Question

Two magnetic poles of strengths $40Am$ and $10Am$ are separated by a distance of $20cm$ in air. The force between them if the distance is reduced to $10cm$ is

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

Answer 1

The correct option is B ${10}^{-3}N,4\times {10}^{-3}N$

$m_1=40Am,m_2=10Am,d_1=20cm$
$d_2=10cm$
$F=\frac{\mu m_1{m}_2}{4\pi d^2}$
$F_1=\frac{{10}^{-7}\times 40\times 10}{(20\times {10}^{-2}{)}^2}$
=${10}^{-3}$
Force is inversely proportional to $d^2$
$F_2=4\times {10}^{-3}$