Physics · Ch 11 — Magnetic Materials
Permanent Magnet and Electromagnet
Permanent Magnet and Electromagnet
Sections 11.5.3 and 11.6 showed that a ferromagnetic material retains some magnetization even after an applied field is removed, and that the size of this retained effect -- along with how hard the material is to demagnetise -- is captured by its retentivity and coercivity, visible directly as the width and height of its hysteresis loop. These two properties, in opposite combinations, are exactly what is exploited to build two very different practical devices.
An electromagnet is built from a SOFT ferromagnetic core (soft iron, having a large permeability, greater than about 1000, together with only a small retained magnetization) placed inside a solenoid. Passing a current through the solenoid magnetises the soft-iron core, and because of its high permeability, the resulting magnetic field is boosted a thousand-fold or more compared with the solenoid alone; switching the current OFF lets the field collapse back to effectively zero, because the soft core's low retentivity means it retains almost none of its magnetisation on its own. This switchable behaviour -- strongly magnetic when the current is on, essentially unmagnetised when it is off -- is exactly what makes electromagnets useful in electric bells, loudspeakers, and circuit breakers, in research laboratories requiring strong but controllable fields, and (built as giant versions) in cranes used to lift heavy iron loads. Superconducting electromagnets, a special case, are used to generate the very highest steady magnetic fields (of the order of a few tesla), such as those required for NMR (Nuclear Magnetic Resonance) spectroscopy. …
Worked out. A soft ferromagnetic material -- the kind used for electromagnet cores and transformer cores -- can be easily magnetised and, just as importantly, easily demagnetised again: it has a thin, tall hysteresis loop, reflecting low coercivity (a small reversed field is enough to bring B back to zero) and low retentivity (little B remains once H returns to zero), together with a high permeability (a large B is reached for a comparatively small H). A hard ferromagnetic material -- the kind used for permanent magnets -- shows the opposite: a wide, fat hysteresis loop, reflecting high coercivity (a large reversed field is needed to demagnetise it) and high retentivity (most of its magnetisation survives even after the applied field is removed entirely). This is exactly the physical reason soft iron and hard steel are chos …