Magnetic materials used for making permanent magnets (P) and magnets in a transformer (T) have different properties of the following, which property best matches for the type of magnet required?

Solution

Physics sir · 4 steps

Ask what each magnet must do: a permanent magnet must hold on, a transformer core must let go every cycle.

1. Two loop properties. Retentivity is the magnetisation left once the magnetising field is switched off. Coercivity is the reverse field needed to wipe that magnetisation out.
2. Permanent magnet. It must stay strongly magnetised with no current around it, so it needs large retentivity. Stray fields, knocks and heat should not demagnetise it, so it needs large coercivity too.
3. Transformer core. With AC, the core is magnetised and demagnetised every cycle. Each cycle wastes energy equal to the hysteresis loop area, so the core needs a thin loop: small retentivity and small coercivity.

   Property	Permanent magnet (P)	Transformer core (T)
   Retentivity	large	small
   Coercivity	large	small
   Hysteresis loop	wide	narrow (low energy loss)
   Typical material	steel, alnico	soft iron

4. So. Only (b) matches. A permanent magnet needs large retentivity and large coercivity.

The idea to keep. Permanent magnet: wide hysteresis loop, so large retentivity and large coercivity. Electromagnet or transformer core: narrow loop, so small values of both and low energy loss per cycle.

Answer from the NTA final answer key. Solution by PrepWiser.