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. Use: R_t = R₀ [1 + α (T - T₀)] . Substitute: R_t = 80 [1 + 1.7 × 10⁻⁴ (300 - 20)] . Calculate: R_t = 80 [1 + 1.7 × 10⁻⁴ × 280] = 80 [1 + 0.0476] = 80 × 1.0476 ≈ 83.81 Ω .