Question

Two bar magnets A and B are placed on the two arms of a deflection magnetometer. When their distances from the centre of the needle are 20 cm and 40 cm respectively, the needle lies in the magnetic meridian. If the moment of the magnet A is 100 Am${}^2$, then the moment of the magnet B is:

• A. 400 Am${}^2$
• B. 800 Am${}^2$
• C. 1200 Am${}^2$
• D. 1600 Am${}^2$

Answer 1

The correct option is B 800 Am${}^2$

The deflection off the magnetic needle in tan A position by a short magnet is given by

$\frac{{\mu }_o}{4\pi }\frac{2M_A}{d^3}=B_{H}tan{\theta }_A$

Since the deflection is zero ,

$\frac{M_A}{M_B}=\frac{d_A^3}{d_B^3}$

$\frac{M_A}{M_B}=\frac{1}{8}$

$M_B=800A{m}^2$