Magnetism

Magnets have many different shapes. The most common are bars and thick disks, squares, or rectangles. A horseshoe magnet is a bar magnet bent into a U shape.

Magnets have a wide variety of uses. Magnets stick to certain metals, which makes them useful as fasteners and latches. Electric tools, appliances, and trains require magnets to run because all electric motors basically consist of a rotating electrical conductor situated between the poles of a stationary magnet. Huge magnets move iron and steel scrap. Tiny magnets on audiotape and videotape store sound and images. Magnets in telephones, radios, and TV sets help change electrical impulses into sounds. Scientists use powerful magnets to hold extremely hot gases in nuclear energy research.

Some rocks, minerals, and meteorites are natural magnets. The earth itself is a giant magnet, and so are the sun and other stars and most of the planets. Some insects, birds, and fish have extremely small magnets in their bodies. Biologists think these magnets may help animals find their way when migrating.

People in ancient Greece and China independently discovered magnetism when they found that the mineral magnetite attracted iron. Scientists could not explain what caused magnetism, however, until the mid-1800's.

Attraction and repulsion. Magnetism causes unlike magnetic poles to attract each other but like poles to repel (push away from) each other. If the north pole of a magnet is brought near the south pole of another magnet, the magnetic force pulls the magnets together. But two north poles or two south poles repel each other. If a bar magnet is suspended between the ends of a horseshoe magnet, it will move so that its north pole faces away from the horseshoe magnet's north pole.

Magnetic fields. The region around a magnet where the force of magnetism can be felt is said to contain a magnetic field. A magnetic field is invisible. You can picture the magnetic field of a bar magnet, however, if you place a piece of paper over the magnet and sprinkle iron filings on the paper. The filings bunch together near the poles and form a pattern around the magnet that corresponds to its magnetic field. A magnetic field can also be thought of as a set of imaginary lines called field lines, flux lines, or lines of force. We think of these lines going out from the north pole of a magnet, looping around, and returning to the magnet at its south pole. The lines lie closest to each other near the poles, where the magnetic field is strongest.

A magnetic field can be shown as imaginary lines that flow out of the north pole and into the south pole of a magnet. The magnetic field of a bar magnet, left, is strongest near the magnet's poles, where the lines lie closest to each other. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram by J. Harlan Hunt.

A magnetic field exerts a force on nearby magnets to make them align along its field lines. The needle of a magnetic compass, for example, is actually a slender bar magnet. It normally points north along one of the earth's magnetic field lines. But a strong bar magnet placed next to the compass will cause the needle to point along one of the bar magnet's field lines.

The strength of a magnetic field is measured in units called gauss or tesla. One tesla equals 10,000 gauss. The earth's magnetic field at its surface is about 1/2 gauss. The field near the poles of a small horseshoe magnet may be several hundred gauss. Fields of magnets used in industry may measure more than 20,000 gauss (2 tesla).

Magnetization. A magnet attracts iron, steel, nickel, and certain other materials. The attracted materials then become magnets themselves in a process called magnetization. A steel nail placed near a magnet, for example, becomes magnetized and can attract a second nail. Magnetization occurs because the magnet causes spinning particles called electrons in the atoms of the nail to align along the magnet's field lines. The atoms with aligned electrons then act like tiny bar magnets.

Temporary magnets are made of such materials as iron and nickel. These materials are known as soft magnetic materials because they usually do not retain their magnetism outside a strong magnetic field. A magnetized iron nail, for example, loses its magnetism if it is removed from a magnetic field.

Permanent magnets keep their magnetism after they have been magnetized. For this reason, they are known as hard magnetic materials. Many strong permanent magnets are alloys (mixtures) of iron, nickel, or cobalt with other elements. These magnetic alloys include Alnico, a group of alloys usually containing a mixture of aluminum, nickel, cobalt, iron, and copper; and an alloy of cobalt and chromium called cobalt-chromium. Alloys containing metallic elements called rare-earth elements have produced some of the strongest permanent magnets. These alloys include samarium-cobalt, a mixture of cobalt and the rare-earth element samarium; and a combination of boron, iron, and the rare-earth element neodymium. Another important group of magnetic alloys, called ferrites, consist of iron, oxygen, and other elements. The best-known natural permanent magnet is a ferrite known as magnetite or lodestone (also spelled loadstone).

