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Asked in GSEB Board August 2020 · Dia, para and ferromagnetism
Idea: an electromagnet has to be strongly magnetised while the current flows and has to lose that magnetisation the moment the current stops. Read the two blanks off those two demands.
'Strongly magnetised' asks for a large permeability, so that the modest H=nI from the winding gives a large B=μ₀μᵣH in the core.
'Loses it at once' asks for a small retentivity, and a small coercivity with it, so that almost nothing is left when H returns to zero.
Soft iron does both: its hysteresis loop is tall and narrow -- high permeability, low retentivity. Steel, with a fat loop, is the opposite and is used for permanent magnets.
So the pair asked for is low retentivity and high permeability.
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