A bar magnet of magnetic moment 2A-m² free to rotate about a vertical axis passing through its centre. The magnet is released from rest from east-west position. Then the kinetic energy of the magnet as it takes North-South position is (horizontal component of the earth's field is 25µT and angle of declination is zero :-
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Question

A bar magnet of magnetic moment 2A-m² free to rotate about a vertical axis passing through its centre. The magnet is released from rest from east-west position. Then the kinetic energy of the magnet as it takes North-South position is (horizontal component of the earth's field is 25µT and angle of declination is zero :-

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G1 Physics · Accepted Answer

Magnetic potential energy is U=−MBₕ cosθ. The magnet goes from θ=90° to 0°, so decrease in U is MBₕ=2×25×10⁻⁶=50 μJ, which appears as kinetic energy. Hence B.