Some soft magnetic materials can be made into weak permanent magnets. An iron needle for a compass, for example, can be permanently magnetized by stroking it in one direction with a magnet.

Electromagnets are temporary magnets produced by electric currents. The simplest electromagnets consist of electric current flowing through a cylindrical coil of wire called a solenoid. One end of the solenoid becomes the north pole of the electromagnet, while the other end becomes the south pole. The poles switch position if the direction of the current is reversed. If the current is shut off, the solenoid loses its magnetism.

Many electromagnets have a cylinder of soft magnetic material, such as iron, within a coil of wire to strengthen the magnetic field the electromagnet produces. When current passes through the coil, the cylinder becomes strongly magnetized. The cylinder loses its magnetism, however, when the current is shut off. This characteristic of electromagnets makes them useful as switches in electric doorbells and telegraphs.

The strength of an electromagnet depends on the number of windings in the coil and the strength of the electric current. More windings and stronger current produce more intense magnetic fields. Fields of about 250,000 gauss (25 tesla) have been produced by passing extremely strong electric current through a coil made of copper plates. These magnets require cooling systems that pump water past the coils, however, to prevent the heat produced by the current from melting the copper plates. Some electromagnets, called superconducting magnets, use coils that conduct current with no loss of energy and, thus, do not heat up. The strongest electromagnets, called hybrid magnets, consist of a water-cooled electromagnet within a superconducting magnet. These devices can produce magnetic fields of about 350,000 gauss (35 tesla).

Audiotape and videotape players have electromagnets called heads that record and read information on tapes covered with many tiny magnetic particles. The magnetic field of a recording head makes the magnetic particles on the tape form patterns that another type of head can read. The second head transforms the magnetic patterns into an electric signal. Magnets in speakers transform the signal into sound by making the speakers vibrate. An electromagnet called a deflection yoke in TV picture tubes helps form images on a screen.

In industry and business, magnets in electric motors help run almost any machine that makes something move or rotate. These devices include cranes, cutters, electric typewriters, fax machines, machine tools, photocopiers, and printing presses. Magnets in computers store information on magnetic tapes and disks. Powerful electromagnets attached to cranes move scrap iron and steel and separate metals for recycling.

One of the most important uses of magnets is electric power production. Generators in power plants rely on magnets similar to those in electric motors to produce electricity. Devices called transformers use electromagnets to change the high-voltage electricity carried by power lines to the lower voltage needed in homes and businesses.

In transportation. All electrified transportation systems depend on magnets in electric motors. These systems include trains, subways, trolleys, monorails, cable cars, escalators, elevators, and moving sidewalks. Electric motors operate windshield wipers, electric windows and doors, door locks, and other devices in automobiles, buses, and airplanes. Electromagnets also produce radio waves in radar systems, an important navigation aid for ships and airplanes.

Scientists and engineers have developed trains that use electromagnets to levitate (float) above a track without touching it. These trains, called magnetic levitation or maglev trains, eliminate the friction of wheels on the track and thus can move at much higher speeds than ordinary trains do.

In science and medicine. Magnets and magnetic fields are widely used in scientific research. Electromagnets in electron microscopes focus a beam of electrons on a sample to be studied. Powerful magnets called bending magnets help control beams of atomic particles that have been boosted to high speed in devices called particle accelerators. In nuclear energy research, physicists make the nuclei of atoms fuse (unite) in extremely hot gases called plasmas. The plasmas are so hot they would melt the walls of any container made of ordinary materials. Therefore, physicists hold the plasmas away from the container's walls in a strong magnetic field that functions as a "magnetic bottle."

