Question

The $10cm$ long magnet has pole strength of $12Am$. The magnetic field at a distance $20cm$ from its centre :

• A. $2.4\times {10}^{-5}T$
• B. $1.28\times {10}^{-5}T$
• C. $3.8\times {10}^{-5}T$
• D. $3.0\times {10}^{-5}T$

Answer 1

Answer: (B)

$1.28\times {10}^{-5}T$

Length of the magnet, $2l=0.1m$

Distance from the center, $d=0.2m$

Pole strength, $M=12Am$

$B=\frac{2{\mu }_{0}Md}{4\pi {(d^2-l^2)}^2}=\frac{2\times 4\pi \times {10}^{-7}\times \left(0.2\right)\times 12}{4\pi {({0.2}^2-{0.05}^2)}^2}$
$B=1.28\times {10}^{-5}T$