Question

The adjacent figure shows the cross-section of a long rod with its length perpendicular to the plane of the paper. It carries a constant current flowing along its length. $B_{1}, B_{2}, B_{3}$ and $B_{4}$ respectively represent the magnetic fields due to the current in the rod at points $1,2,3$ and $4$ lying at different separations from the center $O$, as shown in the figure. Which of the following shall hold true?

• A. $B_{1}>B_{2} \neq 0$
• B. $B_{2}>B_{3} \neq 0$
• C. $B_{1}>B_{2}=B_{3}=0 $
• D. $B_{3}>B_{4} \neq 0$

Answer 1

Answer: (D)

Magnetic field outside the long rod
$\because B=\frac{\mu_{0} i}{2 \pi r}$
$\therefore B_{3}>B_{4} \neq 0$