How an Electric Field Travels at 300,000 km/s with Almost Stationary Electrons

Propagation of the electric field in a conductor — Illustration of the almost instantaneous propagation of an electric field in a copper wire, despite the slow drift velocity of electrons represented by dominoes knocking each other over.
Image source: astronoo.com (new window)

Scientific Summary

This article explains the difference between the drift velocity of electrons and the propagation speed of the electric field in a conductor. The drift velocity of electrons is very low (approximately 0.07 mm/s) because they are constantly slowed by collisions with the crystal lattice. In contrast, the electric field (or electrical signal) propagates as a collective wave at nearly 300,000 km/s (the speed of light). The domino analogy illustrates this phenomenon: the wave travels very fast while each domino (electron) moves only a little. Electricity therefore does not transport electrons from one end of the wire to the other; it is the field disturbances that are transmitted almost instantaneously.

How can an electric field travel at 300,000 km/s while electrons move so slowly?

In a conductor, the electron drift velocity is very low (about 0.07 mm/s) because they are constantly slowed down by collisions. In contrast, the electric field (or electrical signal) propagates as a collective wave at nearly 300,000 km/s (the speed of light). The domino analogy illustrates this: the wave moves very fast while each domino (electron) moves only a very short distance. Electricity does not transport electrons from one end of the wire to the other; it is the field disturbances that are transmitted almost instantaneously.

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Propagation of the Collective Wave of the Field vs. the Slowness of Electrons

The Drift Velocity: Electrons in Slow Motion

In a conductor like copper, the free electrons of the metal move very slowly under the effect of an electric field, as they are constantly slowed down by Brownian collisions with the ions of the crystal lattice and the impurities of the metal. Their average drift velocity is about 0.07 mm/s, comparable to that of a snail. Yet, when an electric field is applied to the end of the wire, the electromagnetic disturbance propagates almost at the speed of light, about 300,000 km/s.

The Electric Signal Does Not Transport Electrons

Electricity does not consist of making electrons travel from one end of the wire to the other. When an electric field is applied, all the electrons in the conductor react at the speed of light, transmitting the disturbance instantaneously on a macroscopic scale.

The Domino Analogy: A Fast Wave, Slow Movements

One can imagine the electrons as reacting locally and almost instantaneously to the applied field, somewhat like a row of dominoes where the wave propagates very quickly, while each domino only moves a very short distance. In this way, the collective wave travels through the conductor almost at the speed of light from one end to the other, while the electrons themselves drift extremely slowly, at a speed comparable to that of a snail, traveling only a few millimeters per second.

Comparison of Speeds in a Conductor

Propagation speed of electrical phenomena in a conductor
Phenomenon	Typical value	Comment
Drift velocity of electrons	~ 0.07 mm/s	Extremely slow, due to frequent and disordered collisions in the crystal lattice
Propagation of the electric field	~ 3 × 10⁸ m/s	Almost instantaneous, like an electromagnetic wave that propagates rapidly through the wire, long before the electrons themselves move
Average thermal velocity of electrons	~ 10⁵ m/s	Brownian motion, does not contribute to the net current

FAQ: Everything about the speed of the electric field versus that of electrons

What is the difference between electron drift velocity and electric field propagation speed?

Electron drift velocity is the average speed at which electrons move through the conductor under the influence of the field. It is extremely slow (about 0.07 mm/s) due to collisions with crystal lattice ions. Electric field propagation is the speed at which the electromagnetic disturbance travels from one end of the wire to the other: about 300,000 km/s (speed of light).

Why do electrons move so slowly in a conductor?

Free electrons in a metal are constantly slowed down by Brownian collisions with crystal lattice ions, impurities, and material defects. These incessant collisions prevent them from accelerating linearly under the electric field. Their overall motion is therefore a slow drift, comparable to a snail's pace, even though their random thermal speed is very high (about 10⁵ m/s).

What is the domino analogy to understand this phenomenon?

The domino analogy is simple: line up dominoes. If you push the first one, the falling wave propagates very quickly along the row, but each domino only falls over a very short distance. Similarly, in an electrical wire, the electric field (the wave) propagates almost instantaneously (at 300,000 km/s) while each electron only moves very slowly (0.07 mm/s) over a short distance before transmitting the disturbance to its neighbor.