Question

The force between two magnetic poles is $F$ when they are separated by certain distance. If the distance between the two poles is doubled and strength of each pole is tripled. The percentage change in the force between the two poles is :

• A. $125\%$
• B. $100\%$
• C. $50\%$
• D. $25\%$

Answer 1

The correct option is A $125\%$

$F_1=\frac{\mu M_1{M}_2}{4\pi d^2}$

$F_2=\frac{\mu 3m_1\times 3m_2}{4\pi (2d{)}^2}$

$\%$ change $=\frac{F_2-F_1}{F_1}\times 100$$=\frac{(\frac{9}{4}-1)}{1}\times 100=125\%$