Why is soft iron used in a transformer core but steel is used for a permanent magnet?
A transformer reverses magnetisation every AC cycle, so its core should magnetise and demagnetise easily with little energy loss.
A transformer reverses magnetisation every AC cycle, so its core should magnetise and demagnetise easily with little energy loss. Soft iron has low coercivity and a narrow hysteresis loop. A permanent magnet must resist demagnetisation, so steel or another hard magnetic material with high coercivity and retentivity is preferred. The correct choice comes from the loop, not merely from 'strong magnetism'.
Key point
Narrow low-coercivity loop for cores; high retentivity/coercivity for permanent magnets.
Common mistake
selecting the material only by saturation magnetisation.
Memory tip
transformer must forget; permanent magnet must remember.
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