Question

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

• A. 2:3
• B. 6:1
• C. 9:1
• D. 18:1

Answer 1

The correct option is C 9:1

$J=\frac{I}{\pi r^2};J\alpha \frac{1}{r^2}\frac{J_C}{J_I}={\left[\frac{r_I}{r_C}\right]}^2={\left[\frac{3}{1}\right]}^2{J}_C:J_I=9:1$