In medicine, many devices for diagnosing diseases use magnets. In a technique known as magnetic resonance imaging (MRI), the patient lies inside a large cylindrical magnet. MRI uses magnetic fields and radio waves to produce images of the head, spine, internal organs, and other body parts. Other diagnostic devices enable physicians to observe magnetic fields generated by the brain, heart, and other internal organs.

The direction of the magnetic field around a straight wire can be determined according to the right-hand rule. If the thumb of the right hand points along the flow of current, the fingers curl around the wire in the direction of the magnetic field.

The right-hand rule shows the direction of the magnetic field around a wire that carries an electric current. If the thumb of the right hand points along the flow of current in a straight wire, left, the fingers curl around the wire in the direction of the field. If a wire carrying current is wound into a coil, the magnetic field is strengthened. Such a coil is called a solenoid. The direction of the magnetic field surrounding a solenoid, right, can be found by wrapping the fingers around the coil in the direction of the current. The thumb then points to the solenoid's north pole and shows the direction of the field. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustrations by Richard Lo.

The right-hand rule also applies to the magnetic field produced by a coil or solenoid. Magnetic field lines flow through the length of a coil. If the fingers of the right hand curl around the coil in the direction of the current, the right thumb points to the coil's north pole and shows the direction of the magnetic field lines.

The right-hand rule is used when the current is thought of as a flow of positive electric charges. In a simple electric circuit connected to a battery, for example, the current is defined as flowing from the battery's positive terminal to its negative terminal.

Magnetism in atoms. Atoms have a small, dense center called a nucleus surrounded by one or more lighter, negatively charged electrons. Nuclei consist of protons, which have positive charges, and neutrons, which have no charge. Under most conditions, the atoms of each element contain an equal number of protons and electrons, and so the atoms are electrically neutral.

The relationship between magnetism and electricity also operates in the atom. The motion of negatively charged electrons around a nucleus makes an electric current, which produces a magnetic field. However, the effect of the electrons moving in one direction equals the effect of the electrons moving in the opposite direction. As a result, the magnetic fields of the moving electrons cancel each other out, and the atom has no magnetic field.

In addition to circling the nucleus, an electron spins on its axis like a top. This motion also produces an electric current and a magnetic field. But in most atoms, one electron spins in one direction for each electron that spins in the opposite direction. The magnetic fields caused by the spinning motion of the paired electrons cancel each other out.

The orbiting motions of paired electrons change slightly when an atom is placed in a magnetic field. The magnetic fields of the electrons then no longer cancel each other out, and their motions produce a weak magnetic field opposite to the external field. This effect is known as diamagnetism (opposite magnetism). The atoms making up most chemical compounds are held together by chemical links called bonds that consist of paired electrons. As a result, most compounds--including water, salt, and sugar--are diamagnetic. Diamagnetic materials are weakly repelled by magnets.

In some atoms, including those of cobalt, iron, nickel, oxygen, and the rare-earth element gadolinium, the spins of some electrons are not paired. As a result, each atom has a magnetic field and acts like a tiny magnet. Such an atom is called an atomic dipole. These atoms, like other magnets, tend to align themselves parallel to the lines of an external magnetic field. This alignment is called paramagnetism (same magnetism) and causes the individual atoms to be weakly attracted to magnets.

Magnetism of materials. In some paramagnetic materials, the atomic dipoles arrange themselves in certain patterns in relation to each other. These arrangements include ferromagnetic, antiferromagnetic, and ferrimagnetic ordering. In ferromagnetic materials, such as iron, an atomic dipole points in the same direction as neighboring dipoles. The ferromagnetic arrangement produces the most strongly magnetic substances. An atomic dipole in an antiferromagnetic material, however, points opposite to its neighbors. Antiferromagnetic materials are weakly magnetic. Ferrimagnetic ordering occurs in materials with several kinds of atoms, including magnetite and ferrite alloys. These materials have more dipoles pointing in one direction than in the other and are strongly magnetic.

