B, H, M, χ and μ_r locked together; the dia/para/ferro table with the Curie law; domain theory; and the hysteresis loop read as a diagram, down to which metal goes in which machine.
Unit 4 carries 9 of the 50 physics marks; with seven chapters sharing them, this one averages a little over one (MEC publishes weights by unit, not by chapter). Its single scope line packs four separate things — the quantities, the three classes of material, domains, and hysteresis — and the loop is the most valuable diagram in the unit because it is examined as a reading exercise rather than as a formula.
The MEC scope line is: Domains, relative permeability, susceptibility and hysteresis in dia-, para- and ferromagnetic materials. The headings below follow that line, in that order.
Three ways it is asked: recall (the sign of χ for a diamagnet, the Curie temperature of iron, the meaning of coercivity); understanding (why a diamagnetic rod sets itself across a field while a paramagnetic one sets along it, why heating destroys ferromagnetism, why the area of the loop is an energy); application (μ_r from χ, a Curie-law ratio when the temperature is halved, the power wasted in a core of stated loop area running at 50 Hz).
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