"Free electrons in a copper wire move to random directions with the speed of 1.3e6m/s even in the case of no electric current, which means it is not in electric field. This velocity is called "Fermi velocity" and it exists even under 0 absolute temperature. It is not heat energy and originated from indefinite theory of quantum mechanics. Since electric current is average flow of free electrons, in other word "drift velocity", electric current doesn't exist under this circumstance.
When voltage is put on both sides of a conductor, free electrons increase the speed in proportion to the electric field and by lattice oscillation, lattice defect, and collision with impurities, they will be scattered to different directions from the electric field and lose the speed to the direction of the electric field. Therefore it doesn't increase the speed infinitely and it will keep certain average velocity. That means collision functions as a kind of friction.
As for copper, the time interval between collisions is 5.26e-45 seconds and average drift velocity is,
4.62e-3 (m/s) / (v/m).
It means that when 1V voltage is put on both ends of 1m long copper wire, the velocity of free electrons to length direction is 4.62 mm/s. It seems amazingly slow but since electric charge of electrons is -1.6e-19c, 12.6A electric current flows in the 0.5mm copper wire with this speed. You see how large the number of free electrons is."
http://www.cartage.org.lb/en/t.../Mysteryofelectric.htm
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