Magnetism is one aspect of the combined electromagnetic force. It refers to physical phenomena arising from the force caused by magnets, objects that produce fields that attract or repel other objects.
A magnetic field exerts a force on particles in the field due to the Lorentz force. The motion of electrically charged particles gives rise to magnetism. 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.
All materials experience magnetism, some more strongly than others. Permanent magnets, made from materials such as iron, experience the strongest effects, known as ferromagnetism. With rare exception, this is the only form of magnetism strong enough to be felt by people.
Magnetic fields are generated by rotating electric charges. Electrons all have a property of angular momentum, or spin. Most electrons tend to form pairs in which one of them is “spin up” and the other is “spin down,” in accordance with the Pauli Exclusion Principle, which states that two electrons cannot occupy the same energy state at the same time. In this case, their magnetic fields are in opposite directions, so they cancel each other. However, some atoms contain one or more unpaired electrons whose spin can produce a directional magnetic field. The direction of their spin determines the direction of the magnetic field. When a significant majority of unpaired electrons are aligned with their spins in the same direction, they combine to produce a magnetic field that is strong enough to be felt on a macroscopic scale.
Magnetic field sources are dipolar, having a north and south magnetic pole. Opposite poles (N and S) attract, and the same poles (N and N, or S and S) repel. This creates a toroidal, or doughnut-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 gets its magnetic field from circulating electric currents within the molten metallic core. A compass points north because the small magnetic needle in it is suspended so that it can spin freely inside its casing to align itself with the planet’s magnetic field. Paradoxically, what we call the Magnetic North Pole is actually a south magnetic pole because it attracts the north magnetic poles of compass needles.
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 surface of the earth, and occasionally, one will be magnetized. These naturally occurring magnets are called lodestones.
People soon learned that they could magnetize an iron needle by stroking it with a lodestone, causing a majority of the unpaired electrons in the needle to line up in one direction. According to NASA, around A.D. 1000, the Chinese discovered that a magnet floating in a bowl of water always lined up in the north-south direction. The magnetic compass thus became a tremendous aid to navigation, particularly during the day and at night when the stars were hidden by clouds.
Other metals besides iron have been found to 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 except for ferromagnetism, they are usually too weak to be observed except by sensitive laboratory instruments or at very low temperatures. Diamagnetism was first discovered in 1778 by Anton Brugnams, who was using permanent magnets in his search for materials containing iron. According to Gerald Küstler, a widely published independent German researcher and inventor, in his paper, “Diamagnetic Levitation — Historical Milestones,” published in the Romanian Journal of Technical Sciences, Brugnams observed, “Only the dark and almost violet-colored bismuth displayed a particular phenomenon in the study; for when I laid a piece of it upon a round sheet of paper floating atop water, it was repelled by both poles of the magnet.”
Bismuth has been determined to have the strongest diamagnetism of all elements, but as Michael Faraday discovered in 1845, it is a property of all matter to be repelled by a magnetic field.
Diamagnetism is caused by the orbital motion of electrons creating tiny current loops, which produce weak magnetic fields. 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, shows even stronger diamagnetism than bismuth, albeit only along one axis, and can actually be levitated above a super-strong rare earth magnet. Certain superconducting materials show even stronger diamagnetism below their critical temperature and so rare-earth magnets can be levitated above them. (In theory, because of their mutual repulsion, one can be levitated above the other.)
Paramagnetism occurs when a material becomes magnetic temporarily when placed in a magnetic field and reverts to its nonmagnetic state as soon as the external field is removed. When a magnetic field is applied, some of the unpaired electron spins align themselves with the field and overwhelm the opposite force produced by diamagnetism. However, the effect is only noticeable at very low temperatures.
Other, more complex, forms include antiferromagnetism, in which the magnetic fields of atoms or molecules align next to each other; and spin glass behavior, which involve both ferromagnetic and antiferromagnetic interactions. Additionally, ferrimagnetism can be thought of as a combination of ferromagnetism and antiferromagnetism due to many similarities shared among them, but it still has its own uniqueness.
When a wire is moved in a magnetic field, the field induces a current in the wire. Conversely, a magnetic field is produced by an electric charge in motion. This is in accordance with Faraday’s Law of Induction, which is the basis for electromagnets, electric motors and generators. A charge moving in a straight line, as through a straight wire, generates a magnetic field that spirals around the wire. When that wire is formed into a loop, the field becomes a doughnut shape, or a torus. This magnetic field can be greatly enhanced by placing a ferromagnetic metal core inside the coil.
In some applications, direct current is used to produce a constant field in one direction that can be switched on and off with the current. This field can then deflect a movable iron lever causing an audible click. This is the basis for the telegraph, invented in the 1830s by Samuel F. B. Morse, which allowed for long-distance communication over wires using a binary code based on long- and short-duration pulses. The pulses were sent by skilled operators who would quickly turn the current on and off using a spring-loaded momentary-contact switch, or key. Another operator on the receiving end would then translate the audible clicks back 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. This is the basis for a number of devices, most notably, the microphone. Sound causes a diaphragm to move in an out with the varying pressure waves. If the diaphragm is connected to a movable magnetic coil around a magnetic core, it will produce a varying current that is analogous to the incident sound waves. This electrical signal can then be amplified, recorded or transmitted as desired. Tiny super-strong rare-earth magnets are now being used to make miniaturized microphones for cell phones.
When this modulated electrical signal is applied to a coil, it produces an oscillating magnetic field, which causes the coil to move in and out over a magnetic core in that same pattern. The coil is then attached to a movable speaker cone so it can reproduce audible sound waves in the air. The first practical application for the microphone and speaker was the telephone, patented by Alexander Graham Bell in 1876. Although this technology has been improved and refined, it is still the basis for recording and reproducing sound.
The applications of electromagnets are nearly countless. Faraday’s Law of Induction forms the basis for many aspects of our modern society including not only electric motors and generators, but electromagnets of all sizes. The same principle used by a giant crane to lift junk cars at a scrap yard is also used to align microscopic magnetic particles on a computer hard disk drive to store binary data, and new applications are being developed every day.