What is magnetism? Facts About Magnetic Fields and Magnetic Force

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Magnetism is a force of nature produced by the movement of electric charges. Sometimes these movements are microscopic and inside a material called magnets. Magnets, or magnetic fields created by moving electric charges, can attract or repel other magnets and change the motion of other charged particles.

A magnetic field exerts a force on particles known as the Lorentz force, according to Georgia State University’s HyperPhysics website. The force acting on an electrically charged particle in a magnetic field depends on the magnitude of the charge, the velocity of the particle, and the strength of the magnetic field. The Lorentz force has the special property of causing particles to move perpendicular to their original motion.

Some materials, like iron, are known as permanent magnets, which means they can withstand a permanent magnetic field. These are the most common forms of magnets encountered in everyday life. Other materials, such as iron, cobalt, and nickel, can receive a temporary magnetic field by placing them inside a larger, more powerful field, but these materials will eventually lose their magnetism.

How magnetism works

Earth’s magnetic field. (Image credit: alxpin via Getty Images)

Magnetic fields are generated by the movement of electric charges, according to HyperPhysics. Electrons all have a fundamental quantum mechanical property of angular momentum, known as “spin”. Inside atoms, most electrons tend to form pairs in which one of them is “spin up” and the other is “spin down”, or in other words their peak angular momentum in opposite directions. In this case, the magnetic fields created by these spins point in opposite directions, so they cancel each other out. However, some atoms contain one or more unpaired electrons, and these unpaired electrons create a tiny magnetic field. The direction of their rotation determines the direction of the magnetic field, according to the Nondestructive Testing (NDT) Resource Center. When a significant majority of unpaired electrons are aligned with their spins in the same direction, they combine to produce a magnetic field strong enough to be observed on a macroscopic scale.

Magnetic field sources are dipole, meaning they have a north pole and a south pole. Opposite poles (N and S) attract and like poles (N and N, or S and S) repel, according to Joseph Becker of San Jose State University. This creates a toroidal or ring-shaped field, as the direction of the field propagates outward from the north pole and enters through the south pole.

The Earth itself is a giant magnet. The planet derives its magnetic field from the flow of electric current through the molten metallic core, according to NASA. A compass points north because the small magnetic needle it contains is suspended so that it can spin freely inside its case to align with the earth’s magnetic field. Paradoxically, what we call the magnetic north pole is actually a magnetic south pole because it attracts the magnetic north poles of compass needles.

history of magnetism

Magnetite (also known as lodestone) is the most magnetic of all naturally occurring minerals on Earth. (Image credit: Aleksandr Pobedimskiy via Shutterstock)

If the alignment of unpaired electrons persists without the application of an external magnetic field or electric current, it produces a permanent magnet. Permanent magnets are the result of ferromagnetism. The prefix “ferro” refers to iron because permanent magnetism was first observed in a form of natural iron ore called magnetite, Fe3O4. Pieces of Magnetite can be found scattered on or near the Earth’s surface, and occasionally one of them will be magnetized. These natural magnets are called magnetites. While scientists don’t know exactly how magnetites form, “most scientists think magnetite is magnetite that has been struck by lightning,” according to the University of Arizona.

People soon learned that they could magnetize an iron needle by stroking it with magnetite, causing the majority of the unpaired electrons in the needle to align in one direction. According to NASA, around the year 1000, the Chinese discovered that a magnet floating in a bowl of water always aligned in the north-south direction. Subsequently, the magnetic compass became a great aid to navigation, especially day and night when the stars were hidden by clouds.

Metals other than iron can have ferromagnetic properties. These include nickel, cobalt and some rare earth metals such as samarium or neodymium, which are used to make super strong permanent magnets.

Other forms of magnetism

Magnetism takes many other forms, but with the exception of ferromagnetism, they are generally too weak to be observed except by sensitive laboratory instruments or at very low temperatures. Anton Brugnams first discovered diamagnetism in 1778 using permanent magnets in his search for iron-containing materials. According to Gerald Küstler, a widely published independent German researcher and inventor, in his article “Diamagnetic Levitation – Historical Milestones”, published in the Romanian Journal of Technical Sciences, Brugnams observed: “Only dark, almost purple bismuth showed a peculiar phenomenon in the study; for when I put a piece of it on a round sheet of paper floating above the water, it was repelled by the two poles of the magnet.

