(credit: Bill McChesney, Flickr)įigure 4 shows that the response of iron filings to a current-carrying coil and to a permanent bar magnet. The patient goes into this “tunnel” on the gurney. The device uses a superconducting cylindrical coil for the main magnetic field. Instrument for magnetic resonance imaging (MRI). Electromagnets are employed for everything from a wrecking yard crane that lifts scrapped cars to controlling the beam of a 90-km-circumference particle accelerator to the magnets in medical imaging machines (See Figure 3). These temporarily induced magnets are called electromagnets. Electromagnetism is the use of electric current to make magnets. This was the first significant evidence that the movement of charges had any connection with magnets. The first significant observation was by the Danish scientist Hans Christian Oersted (1777–1851), who found that a compass needle was deflected by a current-carrying wire. ElectromagnetsĮarly in the 19th century, it was discovered that electrical currents cause magnetic effects. There are several elements and alloys that have Curie temperatures much lower than room temperature and are ferromagnetic only below those temperatures. The Curie temperature for iron is 1043 K (\boldsymbol), which is well above room temperature. There is a well-defined temperature for ferromagnetic materials, which is called the Curie temperature, above which they cannot be magnetized. Increased thermal motion at higher temperature can disrupt and randomize the orientation and the size of the domains. Individual atoms are aligned within domains each atom acts like a tiny bar magnet.Ĭonversely, a permanent magnet can be demagnetized by hard blows or by heating it in the absence of another magnet. (b) When magnetized by an external field, the domains show greater alignment, and some grow at the expense of others. (a) An unmagnetized piece of iron (or other ferromagnetic material) has randomly oriented domains. This induced magnetization can be made permanent if the material is heated and then cooled, or simply tapped in the presence of other magnets. In response to an external magnetic field, the domains may grow to millimeter size, aligning themselves as shown in Figure 2(b). Domains are small and randomly oriented in an unmagnetized ferromagnetic object. Within domains, the poles of individual atoms are aligned. The regions within the material called domains act like small bar magnets. (This results in the attraction of the previously unmagnetized material to the magnet.) What happens on a microscopic scale is illustrated in Figure 2. When a magnet is brought near a previously unmagnetized ferromagnetic material, it causes local magnetization of the material with unlike poles closest, as in Figure 1. Note that there are attractive forces between the magnets. The iron becomes a permanent magnet with the poles aligned as shown: its south pole is adjacent to the north pole of the original magnet, and its north pole is adjacent to the south pole of the original magnet. An unmagnetized piece of iron is placed between two magnets, heated, and then cooled, or simply tapped when cold. 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. Other materials exhibit weak magnetic effects, which are detectable only with sensitive instruments. 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. Such materials are called ferromagnetic, after the Latin word for iron, ferrum.
Only certain materials, such as iron, cobalt, nickel, and gadolinium, exhibit strong magnetic effects. Describe the relationship between electricity and magnetism.Explain the significance of the Curie temperature.Describe the role of magnetic domains in magnetization.
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