Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The intrinsic magnetic moment of an electron is due to a circulating current, as in a current loop.
Reason (R): A current loop placed in a magnetic field is acted on by a torque, just like a magnetic needle.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is not correct but (R) is correct.
Assertion (A) The intrinsic magnetic moment of an electron is due to a circulating current, as in a current loop. — False. An intrinsic magnetic moment is a magnetic moment that the particle carries by itself. Ampere suggested that all magnetism comes from circulating currents. This is only partly true. The electron and the proton carry an intrinsic magnetic moment that circulating currents do not account for.
Reason (R) A current loop placed in a magnetic field is acted on by a torque, just like a magnetic needle. — True. The torque on the loop is \( \vec{\tau}=\vec{m}\times\vec{B} \). A magnetic needle also turns under a torque in a field. This likeness led Ampere to suggest that all magnetism comes from circulating currents. The electron’s own moment is a case his idea does not cover.
NCERT (2026-27):
Circular current loop as a magnetic dipole – “We have shown that a current loop … (ii) is subject to torque like a magnetic needle. This led Ampere to suggest that all magnetism is due to circulating currents.”
Circular current loop as a magnetic dipole – “… However, elementary particles such as an electron or a proton also carry an intrinsic magnetic moment, not accounted by circulating currents.”