A magnetised needle in a uniform magnetic field feels a torque but no net force. An iron nail near a bar magnet, however, feels a force of attraction as well as a torque. This is because
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the nail is in a non-uniform field, and its induced unlike pole is nearer to the pole of the magnet
In a uniform field, the forces on the two poles of a needle are equal and opposite, so only a torque acts. The field of a bar magnet changes from point to point. It also makes the iron nail a magnet by induction.
Option (1) the field near a bar magnet is uniform, so it pulls the nail as a whole — Incorrect. The field of a bar magnet is not uniform. It is strongest at the two ends and weakest near the middle.
Option (2) the nail is in a non-uniform field, and its induced unlike pole is nearer to the pole of the magnet — Correct. The nail gets an induced magnetic moment. Its unlike pole lies closer to the pole of the magnet, where the field is stronger. So the attraction is larger than the repulsion, and a net force acts.
Option (3) the needle in the uniform field also feels an attractive force, but it is too small to notice — Incorrect. A magnet in a uniform field feels zero net force, however carefully it is measured. It feels only a torque.
Option (4) both induced poles of the nail are pulled towards the magnet — Incorrect. The far end of the nail becomes a like pole, so it is pushed away. There is a net pull only because the nearer unlike pole sits in a stronger field.
NCERT (2026-27):
Example 5.1 – “(b) No force if the field is uniform. The iron nail experiences a non-uniform field due to the bar magnet. There is induced magnetic moment in the nail, therefore, it experiences both force and torque. The net force is attractive because the induced south pole (say) in the nail is closer to the north pole of magnet than induced north pole.”