An aluminium (resistivity $\rho =2.2\times {10}^{-8}\Omega -m$ ) wire of a diameter $1.4mm$ is used to make a $4\Omega$  resistor. The length of the wire is

Resistivity $\rho =2.2\times {10}^{-8}\Omega -m$

Diameter $D=1.4\times {10}^{-3}m$

Radius $r=0.7\times {10}^{-3}m$

Resistance $R=4\Omega$

Length $l=?$

Length of the wire is

      $l=\frac{RA}{\rho }$

       $=\frac{4\times \pi \times {\left(0.7\times {10}^{-3}\right)}^2}{2.2\times {10}^{-8}}$

$\therefore l=280m$ .