Q. Two short magnets with pole strengths of 900 ab amp-cm and 100 ab-amp-cm are placed with their axes in the same vertical line, with similar poles facing each other. Each magnet has a length of 1 cm. When separation between the nearer poles is 1 cm, the weight of upper magnet is supported by the repulsive force between the magnets. If g$= 1000cms^{- 2}$, then the mass of upper magnet is
A) 100 g
B) 55 g
C) 45 g
D) 77.5 g
Explanation
(b) In CGS system, $\frac{\mu_{0}}{4\pi} = 1$
In equilibrium, net repulsion due to magnetic interaction = weight of upper magnet. Therefore, as is clear from figure.
[image:5_image60.emf]
$\frac{900(100)}{1^{2}} + \frac{900( - 100)}{2^{2}} - \frac{900(100)}{2^{2}} - \frac{900( - 100)}{3^{2}} = m \times g$
$900 \times 100\left\lbrack \frac{1}{1^{2}} - \frac{1}{2^{2}} - \frac{1}{2^{2}} + \frac{1}{3^{2}} \right\rbrack = m \times 1000$
$90\left( \frac{11}{18} \right) = m$
$ie,m = 55$ g
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.