Physics · Ch 3 — Magnetism and Magnetic Effects of Electric Current
Hysteresis
Hysteresis
When a ferromagnetic material (e.g. iron) is magnetised by a slowly-varying field , its induction does not vary linearly with -- so the ratio is not constant. Starting from an unmagnetised sample, rises with along a curve up to a saturation level. If is then reduced, falls back along a different path, and when , is not zero -- some magnetism is left behind. This residual induction is the retentivity or remanence: the material's ability to retain magnetism after the magnetising field is removed. To bring back to zero, must be increased in the reverse direction; the reverse field magnitude needed is the material's coercivity. Continuing to increase in reverse drives to saturation in the opposite sense, and reversing the cycle again traces the field back through a symmetric path to the starting point. This closed curve is the hysteresis loop, and the phenomenon -- perpetually lagging behind -- is called hysteresis. …
What this figure shows. A closed curve ACDEFGKC is traced on B-H axes. Starting from the origin, B rises along OA up to a saturation point as H increases; reducing H back to zero follows a different path down to D, where B is still positive (this residual value OD is the retentivity/remanence); increasing H in the reverse direction drives B to zero at E (the reverse field OE needed is the coercivity), then on to negative saturation at F; reversing the cycle again traces the path FGKC back to the starting saturation point, closing the loop. The loop never retraces it …
What this figure shows. Two B-H hysteresis loops are drawn on the same axes for comparison: a tall, narrow loop for silicon steel (a soft magnetic material) and a much fatter, wider loop for carbon steel (a hard magnetic material), visually showing that hard materials enclose a far larger loop area -- and therefore have both higher retentivity and higher coercivity, and lose more energy as heat per magnetisation cycle -- than soft material …