A compass needle which is allowed to move in a horizontal plane is taken to a geomagnetic pole. It (a) will become rigif showing no movement (b) will stay in any position (c) will stay in north-south direction only (d) will stay in east-west direction only

Answer: (b). At a magnetic pole the earth's field is vertical, so H = BEcos 90° = 0. A needle free only in the horizontal plane feels no horizontal torque, so it rests in any direction. So the answer is (b) it will stay in any position.

A compass needle which is allowed to move in a horizontal plane is… | NEET 2012 (AIPMT, Prelims)

Q15NEET 2012 · AIPMT · PRELIMSEasy

Earth's Magnetism

A compass needle which is allowed to move in a horizontal plane is taken to a geomagnetic pole. It

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Answer: (b)

At a magnetic pole the earth's field is vertical, so $H = B_E\cos 90^\circ = 0$ .
A needle free only in the horizontal plane feels no horizontal torque, so it rests in any direction.
So the answer is (b) it will stay in any position.

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