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) 1r
(b) 1r2
(c) 1r3
(d) none of these

Answer 1

Option (d) is correct.

Explanation:

Step 1:

Magnetic field B is given by a magnet bar of the magnetic moment M at gap r from the point on the magnet's axis from its centre.

$B=\frac{\mu_{0}}{4 \pi} \frac{2 M r}{\left(r^{2}-l^{2}\right)^{2}}$

Step 2:

Here, the magnet length is 2l.

So, You can clearly see it in the above formula

B proportional to the $\frac{r}{\left(r^{2}-l^{2}\right)^{2}}$

Step 3:

So that  

$B \propto \frac{r}{\left(r^{2}-l^{2}\right)^{2}}$