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When a steady current flows through a conductor, the electrons in it move with a certain average ‘drift speed’. One can calculate this drift speed of electrons for a typical copper wire carrying a small current, and it is found to be actually very small, of the order of 1 mm s-1. How is it then that an electric bulb lights up as soon as we turn the switch on? It cannot be that a current starts only when an electron from one terminal of the electric supply physically reaches the other terminal through the bulb, because the physical drift of electrons in the conducting wires is a very slow process.

What is the exact mechanism of the current flow, which takes place with a speed close to the speed of light ?

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I read the answer to this question 10 years ago. Imagine that you are in a car (the electron) on a road with heavy car traffic. A stop is on its red color, and the cars wait for the green light.

When stop light becomes green, if every driver has instantaneous reactions, you will start driving the moment when you have the green light.

Thus when the electric field is applied, every electron (having almost an instantaneous reaction) start to flow. The Electromagnetic field (the applied voltage or the green light) propagates with the speed of light, the electrons (the cars in the traffic) start to flow a little slower because now and then there are electron collisions with impurities.

The electricity propagates with the speed of light, the electric current propagates with the electron drift velocity.

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When you turn switch on, electromagnetic field from your power plant reaches your bulb hot cathode through wires after $$\Delta t = {L_{wires}}\,/\,c$$ duration, from which electrons in tungsten filament reacts to field and starts drifting, overcoming resistance in cathode, which results in cathode thermal heating, which in turn starts thermionic electron emission. If your power plant say is at $1000 \,\text{km}$ distance from you then EM waves will reach you after $\approx 3.3 \,\text{ms}$ time interval which you will not notice for sure. So resume,- electrons drifts slow, but EM field which forces them to move, reaches bulb electrons fast.

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Often when you want to understand flowing electricity, you can use the metaphor of flowing water. It doesn't always work. They aren't the same and sometimes it can mislead you. But it often works.

Imagine you have a network of pipes full of water that connect to each other, but there are only two places where water can go in or out. You pump in some colored water at one of the openings.

How long does it take the colored water to diffuse to the exit? A long time.

How long does it take the pressure to go up enough to push some other water out the exit? Not long at all.

It isn't instant. It might not happen at the speed of sound in water. The pressure might spread in waves, that can reflect, diffract, etc. All very fast in human terms, in both cases.

The "water hammer" effect tends not to happen much because electrons in conductors are more compressible than water in pipes.

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I'm putting together some animations of how electricity works. Here are the first few- https://www.youtube.com/channel/UCDIr0SMCdoRHNgUhSB_xOfQ Reflections at a short and at an open.

The delivered power is entirely measured and accounted for by the fields outside of the wires, which move at the speed of light for the dielectric medium.

The next series shows how energy propagates in a waveguide, and where group and phase velocities come from.

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Because, simply put, if you “push” an electron into one end of the wire then another electron comes out the other.

So, although electrons themselves don’t move at the speed of light, the light comes on “instantly”...

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The electrons move because of an external electric field.

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  • $\begingroup$ Your very short answer implies that if no external electric field then no electron movement. $\endgroup$ – pasaba por aqui 6 hours ago

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