If a ferromagnetic material is inserted in a current carrying solenoid, the magnetic field inside the solenoid:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → increases significantly
With a core of relative permeability \( \mu_r \), the field inside a long solenoid becomes \( B = \mu_r\mu_0nI \). A ferromagnetic material has \( \mu_r \) of the order of hundreds to thousands, because its domains line up with the applied field.
\( B_{\mathrm{air}} = \mu_0nI \)
\( B_{\mathrm{core}} = \mu_r\mu_0nI,\ \ \mu_r \gg 1 \)
So the field increases significantly; this is why electromagnets and transformers use soft-iron cores.
Why the other options are incorrect
Option (2) remains same — Incorrect. The field is unchanged only with an air (vacuum) core, \( \mu_r = 1 \).
Option (3) decreases significantly — Incorrect. No ordinary material reduces the field by a large amount; even diamagnetic materials have \( \mu_r \) only slightly below 1.
Option (4) decreases slightly — Incorrect. This would happen with a diamagnetic core (\( \mu_r \) slightly less than 1), not a ferromagnetic one.
Note: A similar question was also asked in CUET (UG) 2026 on 24-05-2026 (Slot 2).
NCERT (2026-27):
The Solenoid – “… We shall see in the next chapter that a large field is possible by inserting a soft iron core inside the solenoid.”
Ferromagnetism – “Ferromagnetic substances are those which gets strongly magnetised when placed in an external magnetic field.”
Ferromagnetism – “… There are a number of elements, which are ferromagnetic: iron, cobalt, nickel, gadolinium, etc. The relative magnetic permeability is >1000!”