Question

A short bar magnet has a length $2l$ and a magnetic moment $10A{m}^2$ . Find the magnetic field at a distance of $z=0.1m$ from its centre on the axial line. Here, $l$ is negligible as compared to $z$ .

• A. $4\times 10^{-3}T$
• B. $1\times 10^{-3}T$
• C. $3\times 10^{-3}T$
• D. $2\times 10^{-3}T$

Answer 1

Answer: (D)

If the point lies on the axial line of bar magnet
$B=\frac{\mu .2Md}{4\pi {\left(d^2-l^2\right)}^2}$
If $l<<d$ (as given in the question)
$B=\frac{{\mu }_0}{4\pi }\times \frac{2M}{d^3}$
$=\frac{{\mu }_0}{4\pi }\times 20\times 10^3$
$\left(\frac{{\left(\mu \right)}_0}{4\pi }={\left(10\right)}^{-7}\right)$
$=2\times 10^{-3}T$