Question

A bar magnet is $0.10m$ long and its pole strength is $120Am$. Find magnitude of magnetic field at a point on the its axis at a distance $20cm$ from it.

Answer 1

Magnetic induction on axial line,$B=\frac{{\mu }_0}{4\pi }.\frac{2Md}{{\left(d^2-l^2\right)}^2}$

length of magnet$=2l=0.01m=10cm$

or,$l=5cm$

$M=2ml=2\times 120\times 0.05$

$d=20cm$

$\begin{array}{c}\therefore B=\frac{{\mu }_{\circ }}{4\pi }\times \frac{2\times 2\times 120\times 0.05\times 0.2}{{\left({0.2}^2-{0.05}^2\right)}^2}\\ =341.33\times 1o^{-6}T\\ =3.4\times {10}^{-4}T\end{array}$