Atomic dipoles of ferromagnetic and ferrimagnetic materials settle into an ordered arrangement when the material's temperature falls below its magnetic ordering temperature or Curie point. For antiferromagnetic materials, this temperature is called the Neel temperature. Iron, for example, has a magnetic ordering temperature of 1418 ºF (770 ºC); nickel, 676 ºF (358 ºC); and cobalt, 2050 ºF (1121 ºC). Above this temperature, stronger atomic vibrations prevent the atomic dipoles from arranging themselves in relation to each other. As a result, the materials then show only the weak magnetic attraction of paramagnetism.

In ferromagnetic and ferrimagnetic materials, the atomic dipoles usually align to form larger dipoles called magnetic domains. Domains combine the strength of the individual atomic dipoles. A piece of magnetic material may contain many magnetic domains. The domains often point in different directions, however, and tend to cancel each other out.

Ferromagnetic or ferrimagnetic materials become magnetized when exposed to a strong magnetic field. The domains parallel to the field grow as more atomic dipoles line up with it. If the magnetic field is extremely strong, all the atomic dipoles may align and the entire piece of material may become a single magnetic domain. The domains of a hard magnetic material remain aligned when removed from a magnetic field. Thus, the material becomes a permanent magnet. Soft magnetic materials, however, become demagnetized when removed from the field--that is, their original magnetic domains re-form and cancel each other out.

The magnetic field at the surface of the earth, known as the geomagnetic field, has a strength of about 1/2 gauss. The earth's inner structure creates the geomagnetic field. The earth's crust is the outermost portion on which we live. A rocky mantle lies beneath the crust. Under the mantle is a dense core, which has a solid inner part and a liquid outer part. Scientists believe the motion of electric charges in the liquid outer core produces the geomagnetic field.

Scientists who study the lava of ancient volcanoes have found that the geomagnetic field periodically reverses direction--that is, the earth's north and south magnetic poles switch places. Lava contains small particles of hard magnetic material. When the lava was hot, these magnetic particles were paramagnetic and, thus, were only weakly influenced by the earth's magnetic field. But once the lava cooled below the magnetic ordering temperature, the particles aligned themselves with the geomagnetic field like tiny compass needles. Thus, lava leaves a record of the geomagnetic field at the time when the lava cooled.

The earth's magnetic field also extends into space beyond the atmosphere. There, it is called the magnetosphere. The magnetosphere interacts with a flow of charged particles from the sun called the solar wind. This interaction produces displays of light called auroras and a zone of charged particles around the earth known as the Van Allen belts.

The magnetism of the sun. The sun has an overall magnetic field of about 1 to 2 gauss. But it also has stronger magnetic fields concentrated in relatively cooler areas of its surface called sunspots. These regions have magnetic fields of 250 to 5,000 gauss. Other solar features associated with strong magnetic fields include bright bursts of light called flares and huge arches of gas known as prominences.

The magnetism of other astronomical bodies. The moon has virtually no magnetic field because it does not have a liquid core. But moon rocks brought to the earth by astronauts show that the moon at one time had a stronger magnetic field. This evidence suggests that it once probably had a liquid core. Mercury, Venus, and Mars all have weaker fields than the earth's. But Saturn, Jupiter, Neptune, and Uranus have relatively strong magnetic fields and magnetospheres.

Some types of stars have magnetic fields much stronger than the sun's. These stars include white dwarfs, which can have magnetic fields of more than 1 million gauss. A type of collapsed star called a neutron star can have a field as strong as 10 trillion gauss.

Scientists have also found that some bacteria in water use the geomagnetic field to find their preferred habitat. Each of the bacteria, called magnetotactic bacteria, contains one or more chains of magnetite particles. The bacteria use the particles as tiny compass needles to guide them along geomagnetic field lines.

In 1269, a French soldier named Pierre de Maricourt (also known as Petrus Peregrinus) mapped the magnetic field around a lodestone sphere with a compass. He discovered that the sphere had two magnetic poles. William Gilbert, a physician of Queen Elizabeth I of England, concluded in 1600 that the earth itself is a giant magnet with north and south poles.

