Table of Contents
What is Drift Velocity?
Drift velocity is the average velocity that charge carriers (typically electrons in metals) attain due to an applied electric field in a conductor. Despite the extremely high thermal velocities of electrons (about 10^6 m/s at room temperature), the net drift velocity is remarkably slow, typically on the order of fractions of a millimeter per second for typical currents in household wiring.
The reason electrical signals travel at nearly the speed of light despite slow drift velocity is that the electric field propagates through the conductor almost instantaneously. When you flip a light switch, you do not wait for electrons to physically travel from the switch to the bulb. Instead, the electric field causes all electrons along the circuit to begin drifting almost simultaneously, similar to how pushing one end of a long tube filled with marbles causes the marble at the other end to pop out immediately.
Formula
Where v_d is drift velocity, I is current, n is free electron density, e is electron charge (1.602 x 10^-19 C), A is cross-sectional area, and J is current density.
Typical Drift Velocities
| Conductor | n (m^-3) | 1A in 1mm² wire |
|---|---|---|
| Copper | 8.5 x 10^28 | 0.074 mm/s |
| Aluminum | 6.0 x 10^28 | 0.104 mm/s |
| Silver | 5.9 x 10^28 | 0.106 mm/s |
Speed of Electricity vs Drift Velocity
- Drift velocity in copper wire: ~0.1 mm/s (very slow)
- Signal propagation: ~2/3 speed of light (~200,000 km/s)
- Thermal electron velocity: ~1,000,000 m/s (random directions)
- The key insight: signals are carried by electromagnetic fields, not by individual electron transport
FAQ
Why is drift velocity so slow?
Metals contain an enormous number of free electrons (about 10^28 per cubic meter). Even a small drift velocity applied to this many charges produces a large current. Think of it like a wide, slowly-moving river: the flow rate can be large even though the water moves slowly because the cross-section is enormous.
How long would it take an electron to travel through a wire?
At typical drift velocities of about 0.1 mm/s, an electron would take about 10,000 seconds (nearly 3 hours) to travel 1 meter through copper wire carrying 1A. In a 10-meter circuit, it would take over a day. Yet the light turns on instantly because the electric field propagates at nearly the speed of light.
Does drift velocity depend on temperature?
Indirectly, yes. At higher temperatures, increased lattice vibrations cause more electron scattering, reducing mobility and thus reducing drift velocity for a given electric field. However, for a given current, drift velocity is determined by I/(neA) regardless of temperature, since current is fixed. The voltage required to maintain that current increases with temperature (higher resistance).