Question

A bar magnet of length $3\, cm$ has points $A$ and $B$ along its axis at distances of $24 \,cm$ and $48\, cm$ on the opposite sides. Ratio of magnetic fields at these points will be

• A. 8
• B. $\frac{1}{2\sqrt{2}}$
• C. 3
• D. 4

Answer 1

Answer: (A)

Magnetic field due to a bar magnet at a distance r from the centre of magnet on axial position
$=\frac{\mu_0}{4\pi} \cdot \frac{2M}{r^3}$
$\therefore \frac{B_1}{B_2}=\Bigg(\frac{r_2}{r_1}\Bigg)^3=\Bigg(\frac{48}{24}\Bigg)^3$
$ =\frac{8}{1}=8$