Practice question

**Resistance** R = ρ l/A, ρ resistivity (Ω·m), l length (m), A area (m²), ρ = m/(n e² τ) from Drude model, τ average collision time. Ohm's law V = I R holds when ρ constant, independent of V. Volume constant stretching l→2l implies A→A/2, so R' = ρ·2l/(A/2)=4R, resistance quadruples when length doubles at constant volume. In a diode, resistance depends on the direction and magnitude of the applied voltage due to its semiconductor junction properties. Forward bias reduces resistance, while reverse bias increases it, leading to a non-linear I -versus- V curve. Applying I = n e A v_d, R = ρ l/A,