Diamagnetism is caused by the orbital motion of electrons within atoms creating tiny current loops, which produce weak magnetic fields, according to HyperPhysics. When an external magnetic field is applied to a material, these current loops tend to align in such a way as to oppose the applied field. This causes all materials to be repelled by a permanent magnet; however, the resulting force is usually too weak to be noticeable. There are, however, some notable exceptions.

Pyrolytic carbon, a substance similar to graphite, exhibits even stronger diamagnetism than bismuth, but only along one axis, and can actually levitate above a super strong rare earth magnet. Some superconducting materials exhibit even stronger diamagnetism below their critical temperature (the temperature at which they become superconducting) and so rare-earth magnets can levitate above them. (In theory, due to their mutual repulsion, one can be levitated above the other.)

Paramagnetism occurs when a material temporarily becomes magnetic when placed in a magnetic field and returns to its non-magnetic state as soon as the external field is removed. When a magnetic field is applied, some of the unpaired electron spins align with the field and overwhelm the opposing force produced by diamagnetism. However, the effect is only noticeable at very low temperatures, said Daniel Marsh, professor of physics at Missouri Southern State University.

Other, more complex forms include antiferromagnetism, in which the magnetic fields of atoms or molecules line up next to each other; and spin glass behavior, which involves both ferromagnetic and antiferromagnetic interactions. Additionally, ferrimagnetism can be considered a combination of ferromagnetism and antiferromagnetism due to many shared similarities between them, but it still has its own uniqueness, according to the University of California, Davis.

Electricity and magnetism

Fleming's Right Hand Rule.  Current by direction of magnetic field and force.  fridas via Shutterstock

Infographic showing how Fleming’s right hand rule works. (Image credit: fridas via Shutterstock)

When a conductive wire is moved through a magnetic field, the field induces a current in the wire. Conversely, a magnetic field is produced by a moving electric charge, such as when a wire carries a current. Thus, all the electrical wires in your household produce tiny magnetic fields. This relationship between electricity and magnetism is described by Faraday’s law of induction, which is the basis of electromagnets, electric motors and generators. A charge moving in a straight line, such as through a straight wire, generates a magnetic field that rotates around the wire. When this thread is formed into a loop, the field takes the form of a donut or a torus.

Direct current can also produce a constant field in one direction which can be turned on and off with the current. This field can then deflect a moving iron lever causing an audible click. This is the basis of the telegraph, invented in the 1830s by Samuel FB Morse, which allowed long distance communication over wires using binary code based on long and short pulses, according to the Library of Congress. Skilled operators sent the pulses by quickly turning the power on and off using a spring-loaded momentary contact switch or a key. Another operator on the receiving side would then translate the audible clicks into letters and words.

A coil around a magnet can also be made to move in a pattern of varying frequency and amplitude to induce a current in a coil. It is the basis of a number of devices, notably the microphone. The sound moves a diaphragm in and out with the varying pressure waves. If the diaphragm is connected to a magnetic coil moving around a magnetic core, it will produce a varying current analogous to incident sound waves. This electrical signal can then be amplified, recorded or transmitted at will. Tiny, super-powerful rare-earth magnets are being used to make miniaturized microphones for cell phones, Marsh told Live Science.

When this modulated electrical signal is applied to a coil, it produces an oscillating magnetic field, which moves the coil in and out of a magnetic core in the same pattern. The coil is then attached to a movable speaker cone so that it can reproduce audible sound waves through the air. The first practical application of the microphone and speaker was the telephone, patented by Alexander Graham Bell in 1876, according to the Smithsonian Institution. Although this technology has been improved and refined, it remains the basis of sound recording and reproduction.

The applications of electromagnets are almost innumerable. Faraday’s Law of Induction forms the basis of many aspects of our modern society, including not only electric motors and generators, but also electromagnets of all sizes. The same principle used by a giant crane to lift unwanted cars in a junk yard is also used to align microscopic magnetic particles on a computer hard drive to store binary data, and new applications are being developed every day.

Editor Tanya Lewis contributed to this report.

Additional Resources

Bibliography

NASA, “Earth’s Magnetosphere”, https://www.nasa.gov/magnetosphere

“Magnetism.” Discover science. Gale Research, 1996. Reproduced in Discovering Collection. Farmington Hills, Mich.: Gale Group. December 2000. http://galenet.galegroup.com/servlet/DC/

OnlineGriffiths, David J. (1998). Introduction to electrodynamics (3rd ed.). Prentice Hall. ISBN 978-0-13-805326-0. OCLC 40251748.

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