I can't like that model of the electrons moving slowly but the charges
moving fast. When it comes to simplistic models, I much prefer the one I
saw in the instruction manual of a didactic toy, when I was 8 or 9 years
old and learning electricity. The author of this manual modelled the
electrons in a wire as the cars of a train. As every open-eyed child in
those years had surely observed on more than one occasion, when the
locomotive starts pushing at the end of a standing train, the next thing
that happens is a fast clack-clack-clack, as each car strats moving,
takes out the slack in the joint to the next car, and then that next car
starts moving too. The frontmost car starts moving very briefly after
the locomotive starts pushing, which of course does not mean that the
last car moved fast to the position where the front one was!
So, the MOTION propagates fast along the railway track, while the
individual train cars move slowly. Just the same as in the wire. And the
charge, or load, is transported by the cars/electrons. Since the charge
of each is the same, it doesn't matter whether at the end of the wire we
get the same electrons we put in at the other end, or other electrons.
The same fast motion propagation happens, of course, when the locomotive
pulls instead of pushing, and this fact was used in the booklet to make
clear that electrical phenomena are independent of which way we prefer
to think about them.
About the actual average speed of electrons in a wire, when using a
simplistic/mechanical model, it's of course totally variable, and
depends on the current density in the wire. One can compute it by
calculating how many movable electrons there are in the metal, per
length of wire, and dividing the current, expressed in electrons per
second rather than in ampere, by this number.
And about more advanced models for physical phenomena, including
electricity, I don't think that our current knowledge of quantum
electrodynamics is the last word that will be spoken on the matter. It's
just one more step in the ladder towards a definitive, complete
understanding of physics - which humankind might never achieve.
For the time being, we should be best off by using the simplest model
that adequately explains and predicts results for each particular
situation we have at hand. And we should be aware that these are all
models, not the real thing. And that scientists are gradually getting
closer to the real thing, but are not yet there.
Manfred
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http://ludens.cl
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