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Permanent magnets, Temporary magnets, & Electromagnets

Permanent Magnets
A permanent magnet is one that stays magnetized, such as a magnet used to hold notes on a refrigerator door. Permanent magnets are made from "hard" ferromagnetic materials which are designed to stay magnetized, while "soft" ferromagnetic materials like soft iron are attracted to a magnet but don't tend to stay magnetized.

As we will see, different types of permanent magnets have different characteristics or properties concerning how easily they can be demagnetized, how strong they can be, how their strength varies with temperature, and so on.

Permanent magnets can be made in most any shape imaginable. They can be made into round bars, rectangular bars, horseshoes, rings or donuts, disks, rectangles, multi-fingered rings, and other custom shapes.  Some are cast into a mold and require grinding to achieve final dimensions.  Others start as a powder which is pressed into a mold or pressure bonded or sintered.

There are four classes of permanent magnets:

  • Neodymium Iron Boron (NdFeB or NIB)
  • Samarium Cobalt (SmCo)
  • Alnico
  • Ceramic or Ferrite

 

Temporary Magnets
Temporary magnets are those which act like a permanent magnet when they are within a strong magnetic field, but lose their magnetism when the magnetic field disappears.  Examples would be paperclips and nails and other soft iron items.

Electromagnets
Electromagnet can be made from a coil of wire which acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops. Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.