Q. Two short magnets with their axes horizontal and perpendicular to the magnetic meridian are placed with their centres $40\ cm$ east and $50\ cm$ west of magnetic needle. If the needle remains undeflected, the ratio of their magnetic moments $M_{1}:M_{2}$ is
A) $4\ :5$
B) $16\ :25$
C) $64\ :125$
D) $2\ :\sqrt{5}$
Explanation
(c) For null deflection $\frac{M_{1}}{M_{2}} = \left( \frac{d_{1}}{d_{2}} \right)^{3} = \left( \frac{40}{50} \right)^{3} = \frac{64}{125}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.