Current Electricity · Drift Velocity

Trace the current through a metal wire and compute the electron drift speed

A 100 m metal wire is connected across a 2 V battery with internal resistance 1 Ω. Use conductivity to find the wire resistance, atomic data to find the free-electron density, and then connect current density to drift velocity.

2026 Paper 2 Q1 · drift velocity lab
Electrons drift opposite conventional current · motion is magnified for visibility
Calculation stage
Electron motion scale 1.0×
Wire resistance
The wire resistance is R = L/(σA). With L = 100 m, σ = 2×10⁸ mho m⁻¹ and A = 0.5 mm² = 5×10⁻⁷ m², the wire has resistance 1 Ω.
ExpressionR = L/(σA)
ValueR = 1 Ω
Conventional currentfrom battery positive terminal through the wire
Electron driftopposite current, shown magnified
Final result0.208 mm s⁻¹, option (C)

Calculation sequence

01 / RESISTANCE

The long wire contributes 1 Ω

Conductivity gives the resistance of the metal wire directly.

Rwire = L/(σA) = 1 Ω
02 / CURRENT

Total resistance is 2 Ω

The battery has internal resistance 1 Ω, so the circuit current is 2 V divided by 2 Ω.

I = 2 / (1+1) = 1 A
03 / ELECTRON DENSITY

One conduction electron per atom

The metal has 10⁵ moles per cubic metre, giving 6×10²⁸ free electrons per cubic metre.

n = 6×10²⁸ m⁻³
Drift velocity
vd = I/(neA) = 1 / [(6×10²⁸)(1.6×10⁻¹⁹)(5×10⁻⁷)]
vd = 0.208 mm s⁻¹ · Correct option (C)