An aluminium (resistivity ρ=2.2×10−8Ω−m ) wire of a diameter 1.4 mm is used to make a 4Ω  resistor. The length of the wire is

Resistivity ρ=2.2×10−8 Ω−m Diameter D=1.4×10−3 m Radius r=0.7×10−3 m Resistance R=4 Ω Length l=? Length of the wire is      l=R Aρ        =4×π×(0.7×10−3)22.2×10−8 ∴  l=280 m .