Equal potentials are applied on an iron and copper wire of same length. In order to have the same current flow in the two wires, the ratio r (iron)/r(copper) of their radii must be (Given that specific resistance of iron `=1.0 xx 10^(-7) ohm-m` and specific resistance of copper `=1.7 xx 10^(-8) ohm-m`)

Options

A. About 1.2
B. About 2.4
C. About 3.6
D. About 4.8

Answer

B

Explanation

`rho_("iron") = 1 xx 10^(-7) Omega m, rho_(cu) = 1.7 xx 10^(-8) Omega m`
`R = (rhol)/A rArr R prop rho/A`
`rArr 1 = (1 xx 10^(-7))/(1.7 xx 10^(-8)) xx r_("Cu")^(2)/r_("Fe")^(2)`
` 10/1.7 rArr r_(Fe)/r_(Cu) = 2.42 = 2.4`