Question

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

• A. 18 : 1
• B. 9 : 1
• C. 6 : 1
• D. 2 : 3

Answer 1

The correct option is B 9 : 1

We know $J=\frac{i}{A}$ when i = constant $J\infty \frac{1}{A}\Rightarrow \frac{J_c}{J_i}=\frac{A_i}{A_c}={\left(\frac{r_i}{r_c}\right)}^2={\left(\frac{3}{1}\right)}^2=\frac{9}{1}$