Electricity and magnetism. In 1820, Hans Christian Oersted, a Danish physicist and chemist, observed that an electric current flowing in a wire caused the needle of a magnetic compass to rotate. His discovery proved that electricity and magnetism were related. The French physicist Andre Marie Ampere worked out the mathematical relationship between current and the strength of the magnetic field during the 1820's. He also proposed that electric current in atoms caused magnetism. In the early 1830's, the English scientist Michael Faraday and the American physicist Joseph Henry independently discovered that a changing magnetic field induced a current in a coil of wire. In 1864, James Clerk Maxwell, a Scottish scientist, developed the mathematical theory that described the laws of electricity and magnetism.

The magnetic properties of materials became a focus of research in the late 1800's. The French physicist Pierre Curie found that ferromagnetic materials lose their ferromagnetism above a certain temperature, which became known as the Curie point.

In the early 1900's, a number of physicists developed the theory of quantum mechanics, which describes the behavior of electrons and other particles. The pioneers of quantum theory included Niels Bohr of Denmark, Wolfgang Pauli of Austria, and the German physicists Albert Einstein, Werner Heisenberg, Max Planck, and Erwin Schrodinger. Richard P. Feynman and Julian S. Schwinger of the United States and Sin-itiro Tomonaga of Japan later developed an improved theory of quantum electrodynamics. Their work led to a better understanding of the interaction between charged particles and an electromagnetic field. John H. Van Vleck of the United States and Louis E. F. Neel of France applied quantum mechanics to understand the magnetic properties of atoms and molecules.

Modern research in magnetism. In the 1940's, the American physicists Edward M. Purcell and Felix Bloch independently developed a way to measure the magnetic field of nuclei. They placed a substance in a strong magnetic field and exposed it to radio waves. They discovered that the waves interacted with the nuclei of the substance's atoms. This discovery, known as nuclear magnetic resonance, led to magnetic resonance imaging and other methods for studying the structure of living tissues.

The American physicist Francis Bitter pioneered in developing stronger magnets for research. In the 1930's, he developed electromagnets made of water-cooled copper plates that generated powerful magnetic fields. In the 1960's and 1970's, scientists developed superconducting materials. When cooled to near absolute zero (-459.67 ºF or -273.15 ºC), these materials could be used in magnets to generate fields as high as 200,000 gauss. Superconducting magnets are used in maglev trains and in nuclear research. In the 1980's and 1990's, researchers discovered materials that become superconducting at higher, though still extremely cold, temperatures--about -280 ºF (-173 ºC). These new superconductors will enable scientists to generate even stronger fields.

Contributor: Richard B. Frankel, Ph.D., Prof. of Physics, California Polytechnic State Univ. and Brian B. Schwartz, Ph.D., Prof. of Physics, Brooklyn College; Associate Executive Officer, The American Physical Society.

Related articles include:

Armature; Electric Generator; Electric Motor; Electricity; Electromagnet; Electromagnetism; Hall Effect; Lenz's Law; Linear Electric Motor; Magnetic Amplifier; Magneto; Quantum Mechanics; Sun; Superconductivity.

What do scientists think causes the earth's magnetic field?

What is a magnetic dipole?

How are atomic dipoles arranged in ferromagnetic materials?

Who developed the mathematical theory that describes the relationship between magnetism and electricity?

What is the right-hand rule?

Why are ferromagnetic and ferrimagnetic materials useful in making permanent magnets?

Who discovered magnetism?

What is the magnetic ordering temperature or Curie point?

How is a "magnetic bottle" used in nuclear energy research?

Gardner, Robert. Electricity and Magnetism. 21st Century Bks., 1994.

VanCleave, Janice P. Janice VanCleave's Magnets. Wiley, 1993. Younger readers.

Wong, Ovid K. Experimenting with Electricity and Magnetism. Watts, 1993.

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