A copper wire of 1km has a radius of 1 mm. The resistivity of the copper wire is 1.72 $\times$10^-8 $\mathrm{\Omega }$m. Find the resistance of the wire.

Length of copper wire = 1km = 1000 m 

Radius = 1 mm or 10^-3 m and 

Resistivity $\mathrm{\rho }=1.72\times {10}^{-8} \Omega \mathrm{m}$, 
$\mathrm{R}=\mathrm{\rho }\frac{\mathrm{l}}{\mathrm{A}}$
For a wire, the cross-sectional area is the area of the circular cross-section. Hence, the relation can be written as
$\mathrm{R}=\mathrm{\rho }\frac{\mathrm{l}}{\mathrm{\pi r}²}$
On substituting the given values, we get the resistance as
$\Rightarrow \mathrm{R}=\left(1.72\times {10}^{-8}\right)\times \frac{1000}{3.14\times {\left(1\times {10}^{-3}\right)}^2}$
$\Rightarrow \mathrm{R}=5.47\mathrm{ \Omega }$
Hence, the resistance of the wire is $5.47\mathrm{ \Omega }$.