[Amps] "Conventional" current flow

[Manfred Mornhinweg]

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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