JEE Main 2021 · 24 February · evening · Single correct · +4 / −1
A soft ferromagnetic material is placed in an external magnetic field. The magnetic domains :
- (a) may increase or decrease in size and change its orientation.
- (b) increase in size but no change in orientation.
- (c) decrease in size and changes orientation.
- (d) have no relation with external magnetic field.
Answer and solution
Answer (a) may increase or decrease in size and change its orientation.
Solution
Physics sir · 4 steps
Domains aligned with the field grow, the others shrink, and their moments turn towards the field.
- What a domain is. A ferromagnet is made of small regions called domains. Inside each one, all the atomic magnetic moments point the same way, but different domains point in different directions.
- Boundaries move. In an external field, domains whose moments are roughly along the field grow. They do this by pushing their walls into the neighbouring domains, which shrink. In a soft material the walls move easily, so this happens even in a weak field.
- Moments rotate. In a stronger field, the moments inside the remaining domains also rotate towards the field direction. So orientation changes as well as size.
- So. Some domains increase in size, others decrease, and their orientation changes. That is option (a).
The idea to keep. Magnetising a ferromagnet works in two ways: favourably aligned domains grow at the expense of the others, and then the domain moments rotate towards the field. Size and direction both change.
Answer from the NTA final answer key. Solution by PrepWiser.