Q. A long solenoid has a radius $a$ and number of turns per unit length $n$. If it carries a current $i$, then the magnetic field on its axis is directly proportional to
A) $ani$
B) $ni$
C) $\frac{ni}{a}$
D) $n^{2}i$
Explanation
(b) Because $B = \mu_{0}ni\ \Rightarrow B \propto ni$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.