The length of copper wire is 120 m, and its radius is 1 mm. Calculate its resistance if the resistivity of copper is 1.72 × 10⁻⁸ Ω m.

Length of copper wire (l) = 120 m.

Area of cross-section (a) = πr² = 3.14 × (10^-3m) ² = 3.14 × 10^-6m²

Resistivity (ρ) of copper = 1.72 × 10⁻⁸ Ω m

The resistance offered by a conductor is given by

$$\begin{gathered} \mathrm{R}=\mathrm{\rho }\frac{\mathrm{\ell }}{\mathrm{A}} \\ =\frac{1.72\times {10}^{-8}\times 120}{3.14\times {10}^{-6}}=0.66\mathrm{\Omega } \end{gathered}$$