22.2 Ferromagnets and electromagnets

Learning objectives

By the end of this section, you will be able to:

• Define ferromagnet.
• Describe the role of magnetic domains in magnetization.
• Explain the significance of the Curie temperature.
• Describe the relationship between electricity and magnetism.

The information presented in this section supports the following AP® learning objectives and science practices:

• 2.D.3.1 The student is able to describe the orientation of a magnetic dipole placed in a magnetic field in general and the particular cases of a compass in the magnetic field of the Earth and iron filings surrounding a bar magnet. (S.P. 1.2)
• 2.D.4.1 The student is able to use the representation of magnetic domains to qualitatively analyze the magnetic behavior of a bar magnet composed of ferromagnetic material. (S.P. 1.4)
• 4.E.1.1 The student is able to use representations and models to qualitatively describe the magnetic properties of some materials that can be affected by magnetic properties of other objects in the system. (S.P. 1.1, 1.4, 2.2)

Ferromagnets

Only certain materials, such as iron, cobalt, nickel, and gadolinium, exhibit strong magnetic effects. Such materials are called ferromagnetic    , after the Latin word for iron, ferrum . A group of materials made from the alloys of the rare earth elements are also used as strong and permanent magnets; a popular one is neodymium. Other materials exhibit weak magnetic effects, which are detectable only with sensitive instruments. Not only do ferromagnetic materials respond strongly to magnets (the way iron is attracted to magnets), they can also be magnetized    themselves—that is, they can be induced to be magnetic or made into permanent magnets.

When a magnet is brought near a previously unmagnetized ferromagnetic material, it causes local magnetization of the material with unlike poles closest, as in [link] . (This results in the attraction of the previously unmagnetized material to the magnet.) What happens on a microscopic scale is illustrated in [link] . The regions within the material called domains    act like small bar magnets. Within domains, the poles of individual atoms are aligned. Each atom acts like a tiny bar magnet. Domains are small and randomly oriented in an unmagnetized ferromagnetic object. In response to an external magnetic field, the domains may grow to millimeter size, aligning themselves as shown in [link] (b). This induced magnetization can be made permanent if the material is heated and then cooled, or simply tapped in the presence of other magnets.