Magnetism is an aspect of the combined electromagnetic force. Magnets, which have physical phenomena due to the force it causes, are objects that produce fields that attract or repel other objects.
The magnetic field exerts a force on the particles in the field due to the Lorentz force. The movement of electrically charged particles causes 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 experience magnetism, some stronger than others. Permanent magnets, made from materials such as iron, experience the strongest effects known as ferromagnetism. With rare exceptions, this is the only form of magnetism strong enough to be felt by humans.
Attraction of Opposites
Magnetic fields are generated by rotating electrical charges. Electrons all have an angular momentum or spin property. According to the Pauli Exclusion Principle, it tends to form pairs that state that two electrons cannot occupy the same energy state at the same time. In this case, they cancel each other out because their magnetic fields are in opposite directions. However, some atoms contain one or more unpaired electrons that can create a rotational magnetic field. The direction of their rotation determines the direction of the magnetic field. When a significant majority of the unpaired electrons are aligned with their rotation in the same direction, they combine to create a magnetic field strong enough to be felt on a microscopic scale.
Magnetic field sources are bipolar with north and south magnetic poles. It attracts opposing poles (N and S) and repels similar poles (N and N or S and S). This creates a toroidal or annular field as the direction of the field radiates outward from the north pole and enters from the south pole.
The world itself is a giant magnet. The planet gets its magnetic field from electric currents circulating within the molten metallic core. A compass points north because the small magnetic needle inside is hanging so it can rotate freely inside its case 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 the compass needles.
Ferromagnetism
If the alignment of the unpaired electrons continues 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 the form of magnetite, a natural iron ore called Fe3 O4. Pieces of magnetite can be found scattered above or near the earth, and sometimes one can be magnetized. These naturally occurring magnets are called lodestones. According to many studies, most scientists believed that the limestone was a magnetite struck by lightning.
People soon learned that they could magnetize an iron needle by rubbing it with a limestone, causing most of the unpaired electrons in the needle to align in one direction. Around 1000 AD, the Chinese discovered that a magnet floating in a bowl of water was always lined up in a north-south direction. Thus, the magnetic compass was a tremendous aid to navigation, especially during the day and night when the stars were hidden by clouds.
In addition to iron, other metals 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 apart from ferromagnetism, it is often too weak to be observed except in sensitive laboratory devices or at very low temperatures. Diamagnetism was first discovered by Anton Brugnams in 1778. Brugnams used permanent magnets when searching for ferrous materials
Bismuth was determined to have the strongest diamagnetism of all elements, but as Michael Faraday discovered in 1845, it is a property of all matter that will be repelled by a magnetic field.
Diamagnetism is caused by the orbital motion of electrons, which creates small circuits of current that produce weak magnetic fields. When an external magnetic field is applied to a material, these current cycles tend to be aligned to oppose the applied field. This causes all the materials to be pushed by a permanent magnet, but the resulting force is often too weak to be noticed. However, there are some notable exceptions.
Pyrolytic carbon, a graphite-like substance, shows a diamagnetism even stronger than bismuth, albeit along only one axis, and can actually rise above a super-strong rare-earth magnet. Some superconducting materials exhibit an even stronger diamagnetism below their critical temperature, and rare-earth magnets can therefore be lifted off their tops. (In theory, one can rise above the other due to their mutual repulsion.)
Paramagnetism occurs when a material becomes temporarily magnetic when placed in a magnetic field and returns to its non-magnetic state as soon as the outer field is removed. When a magnetic field is applied, some of the unpaired electron spins align themselves with the field and suppress the opposing force generated by diamagnetism. However, the effect is only noticeable at very low temperatures.
Other forms are antiferromagnetism, in which the magnetic fields of atoms or molecules are aligned side by side between more complex forms; and rotary glass behavior, which involves both ferromagnetic and antiferromagnetic interactions. In addition, ferromagnetism can be considered a combination of ferromagnetism and antiferromagnetism due to the many similarities shared between them.
Electromagnetism
When a wire is moved in a magnetic field, the field induces a current in the wire. Conversely, a magnetic field is generated by an electric charge in motion. This is in accordance with Faraday’s Law of Induction, which is the basis of electromagnets, electric motors and generators. A charge moving in a straight line, such as along a straight wire, creates a magnetic field that revolves around the wire. When this wire is formed into a loop, the field becomes a ring shape or a bagel. This magnetic field can be greatly increased by inserting a ferromagnetic metal core into the coil.
In some embodiments, the direct current is used to create a constant field in one direction that can be switched on and off with the current. This area can then make an audible click by deflecting a moving iron arm. This is the basis of the telegraph, invented by Samuel FB Morse in the 1830s, which allowed long-distance communication over wires using a binary code based on long- and short-term pulses. The pulses were sent by skilled operators who quickly turned the current on and off using a spring-loaded momentary ignition switch or wrench. Another operator on the receiving side would then translate audible clicks into letters and words.
A coil around a magnet can also be made to move in a changing pattern of frequency and amplitude to induce a current in a coil. This is the basis of a number of devices, mainly microphones. The sound causes a diaphragm to protrude with varying pressure waves. If the diaphragm is connected to a moving magnetic coil around a magnetic core, it will produce a varying current similar to incoming sound waves. This electrical signal can then be amplified, recorded or transmitted as desired. Small super powerful rare earth magnets are now used to make miniature microphones for mobile phones.
When this modulated electrical signal is applied to a coil, it generates an oscillating magnetic field that causes the coil to move in and out on a magnetic core of the same pattern. The coil is then connected to a moving speaker cone so that it can produce 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 developed and improved, it forms the basis for voice recording and reproduction.
The applications of electromagnets are almost countless. Faraday’s Law of Induction underpins 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 scrap cars at a scrap yard is also used to align microscopic magnetic particles on a computer’s hard disk drive to store binary data, and new applications are being developed every day.
