Question

Let r be the distance of a point on the axis of a magnetic dipole 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

`1/r^3`

Magnetic field B due to a bar magnet of magnetic moment M at distance r of the point on the axis from its centre is given by `B = (u_0 2Mr)/(4pi(r^2 - l^2)^2`

Here, 2l  is the length of the magnet.
When the distance of the point where the magnetic field has to be calculated is greater than the length of the magnet, i.e r >> l , the bar magnet acts like a magnetic dipole whose magnetic field is `B ∝ 1/r^3`

Now, l in the denominator can be neglected.
So, the correct option is (c).