NEET-XII-Physics
36: Permanent Magnets
- #5Let 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
(a)
1r
(b)
1r2
(c)
1r3
(d) none of thesedigAnsr: cAns : (c) `` \frac{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=\frac{{\mu }_{\mathit{0}}2Mr}{4\,\mathrm{\,\pi \,}{\left({r}^{2}-{l}^{2}\right)}^{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`` \propto \frac{1}{{r}^{3}}``
Now, l in the denominator can be neglected.
So, the correct option is
(c).
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