Practice question

**Potentiometer** measures potential difference without drawing current, using null deflection, principle V ∝ l, l balance length, accurate because no I r drop. Potential drop across resistor V = I R arises because electric field does work on charges, energy converted to heat, maintaining E = -dV/dx along wire. Use: rho_t = rho₀ [1 + α (T - T₀)] . Substitute: rho_t = 8 × 10⁻⁸ [1 + 4 × 10⁻³ (70 - 20)] . Calculate: rho_t = 8 × 10⁻⁸ [1 + 0.2] = 8 × 10⁻⁸ × 1.2 = 9.6 × 10⁻⁸ Ω m . Applying I = n e A v_d, R