Magnetism as a Voltage Source
This series provides a look at different phenomena that can produce electrical energy. In this article, the seventh in our series, we’ll discuss how magnetism is used as one of the primary mechanisms of producing electrical energy.
Magnetism is used as one of the primary mechanisms of producing electrical energy. Most commercially produced electrical power is generated using magnetism. Knowledge about magnetism dates to ancient times. Thales of Miletus (634–546 BC) recorded that iron was strangely attracted to a substance known as lodestone. Lodestone was discovered at Magnesia in Asia Minor and is now called magnetite. Magnetite is the naturally occurring magnet that we know as iron oxide (Fe3O4). Magnetite attracts articles that have iron in their molecular structure. In early times, the Chinese discovered that if a piece of magnetite were suspended on a string and left free to turn, it always aligned itself in a north-south direction.
A magnet is a substance that attracts iron and produces a magnetic field. Magnetism is the invisible force exerted by a magnet. Materials that are strongly attracted by a magnet are referred to as ferromagnetic materials. Besides iron, only nickel and cobalt occur naturally. However, alloys such as steel also exhibit magnetic characteristics.
Paramagnetic materials also exist. A paramagnetic material is a partially magnetic material. Aluminum, chromium, platinum, and air are paramagnetic materials. Paramagnetic materials are slightly attracted to a magnetic field. In addition, diamagnetic materials can be slightly magnetized under the influence of a strong magnetic field. A diamagnetic material is a material only slightly repelled by a magnetic field. Diamagnetic materials include copper, silver, gold, and mercury. All materials have some magnetic properties, but the magnetic characteristics of most materials are not easily observable. Paramagnetic and diamagnetic substances are usually placed in this nonmagnetic classification since these materials do not retain their magnetic properties once an external field is removed.
Every magnet has two poles, defined as a north pole and a south pole. The north pole of a magnet is the pole that points to the magnetic north pole of the Earth (see Figure 1).
Figure 1. The north pole of a magnet is the pole that points to the magnetic north pole of the Earth. Image courtesy of MagLab.org
How are Artificial Magnets Classified?
Artificial magnets are classified as either permanent (hard) magnets or temporary (soft) magnets. A permanent magnet is a magnet that retains its magnetism after the magnetizing force has been removed. Permanent magnets are made of special steels and alloys such as alnico, which consists principally of aluminum, nickel, and cobalt. Permanent magnets have a high degree of retentivity (ability to retain residual magnetism). A temporary magnet is a magnet that loses its magnetism when the magnetizing force is removed. Temporary magnets have low retentivity.
Alnico, hardened steels and several other alloys are difficult to magnetize. These alloys are said to have low permeability. Once magnetized, however, these materials maintain their magnetic characteristics and force. Conversely, soft iron and annealed silicon steel have a high permeability; they are easy to magnetize. When the magnetizing force is removed, however, they retain only a small part of their magnetic force. The small amount of magnetism that remains in these materials is called residual magnetism. Relative permeability is the ability of a material to conduct magnetic lines of force as compared to air, which has a permeability value of 1. Paramagnetic substances have a permeability slightly greater than 1. Diamagnetic materials have a permeability of less than 1. Both permanent and temporary magnets have applications in modern technology.
Artificial magnets are manufactured in various shapes, sizes, and strengths depending on their intended use. Most magnets are bar, horseshoe, or ring magnets. See Figure 2. Bar magnets are often used to study the characteristics and effects of magnetism. Bar magnets may be square, rectangular, round, or other desired shapes. A horseshoe magnet takes its name from its shape. Horseshoe magnets have greater strength than bar magnets of the same size and material. Horseshoe magnets are often used in electrical meters. A closed ring magnet has its field confined to the ring. By cutting a slot in the ring, the magnetic field can be detected in the gap. Ring magnets were used as data storage devices in early computers. Temporary ring magnets are often used as magnetic shields.
Figure 2. Common manufactured magnet shapes include a bar, horseshoe, and ring magnets. Image courtesy of Arnold Magnetic Technologies
The attraction iron has to a bar magnet can be observed by placing the magnet into a pile of iron filings. When removed, most of the iron filings collect near both ends of the magnet. This shows that the strongest magnetic force exists at each end. The ends of the magnet are called its poles. All magnets have two poles. These are the north and south poles. Usually, the north pole is marked in some way by the original equipment manufacturer (OEM).
Figure 3. A magnet will always have a north and a south pole regardless of the number of times it is divided. Image courtesy of Lumen Learning
If a bar magnet is cut in half, each half has a north and a south pole. If a bar magnet is cut into thirds, each piece still maintains a north and a south pole. A single-pole magnet does not exist. If a north pole is present, then a magnet must also have at least one south pole. See Figure 3.