Question

Let $r$ be the distance of a point on the axis of a bar magnet from its centre. The magnetic field at such a point is proportional to :

• A. $\frac{1}{r}$
• B. $\frac{1}{r^2}$
• C. $\frac{1}{r^3}$
• D. none of these

Answer 1

The correct option is D none of these

$B$ $=$ $\left(\frac{{\mu }_0}{4\pi }\right)\frac{2M}{d^3}$

The formula is valid for $d\gg l$ where l is the length of the magnet and d is the distance from the center of the magnet.

If the above condition is not satisfied, B will not be proportional to any of ,

$\frac{1}{d^3}$ or $\frac{1}{d^2}$ or $\frac{1}{d}$