Q. Two short bar magnets with magnetic moments 400 ab-amp $cm^{2}\ $and 800 ab-amp $cm^{2}\ are$ placed with their axis in the same straight line with similar poles facing each other and with their centers at 20 cm from each other. Then the force of repulsion is
A) 12 dyne
B) 6 dyne
C) 800 dyne
D) 150 dyne
Explanation
(a) $F = \frac{\mu_{0}}{4\pi}.\frac{6M_{1}M_{2}}{r^{4}} = \frac{6M_{1}M_{2}}{r^{4}}$
$\therefore\left\lbrack \ In\ CGS\ system,\ \frac{\mu_{0}}{4\ \pi} = 1 \right\rbrack$
$= \frac{6 \times 800 \times 400}{20 \times 20 \times 20 \times 20} =$ 12 dyne.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.