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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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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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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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