Two magnetic poles one of which is 4 times as strong as the other exert a force of 64 mg.wt. When placed 10 cm apart in air. Strength of the poles are (g = 10ms^–2)

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3A–m and 12Am

4A–m and 16Am

5Am and 20Am

6Am and 10 Am

answer is B.

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Detailed Solution

$m_{1} = m, m_{2} = 4 m; 1 mg = 10^{- 6} kg F = \frac{μ_{0}}{4 π} \frac{m .4 m}{r^{2}} = 64 \times 10^{- 6} \times 10 N \Rightarrow \frac{10^{- 7} \times 4 m^{2}}{100 \times 10^{- 4}} = 64 \times 10^{- 5} \Rightarrow m^{2} = 16, \Rightarrow m = 4$

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