Question

what is the resistance of 60m long iron wire, diameter of cross section is 2 mm​

Answer 1

Resistance = 1 / cross sectional area.

Resistance = 1/ 0.002 m

Answer 2

Resistance(R)=P×(l/a)

where P=resistivity of a substance

           l=length

           a=cross sectional area

value of P for iron=9.71 × $10^{-8}$m

so now, cross sectional area(a)= π/4×d²

 where d= diameter of the cross section

               a=(3.14/4)×(0.002m)²

               a=0.785×0.000004

               a=0.00000314 m²

now R=9.71 × $10^{-8}$m×(60m/0.00000314m²)

          =9.71×0.191

          =1.855 ohm