Drift Velocity Calculator
Enter conductivity σ, carrier density n and charge q to compute the drift velocity v = μE = σE/(nq) under an applied field E.
Input Data
Results
At a glance:In a conductor, free carriers are pushed by an electric field and acquire a small average drift speed v opposite to E. Mobility μ = σ/(nq) links them: v = μE = σE/(nq). Despite the tiny carrier speed, the electromagnetic signal travels near light speed, so a lamp lights instantly although electrons barely move. This resolves the common confusion between signal speed and carrier drift speed.
Formula
Drift velocity: v = σE/(nq).
Mobility: μ = σ/(nq).
Current density: J = nqv = σE.
$$v = \frac{\sigma E}{nq}$$$$\mu = \frac{\sigma}{nq}$$$$J = nqv = \sigma E$$How to Use
- Enter conductivity σ, carrier density n, charge q and field E.
- The tool returns drift velocity v and mobility μ.
Case Studies
Copper wire at 0.01 V/m
σ=5.8×10⁷, n=8.5×10²⁸, q=1.602×10⁻¹⁹, E=0.01 V/m.
v = 5.8e7×0.01/(8.5e28×1.602e-19) ≈ 4.3×10⁻⁴ m/s.
Carriers drift ≈ 0.43 mm/s while the signal moves near light speed.
FAQ
Why do lights turn on instantly if drift is so slow?
The electric field propagates at near light speed, pushing carriers everywhere almost at once; the drift speed is just the average carrier creep, not the signal speed.
What is mobility?
Mobility μ = σ/(nq) is how fast carriers drift per unit field; higher mobility means faster drift for the same field.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.