drift velocity
/ DRIFT vuh-LOSS-ih-tee /
Watch a leaf on a babbling stream. It darts this way and that as eddies push it around, yet over time it still creeps downstream. The electrons in a wire are like a swarm of such leaves: they zip around wildly at huge speed in all directions, but an applied voltage gives the whole crowd a faint, steady lean in one direction. That gentle net creep is the drift velocity.
Precisely, drift velocity is the average speed at which charge carriers move along a wire because an electric field is pushing them. Without a field, electrons fly about chaotically and their motions cancel, so no current flows. Switch on a voltage and a tiny bias is added to all their wandering: between collisions each electron accelerates a little along the field, then a bump randomizes it, then it accelerates again. The accumulated lean is the drift velocity, and it is astonishingly slow — far slower than the electrons' own frantic thermal speed.
Drift velocity matters because it is what actually carries electric current: current equals how many carriers there are, times their charge, times this drift speed. The famous surprise is how sluggish it is — in a typical household wire, electrons drift at well under a millimetre per second, slower than a strolling ant. The common confusion is to think this means electricity itself is slow; in fact the electric signal, the push that makes all electrons start drifting at once, travels near the speed of light, which is why a lamp lights the instant you flip the switch.
In a copper wire carrying the current of a household lamp, electrons drift forward at roughly a tenth of a millimetre per second — it would take an electron over an hour to crawl the length of a single power cord. Yet the lamp glows the instant the switch closes, because the signal, not the electrons, races ahead.
Electrons drift slower than an ant walks, yet the light comes on instantly — the signal outruns the carriers.
Drift velocity is the slow, directed crawl caused by the field; the Fermi velocity is the huge, random speed the conduction electrons already have. Current rides on the tiny directed part, not on the fast chaotic part.