[Amps] Zener Screen Regulators

[Manfred Mornhinweg]

Hi Jim,

> Two minor points. The "flow" of current is a useful concept when visualizing
> circuits, but in AC circuits there actually is no flow of current along a
> wire. 

There is!

> The conduction electrons in a wire just slosh back an forth a fraction
> of a micron and never go anywhere. In DC circuits, there is a flow, although
> the distance traveled is very small and the speed of the charge flow is very
> very slow -- tiny fractions of a mm per second 

I don't see why we have to make any difference here between DC and AC. 
In either case, the actual overall motion of the electron "cloud" in the 
metal is extremely slow, and is of similar speed in both cases. Only 
that with AC it goes forth and back, moving only a tiny total amount, as 
you correctly say, while in DC it moves along, to any distance you want, 
as long as you give it enough time.  Still there is current flow, in the 
form of electrons actually moving, even in AC.

Very generally I dislike making any difference at all between AC and DC, 
for analysis purposes. Because if you look over a time window short 
enough (much shorter than one cycle), AC and DC is the same, and on the 
other hand any DC must have gotten started at some time, and will 
eventually stop, and this implies an AC component in it.

 > hardly the speed of light, which many people presume!

Yes, many people think that the electrons shoot at the speed of light 
through a wire... But that's only because they have never thought about 
it. As soon as they start thinking, they realize that this isn't really 
possible, given the amount of movable electrons in a metal, and how many 
electrons per second have to pass a spot to make a certain current.

I like to explain this matter by means of the old analogy with a train: 
There is always some little play between the train cars. When the 
locomotive starts going, the last car of the train starts going slightly 
later, maybe a few seconds, when all teh slack between teh cars has been 
taken up. So the impulse from the locomotive propagates along the train 
pretty fast, spanning the whole length of the train in just a very few 
seconds, even while the train as a whole, and each car of it separately, 
are traveling very, very slowly.

What propagates at the speed of light in a wire, or very close to it, is 
this impulse between electrons, not the electrons themselves. It's a 
simple mechanical model, that has its pitfalls too, but is very useful 
to explain most things that happen with electrical current.

Manfred

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