At 20^oC the resistance of a copper wire of length 30m and diameter o.1 cm is $0.65\Omega$.The resistivity of copper wire at 20^oC is
Answer & explanation
Correct answer: option 2
$\rho = \frac{RA}{L} $
$ A = \frac{\pi d^2}{4} = \frac{\pi 10^{-6}}{4} = 0.78\times 10^{-6}$
$ \rho = \frac{0.65 \times 0.78\times 10^{-6}}{30} = 1.702\times 10^{-8} $