Question

A bar magnet of length $3\, cm$ has a point $A$ and $B$ along axis at a distance of $24\, cm$ and $48\, cm$ on the opposite ends. Ratio of magnetic fields at these points will be

• A. 8
• B. 3
• C. 4
• D. $1 / 2 \sqrt{2}$

Answer 1

Answer: (A)

Here, $2 l=3\, cm , d_{1}=24\, cm$ and $d_{2}=48\, cm$
As, the magnet is short, $\frac{B_{1}}{B_{2}}=\frac{d_{2}^{3}}{d_{1}^{3}}$
$=\left(\frac{48\, cm }{24\, cm }\right)^{3}=8$