What Is Magnetism? Definition, Examples, Facts

History

Ancient people used lodestones, natural magnets made of the iron mineral magnetite. In fact, the word "magnet" comes from the Greek words magnetis lithos, which means "Magnesian stone" or lodestone. Thales of Miletus investigated the properties of magnetism around 625 BCE to 545 BCE. The Indian surgeon Sushruta used magnets for surgical purposes around the same time. The Chinese wrote about magnetism in the fourth century BCE and described using a lodestone to attract a needle in the first century. However, the compass didn't come into use for navigation until the 11th century in China and 1187 in Europe.

While magnets were known, there wasn't an explanation for their function until 1819, when Hans Christian Ørsted accidentally discovered magnetic fields around live wires. The relationship between electricity and magnetism was described by James Clerk Maxwell in 1873 and incorporated into Einstein's theory of special relativity in 1905.

Causes of Magnetism

So, what is this invisible force? Magnetism is caused by the electromagnetic force, which is one of the four fundamental forces of nature. Any moving electric charge (electric current) generates a magnetic field perpendicular to it.

In addition to current traveling through a wire, magnetism is produced by the spin magnetic moments of elementary particles, such as electrons. Thus, all matter is magnetic to some degree because electrons orbiting an atomic nucleus produce a magnetic field. In the presence of an electric field, atoms and molecules form electric dipoles, with positive-charged nuclei moving a tiny bit in the direction of the field and negative-charged electrons moving the other way.

Magnetic Materials

All materials exhibit magnetism but magnetic behavior depends on the electron configuration of the atoms and the temperature. The electron configuration can cause magnetic moments to cancel each other out (making the material less magnetic) or align (making it more magnetic). Increasing temperature increases random thermal motion, making it harder for electrons to align, and typically decreasing the strength of a magnet.

Magnetism may be classified according to its cause and behavior. The main types of magnetism are:

Diamagnetism: All materials display diamagnetism, which is the tendency to be repelled by a magnetic field. However, other types of magnetism can be stronger than diamagnetism, so it is only observed in materials that contain no unpaired electrons. When electrons pairs are present, their "spin" magnetic moments cancel each other out. In a magnetic field, diamagnetic materials are weakly magnetized in the opposite direction of the applied field. Examples of diamagnetic materials include gold, quartz, water, copper, and air.

Paramagnetism: In a paramagnetic material, there are unpaired electrons. The unpaired electrons are free to align their magnetic moments. In a magnetic field, the magnetic moments align and are magnetized in the direction of the applied field, reinforcing it. Examples of paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.

Ferromagnetism: Ferromagnetic materials can form permanent magnets and are attracted to magnets. A ferromagnet has unpaired electrons, plus the magnetic moments of the electrons tend to remain aligned even when removed from a magnetic field. Examples of ferromagnetic materials include iron, cobalt, nickel, alloys of these metals, some rare earth alloys, and some manganese alloys.

Antiferromagnetism: In contrast to ferromagnets, the intrinsic magnetic moments of valence electrons in an antiferromagnet point in opposite directions (anti-parallel). The result is no net magnetic moment or magnetic field. Antiferromagnetism is seen in transition metal compounds, such as hematite, iron manganese, and nickel oxide.

Ferrimagnetism: Like ferromagnets, ferrimagnets retain magnetization when removed from a magnetic field but neighboring pairs of electron spins point in opposite directions. The lattice arrangement of the material makes the magnetic moment pointing in one direction stronger than that pointing in the other direction. Ferrimagnetism occurs in magnetite and other ferrites. Like ferromagnets, ferrimagnets are attracted to magnets.

There are other types of magnetism, too, including superparamagnetism, metamagnetism, and spin glass.

Properties of Magnets

Magnets form when ferromagnetic or ferrimagnetic materials are exposed to an electromagnetic field. Magnets display certain characteristics:

• There is a magnetic field surrounding a magnet.
• Magnets attract ferromagnetic and ferrimagnetic materials and can turn them into magnets.
• A magnet has two poles that repel like poles and attract opposite poles. The north pole is repelled by north poles of other magnets and attracted to south poles. The south pole is repelled by the south pole of another magnet but is attracted to its north pole.
• Magnets always exist as dipoles. In other words, you can't cut a magnet in half to separate north and south. Cutting a magnet makes two smaller magnets, which each have north and south poles.
• The north pole of a magnet is attracted to Earth's north magnetic pole, while the south pole of a magnet is attracted to Earth's south magnetic pole. This can be kind of confusing if you stop to consider the magnetic poles of other planets. For a compass to function, a planet's north pole is essentially the south pole if the world was a giant magnet!

Magnetism in Living Organisms

Some living organisms detect and use magnetic fields. The ability to sense a magnetic field is called magnetoception. Examples of creatures capable of magnetoception include bacteria, mollusks, arthropods, and birds. The human eye contains a cryptochrome protein which may allow some degree of magnetoception in people.

Many creatures use magnetism, which is a process known as biomagnetism. For example, chitons are mollusks that use magnetite to harden their teeth. Humans also produce magnetite in tissue, which may affect the immune and nervous system functions.

Magnetism Key Takeaways

• Magnetism arises from the electromagnetic force of a moving electric charge.
• A magnet has an invisible magnetic field surrounding it and two ends called poles. The north pole points toward Earth's north magnetic field. The south pole points toward the Earth's south magnetic field.
• The north pole of a magnet is attracted to the south pole of any other magnet and repelled by the north pole of another magnet.
• Cutting a magnet forms two new magnets, each with north and south poles.

Sources

• Du Trémolet de Lacheisserie, Étienne; Gignoux, Damien; Schlenker, Michel. "Magnetism: Fundamentals”. Springer. pp. 3–6. ISBN 0-387-22967-1. (2005)
• Kirschvink, Joseph L.; Kobayashi-Kirshvink, Atsuko; Diaz-Ricci, Juan C.; Kirschvink, Steven J. "Magnetite in Human Tissues: A Mechanism for the Biological Effects of Weak ELF Magnetic Fields". Bioelectromagnetics Supplement. 1: 101–113. (1992)