Two iron bars A and B look alike. At least one of them is magnetised. The two bars never repel each other. One end of A is brought near one end of B, and A is attracted. The same end of A is then brought near the middle of B, and A feels no force. Which of the following is correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Only bar B is magnetised.
The field of a bar magnet is strongest at its two ends (poles) and weakest at its middle. A piece of unmagnetised iron has no poles, so a magnet attracts every part of it.
Option (1) Only bar A is magnetised. — Incorrect. If A were the magnet, its end would attract the iron of B at the middle as well as at the end.
Option (2) Only bar B is magnetised. — Correct. B pulls A at its end but not at its middle. The field of a magnet is strong at its ends and weak at its middle, so B is the magnet.
Option (3) Both bars are magnetised. — Incorrect. Two magnets would repel when like poles are brought close. The bars never repel, so one of them is not magnetised.
Option (4) The test cannot tell which bar is magnetised. — Incorrect. The change in force from the end to the middle of B is enough to decide which bar is the magnet.
NCERT (2026-27):
Example 5.1 – “(d) Try to bring different ends of the bars closer. A repulsive force in some situation establishes that both are magnetised. If it is always attractive, then one of them is not magnetised. In a bar magnet the intensity of the magnetic field is the strongest at the two ends (poles) and weakest at the central region. … If you notice that in the middle of B, A experiences no force, then B is magnetised. …”