Drift Velocity Versus Signal Speed

A wire is packed with free electrons already, roughly 10²⁸ to 10²⁹ per cubic meter in copper. When you close a switch, you are not waiting for an electron to travel from the switch to the bulb. You are setting up an electric field along the wire, and that field spreads through the conductor at a speed close to the speed of light. Every electron in the wire feels the field at nearly the same moment, so the whole sea of electrons starts drifting together almost instantly, even though each individual electron creeps along slowly.

The field pushes every electron in the wire at once because the field fills the wire's interior continuously, not point by point like a chain reaction. So the signal speed, how fast the bulb responds, has almost nothing to do with the drift speed, how fast any single electron physically moves. Drift speed stays small because there are so many charge carriers available: the current can be large even though each carrier only inches forward, since current depends on the number of carriers times their speed times the wire's cross section.

This distinction sets up the rest of circuit analysis. When you later calculate current, resistance, or power, you are describing the collective, slow drift of charge, while the response time of a circuit, like a light turning on, depends on the near-instant field. Keeping these two speeds separate also matters for understanding why charge takes time to build up on a capacitor even though the circuit itself responds quickly.