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Drift velocity
Intermediate
Slow average speed of charge carriers.
I = nAev_dElectrons in a wire drift slowly: I = nAev_d. In copper v_d is about 0.1 mm/s even for amps of current, yet the signal travels near the speed of light because the field is established everywhere almost at once.
1 A in 1 mm² copper: v_d ≈ 0.074 mm/s.
The lamp lights instantly because charges are already present throughout the wire.
In more detail
Drift velocity is the average velocity of electrons moving through a conductor under an electric field. It is typically only about a millimetre per second, even though the signal propagates much faster.
Real-world uses
- Estimating electron density in metals from measured currents.
- Understanding why switches respond instantly despite slow electron motion.
- Engineers estimate that electrons in a copper wire drift at only a few millimetres per second, even though the signal travels near the speed of light.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Why does the light turn on instantly if electrons drift so slowly?
The electric field spreads through the wire near the speed of light, so all electrons start moving almost at once.
What is the formula?
I = nAvq, where n is the electron density, A the cross-section, v the drift velocity and q the charge.
If electrons move so slowly, why is the bulb instant?
The electric field that drives the current travels at close to the speed of light, so all the electrons start moving almost together.
Related topics in Current Electricity & Circuits
Electric current · Direction of current · Ohm's law · Resistance · Resistivity