Q. A Copper wire of length 1 m 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 wires. The ratio of the current density in the copper and iron wires is
A) $2:3$
B) $6:1$
C) $9:1$
D) $18:1$
Explanation
(c) $\frac{J_{1}}{J_{2}} = \frac{I/\pi r_{1}^{2}}{I/\pi r_{2}^{2}} = \left( \frac{r_{2}}{r_{1}} \right)^{2} = \left( \frac{3}{1} \right)^{2} = \frac{9}{1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.