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

Options

• `1/r`

• `1/r^2`

• `1/r^3`

• none of these 

Answer 1

None of these

Magnetic field B due to a bar magnet of magnetic moment M at distance r of the point on the axis of the magnet from its centre is given by

`B = (u_0)/(4pi) (2Mr)/(r^2 - l^2)^2`

Here, 2l  is the length of the magnet.

So, from the above formula, it can be easily seen that `B ∝ r/(r^2 - l^2)^2`