A straight wire of length 30cm and mass 60 mg lies in a direction 30° east of north. The earth’s magnetic field at this site is horizontal and has a magnitude of 0.8 G. The current must be passed through the wire so that it may float in air is (g = 10 m/s²)

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25A

75A

50A

answer is C.

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Detailed Solution

$\begin{array}{l} F = Bilsinθ; mg = B_{H} i \times l \times \sin θ \\ 60 \times 10^{- 6} \times 10 = 0 .8 \times 10^{- 4} \times i \times 30 \times 10^{- 2} \times \frac{1}{2} \end{array}$
i = 50A

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