Question

The north poles of pole strengths, $m$, $9m$ are kept apart by certain distance '$d$'. Find distance of null point from larger pole.

• A. $2d/4$
• B. $2d/3$
• C. $5d/4$
• D. $3d/2$

Answer 1

The correct option is B $2d/3$

Let $B_1$ due to large pole and $B_2$ due to smaller pole

$B_1=\frac{{\mu }_0}{4\pi }\frac{2\times 9m}{x^3}$

$B_2=\frac{{\mu }_0}{4\pi }\frac{2\times m}{{(d-x)}^3}$

if P is null point then,

$B_1=B_2$

$\frac{9}{x^3}=\frac{1}{{(d-x)}^3}$

${\left(\frac{x}{d-x}\right)}^3=9$

$\frac{x}{d-x}=2.08x=\frac{2.08}{3.08}d$