The resistivity os copper is `1.76 xx 10^-8` ohm m. The radius of the wire is `1 mm`. Calculate the length of a telegraph wire needed for having a resistance of `10.5 Omega`.

Answer

`1.873 km`

Explanation

As `R = (rho l)/(A), l = (RA)/(rho) = (R(pi r^2))/(rho) = (10.5 Omega xx 3.14 xx (10^-3 m)^2)/(1.76 xx 10^-8 Omega m)=1.873 xx 10^3m = 1.873 km`.