A ferromagnetic material is placed in an external magnetic field. The magnetic domains oriented in the direction of the field
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → increase in size
A ferromagnetic material is made up of domains, regions in which the atomic dipole moments are aligned in a common direction. With no external field the domains point at random and there is no bulk magnetisation.
Option (1) increase in size — Correct. In an external field \( B_0 \) the domains turn towards the field. At the same time the domains oriented along \( B_0 \) grow in size. In the end the aligned domains join into a single ‘giant’ domain, and the sample is strongly magnetised.
Option (2) decrease in size — Incorrect. If the domains along the field shrank, the magnetisation along the field would fall. But a ferromagnet becomes strongly magnetised in the direction of the field.
Option (3) may increase or decrease in size — Incorrect. The change does not happen by chance. The domains along the field always grow, and this gives the material its large magnetisation along the field.
Option (4) have no relation with the field — Incorrect. The field turns the domains towards itself and makes the ones along it grow. So the domain pattern changes as soon as the field is applied.
NCERT (2026-27):
Ferromagnetism – “… When we apply an external magnetic field B0, the domains orient themselves in the direction of B0 and simultaneously the domain oriented in the direction of B0 grow in size.”