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At some location on earth the horizontal component of earth’s magnetic field is 18 × 10 –6 T. At this location, magnetic needle of length 0.12 m and pole strength 1.8 Am is suspended from its mid-point using a thread, it makes 45° angle with horizontal in equilibrium. To keep this needle horizontal, the vertical force that should be applied at one of its ends is -

1. 3.6 × 10 –5 N
2. 1.8 × 10 –5 N
3. 1.3 × 10 –5 N
4. 6.5 × 10 –5 N

Correct answer: 6.5 × 10 –5 N

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