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What is the difference between permanent and temporary magnets?
Dec 23, 2018

Physics recognizes permanent magnets and electromagnets. The most common permanent magnets contain iron. Steel is a general term for an iron-based metallic alloy. Without question, there are steel alloys that make decent permanent magnets. However, other steel alloys such as stainless steel make terrible permanent magnets.

A permanent magnet is composed of metal whose outer shell electrons are unpaired and that align parallel over microscopic volumes known as domains. [Compared to the crystal lattice, the microscopic scale is huge.] These domains may be randomly aligned. If the metal is struck with a hammer or some other tool or if it is placed in a external magnetic field, then its domains may be aligned parallel. If the ambient temperature is less than the Curie temperature, then the domains will remain aligned after the external magnetic field is removed. Thus, the metal is a permanent magnet.

In addition to iron, permanent magnets may be constructed using nickel or cobalt. They may also be constructed using alloys. Rare earth elements and even ceramics are popular constituents of high-field permanent magnets.

Permanent magnets are called permanent, but the term is misleading. Over time, their magnetic domains lose alignment. The alignment loss rate is of the order of 1% per century.

Electromagnets depend of current-carrying conductors to generate their magnetic fields. The highest magnetic field electromagnets rely on water-cooled stacks of split copper rings. Superconducting coils of wire are used to generate high magnetic fields in many science labs and hospital radiology units. Where only small currents are available, the coil may be wrapped around a core of material with large magnetic susceptibility such as “soft” iron. One of the issues with this type of electromagnet is that the iron in its core will retain a small remnant [permanent] magnetic field.


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