A copper wire of length `1 m` and radius is joined in series with an iron wire of length `2 m` and radius `3 mm` and a current is passed through the wire. The ratio of the current density in the copper and iron wires is

Options

A. `3//1`
B. `1//3`
C. `9//1`
D. `1//9`

Answer

C

Explanation

In series circuit, current is same
`J=(I)/(A) prop (1)/(r^(2))`
`:.J_(1)r_(1)^(2)= J_(2)r_(2)^(2) :.(J_(1))/(J_(2))=(9)/(1)`