Worked example: A copper wire of cross-section A = 1 mm² carries I = 2 A. Copper has n = 8.5 × 10²⁸ m⁻³ free electrons. (a) Find the drift velocity. (b) How long does an electron take to travel 1 m? Step 1: I = n·e·A·vd, so vd = I/(neA) = 2/[(8.5 × 10²⁸)(1.6 × 10⁻¹⁹)(10⁻⁶)] = 2/(1.36 × 10⁴) ≈ 1.47 × 10⁻⁴ m/s ≈ 0.15 mm/s. Step 2: Time for 1 m = 1/vd ≈ 6800 s ≈ 1.9 hours! An electron takes nearly two hours to cross one metre of wire, yet your bulb lights instantly when you flip the switch — because the field establishes itself along the whole wire at nearly light speed, and every electron starts drifting simultaneously. Step 3 (mobility check): with E = ρJ and ρ_Cu = 1.7 × 10⁻⁸ Ω·m, J = 2 × 10⁶ A/m², E = 0.034 V/m, μ = vd/E ≈ 4.3 × 10⁻³ m²/(V·s). Consistent.