Q. Calculate the resistance of 1 km long copper wire of radius 1 mm.
(specific resistance of copper is 1.72 $\times$10^-8 $\Omega$ m )

Length=1 km=1000m Radius=1 mm=1*10⁻³ m Resistivity of copper=1.72x10⁻⁸ R=(Resistivity of copper * Length)/(Area)                   =(Resistivity of copper * Length)/(r²) R=[1.72*10⁻⁸*1000]/[22/7*(1*10⁻³)² R=5.5 ohm