept, if the fiberglass was packed densely,
could also incorporate the idea of the high "friction" of wires. But
"air" implies low density and compressibility, while in wires, the density
of carriers is equal or greater than the density of atoms, no? The
"charge" indeed is more like an electric fluid than an electric gas. Oh.
Right. Triple point material. That's good!

If the "friction" in a wire is so high that a flow rate on the order of
1cm/sec causes immediate incandescence, then the fiberglas in the above
must be packed as densely as balsa wood. If we could apply enough
pressure across balsa wood that air would flow through it at cm/sec rate,
I bet it would incandesce like a lightbulb filament!

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