Match List-I with List-II.
| List-I | List-II |
|---|---|
| (A) Direction of the magnetic force on a charge moving in a magnetic field \(\vec{B}\) | (I) Zero |
| (B) Direction of the net magnetic moment of a diamagnetic substance placed in a field \(\vec{B}\) | (II) Opposite to \(\vec{B}\) |
| (C) Net magnetic moment of an infinitely long straight wire carrying a current | (III) Can be non-zero |
| (D) Net magnetic moment of a system whose net charge is zero | (IV) Normal to \(\vec{B}\) |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A)-(IV), (B)-(II), (C)-(I), (D)-(III)
(A) Direction of the magnetic force on a charge moving in a magnetic field \( \vec{B} \) — This matches (IV) normal to \( \vec{B} \). The force is \( q\vec{v}\times\vec{B} \), which is always perpendicular to \( \vec{B} \). So magnetic field lines are not lines of force.
(B) Direction of the net magnetic moment of a diamagnetic substance placed in a field \( \vec{B} \) — This matches (II) opposite to \( \vec{B} \). The applied field slows down some orbiting electrons and speeds up others. The substance gets a net moment against the field, and so it is repelled.
(C) Net magnetic moment of an infinitely long straight wire carrying a current — This matches (I) zero. A straight infinite conductor, like a toroid, has no net magnetic moment. Its field lines are circles around the wire. So, unlike a bar magnet, it has no north pole and no south pole.
(D) Net magnetic moment of a system whose net charge is zero — This matches (III) can be non-zero. The charges may add up to zero, yet the current loops inside can give a net moment. Atoms of a paramagnetic material are an example.
NCERT (2026-27):
Example 5.4 – “(a) No. The magnetic force is always normal to B (remember magnetic force = qv × B). It is misleading to call magnetic field lines as lines of force.”
Diamagnetism – “… Thus, the substance develops a net magnetic moment in direction opposite to that of the applied field and hence repulsion.”
Example 5.1 – “(c) Not necessarily. True only if the source of the field has a net non-zero magnetic moment. This is not so for a toroid or even for a straight infinite conductor.”
Example 5.4 – “(d) Yes. The average of the charge in the system may be zero. Yet, the mean of the magnetic moments due to various current loops may not be zero. …”