What Electrical Circuits Look Like to Me

Eudaimonium said:

Technically, speed of individual electrons is about 1 cm per second IIRC, everything happens very fast because every electron "pushes" one next to him, like a PCB tube full of marbles - you push the end marle and first marble falls out instanty, and speed of individual marbles is very small actually.

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Gryphon said:

Seriously!? Thats sweet! I knew the whole they push the one next to them so theres basically no delay in
electrical transfer, but i didn't know that the speed at which electrons are displaced was that slow.

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How fast do electrons drift?

We can get an idea of how fast the drift velocity is by taking some typical values of current and wire dimensions.

Let's think of a current of 5 A that is flowing in a copper wire with a cross section of 0.5 mm2 (= 0.5 * 10-6 m2)

For copper, n = 8.5 × 1028 per m3
The charge on an electron, Q = 1.6 × 10-19 C

So,
I = n A v Q

5 = 8.5 × 1028 × 0.5 × 10-6 × v × 1.6 × 10-19

5 = 27 200 v

v = 7.35 × 10-4 m s-1

So, for this current, the drift velocity of electrons is about a tenth of a millimetre per second: pretty slow!

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pullbangdead said:

Individuals electrons are moving very slowly, here's an example calculation of the drift velocity:



The analogy of marbles pushing through isn't a complete analogy though.

The real explanation is that electrons are being accelerated by the applied field, and they accelerate until they hit something, ie scatter, which slows them back down again. So you get electrons accelerating, scattering, accelerating again, scattering again, giving you an average drift velocity. The field is moving through the conductor fast, at almost the speed of light-ish, but the drift velocity of the electrons is much smaller through the material due to scattering. That's why conductivity of metals decreases as temperature increases: higher temperature gives more thermal vibrations of atoms in the material, which increases the frequency of scattering events, which means the average drift velocity of the electrons goes down.

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