A solenoid has a core of a magnetic material. Its windings are insulated from the core and carry a current \(I\). The magnetising current \(I_M\) of this solenoid is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → the extra current needed in the windings, with the core removed, to give the same B
A magnetised core makes the field inside the solenoid larger than \( \mu_0 nI \). The magnetising current describes this extra field as if it came from an extra current in the windings.
Option (1) the extra current needed in the windings, with the core removed, to give the same B — Correct. Without the core, the windings would need a current \( I+I_M \) to give the same field. So \( B=\mu_0 n(I+I_M) \), and \( I_M \) is the extra part.
Option (2) the current that flows through the material of the core itself — Incorrect. The windings are insulated from the core, so no conduction current flows into it. \( I_M \) is not a current in the core material.
Option (3) the current needed in the windings to make the magnetisation of the core zero — Incorrect. \( I_M \) does not remove the magnetisation. It is the current that would produce the extra field \( \mu_0 M \) of the core, with no core present.
Option (4) the current in the windings that produces the magnetic intensity \(H\) — Incorrect. The magnetic intensity \( H=nI \) comes from the real current \( I \) alone. \( I_M \) is added on top of \( I \).
NCERT (2026-27):
Example 5.5 – “(d) The magnetising current IM is the additional current that needs to be passed through the windings of the solenoid in the absence of the core which would give a B value as in the presence of the core. …”