Discussion
Demagnetisation of a permanent magnet in a machine is caused by
Answer: D
Correct answer: excessive temperature or an excessive opposing field
Both mechanisms drive the magnet past the knee of its demagnetisation curve, after which the loss is permanent. Heat lowers the coercivity so that a field which was safe when cool is not, which is why the two causes so often act together - a stall current in an already hot machine being the classic case.
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