What is magnetism? | Magnetic fields and magnetic force

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Magnetism is one aspect of the combined electromagnetic force. It refers to physical phenomena resulting from the force caused by magnets, objects that produce fields that attract or repel other objects.

A magnetic field exerts a force on the particles in the field due to the Lorentz force, according to the Georgia State University HyperPhysics website. The movement 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 speed of the particle, and the strength of the magnetic field.

All materials are subject to magnetism, some more strongly than others. Permanent magnets, made from materials such as iron, experience the strongest effects known as ferromagnetism. With rare exceptions, it is the only form of magnetism strong enough to be felt by people.

Opposites attract

Magnetic fields are generated by rotating electrical charges, according to HyperPhysics. 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 “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 out. 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, according to the Resource Center for Non-Destructive Testing (NDT). 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 felt on a macroscopic scale.

Magnetic field sources are dipolar, having a north and south magnetic pole. Opposite poles (N and S) attract, and like poles (N and N, or S and S) repel each other, according to Joseph Becker of San José State University. This creates a toroidal or donut-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 electric currents flowing through the molten metal core, according to Hyperphysical. 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 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 the needles of the compasses.

Ferromagnetism

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 naturally occurring iron ore called magnetite, Fe3O4. Pieces of magnetite can be found strewn on or near the surface of the earth, and sometimes one will be magnetized. These natural magnets are called magnetites. “We are still not certain of their origin, but most scientists believe that 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 a magnetite, causing the majority of 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 was still aligned in a north-south direction. The magnetic compass thus becomes a formidable navigation aid, especially day and night when the stars are hidden by the clouds.

Besides iron, other metals have ferromagnetic properties. These include nickel, cobalt and certain rare earth metals like 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. Diamagnetism was first discovered in 1778 by Anton Brugnams, who used permanent magnets in his search for materials containing iron. 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 and almost purple colored bismuth exhibited a peculiar phenomenon in the study; because 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.

Bismuth was determined to have the strongest diamagnetism of all the 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, 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 generally too weak to be noticeable. There are, however, a few notable exceptions.

Pyrolytic carbon, a substance similar to graphite, exhibits even stronger diamagnetism than bismuth, albeit along a single axis, and can in fact levitate above a super strong rare earth magnet. Some superconducting materials exhibit even greater diamagnetism below their critical temperature, and rare earth magnets may therefore be levitating above them. (In theory, due to their mutual repulsion, one can be levitating above the other.)

Paramagnetism occurs when a material becomes temporarily magnetic when placed in a magnetic field and reverts to its non-magnetic state as soon as the external field is removed. When a magnetic field is applied, some of the unpaired electronic spins align with the field and crush the opposing force produced by diamagnetism. However, the effect is only noticeable at very low temperatures, according to 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 the behavior of spin glass, which involves both ferromagnetic and antiferromagnetic interactions. In addition, ferrimagnetism can be considered as 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.

Electromagnetism

When a wire is moved in a magnetic field, the field induces a current in the wire. Conversely, a magnetic field is produced by a moving electric charge. This is in accordance with Faraday’s law of induction, which is the basis of electromagnets, electric motors and generators. A load moving in a straight line, such as through a straight wire, generates a magnetic field that wraps around the wire. When this wire is formed into a loop, the field takes the form of a donut or a torus. According to Magnetic recording manual (Springer, 1998) by Marvin Cameras, 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 turned on and off with current. This field can then deflect a movable iron lever causing an audible click. It is the basis of the telegraph, invented in the 1830s by Samuel FB Morse, which allowed long distance communication over wires using a binary code based on long and short duration pulses. The pulses were sent by skilled operators who quickly turned the power on and off using a spring-loaded momentary contact switch or 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 caused to move in a pattern of varying frequency and amplitude to induce current in a coil. It is the basis of a number of devices, in particular the microphone. The sound causes a diaphragm to move inward and outward with the different pressure waves. If the diaphragm is connected to a moving magnetic coil around a magnetic core, it will produce a variable current analogous to the incident sound waves. This electrical signal can then be amplified, recorded or transmitted at will. Super powerful tiny rare earth magnets are now used to make miniaturized cell phone microphones, Marsh told Live Science.

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 of a magnetic core in the same pattern. The coil is then attached to a moving speaker cone so that it can reproduce audible sound waves in the air. The first practical application of the microphone and speaker was the Phone, patented by Alexander Graham Bell in 1876. Although this technology has been improved and refined, it remains the basis for recording and reproducing sound.

The applications of electromagnets are almost innumerable. Faraday’s Law of Induction forms the basis for 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 for lifting unwanted cars in a junkyard is also used to align microscopic magnetic particles on a computer hard drive to store binary data, and new applications are developed every day.

Editor-in-chief Tanya Lewis contributed to this report.

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