Magnet

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Magnets are materials that produce a magnetic field of their own. Extreme examples of magnets are (1) "hard" or "permanent" magnets (like refrigerator magnets), which remember how they have been magnetized, and (2) "soft" or "impermanent" magnets (like the material of the refrigerator door), which lose their memory of previous magnetizations. "Soft" magnets are often used in electromagnets to enhance (often by factors of hundreds or thousands) the magnetic field of a current-carrying wire that has been wrapped around the magnet; when the current increases, so does the field of the "soft" magnet, which is much larger than the field due to the current. Permanent magnets occur naturally in some rocks, particularly lodestone, but they are now more commonly manufactured.

Materials without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic), or repelled (diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic. Paramagnets tend to intensify the magnetic field in their vicinity, whereas diamagnets tend to weaken the magnetic field in their vicinity. "Soft" magnets, which are strongly attracted to magnetic fields, can be thought of as strongly paramagnetic; superconductors, which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.

Contents 1 Physical origin of magnetism 1.1 Permanent magnets 1.2 Electromagnets 2 Characteristics of magnets 2.1 Permanent magnets and dipoles 2.2 North-south pole designation and the Earth's magnetic field 3 Common uses for magnets and electromagnets 4 Magnetization of materials 5 Demagnetizing materials 6 Types of permanent magnets 7 Magnetic forces 7.1 Magnets and ferromagnetic materials 7.2 Magnets and diamagnetic materials 7.3 Magnets and paramagnetic materials 8 Units in Magnetism 8.1 Calculating the magnetic force 8.1.1 Force between two magnetic poles 8.1.2 Force between two nearby attracting surfaces of area A and equal but opposite magnetizations M 8.1.3 Force between two bar magnets 9 See also 10 Online references 11 Printed references 12 External links

[edit] Physical origin of magnetism

Magnetism ultimately is due to the motion of electric charge. For a macroscopic object, like a wire loop, an electric current flowing through it has a magnetic moment. Far from the loop there is a magnetic field proportional in strength to its magnetic moment.

For a microscopic object, the physical picture is more complex. An electron within an atom can have orbital angular momentum and a magnetic moment proportional to that orbital angular momentum; the electron also has intrinsic angular momentum, or spin, and a magnetic moment proportional to that spin angular momentum. The orbital and spin angular momentum of an electron are comparable in magnitude, as are their magnetic moments. Far from the electron there is a magnetic field proportional in strength to its magnetic moment.

In addition, within the atomic nucleus are both neutrons and protons, and these too have orbital and spin angular momentum, and associated magnetic moments. However, the nuclear magnetic moment typically is much smaller than the electron magnetic moment, because magnetic moment is proportional to angular momentum but inversely proportional to mass. Nevertheless, it is this relatively small nuclear magnetic moment that is responsible for nuclear magnetic resonance (NMR), which is the basis for magnetic resonance imaging (MRI).

Although most atoms and molecules have a net magnetic moment at temperatures well below room temperature, at room temperature they typically have no net magnetic moment. However, they can often be magnetized. If the orbital magnetic properties dominate, the response typically will be diamagnetic; if the intrinsic magnetic properties dominate, the response typically will be paramagnetic.

Solids are collections of atoms and molecules. At room temperature most solids are either diamagnetic or paramagnetic.

Although for many purposes it is convenient to think of a magnet as having magnetic poles, it must be remembered that no isolated magnetic pole has ever been observed. As indicated above, the proper description is ultimately one due to electrical currents. For a magnet, these currents should be thought of as circulating about its atoms, and flowing without any electrical resistance. This physical picture is due to André-Marie Ampère, and these atomic currents are known as Amperian currents. For a uniformly magnetized bar magnet in the shape of a cylinder, the net effect of the atomic currents is to make the magnet behave as if there is a sheet of current flowing around the cylinder, with local flow direction normal to the cylinder axis.

[edit] Permanent magnets

A few elements -- especially iron, cobalt, and nickel -- are ferromagnetic at room temperature. When quantum mechanics and the Pauli Exclusion Principle are accounted for, the electrical energy within these atoms is found to be lower if the magnetic moments of the valence electrons are aligned. This makes them ferromagnetic. Every ferromagnet has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy lowering due to ferromagnetic order. A perfectly aligned ferromagnet is said to have long-range order because all of its atoms have their magnetic moments pointing in the same direction. Real ferromagnets are not perfectly aligned, but rather contain perfectly aligned regions, called magnetic domains, which have their own magnetization directions.

A long bar magnet has a north pole at one end and a south pole at the other. Near either end the magnetic field falls off inversely with the square of the distance from that pole.

For a magnet of any shape, at distances large compared to its size, the strength of the magnetic field falls off inversely with the cube of the distance from the magnet's center.

[edit] Electromagnets

An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid. When electric current flows through the wire, a magnetic field is generated. It is concentrated near the coil, and its field lines are very similar to those for a magnet. The orientation of this effective magnet is determined via the right hand rule. The more loops of wire, the greater the cross-section of each loop, and the greater the current passing through the wire, the stronger the field.

If the coil of wire is wrapped around a material with no special magnetic properties (i.e., cardboard), it will tend to generate a very weak field. However, if it is wrapped around a "soft" ferromagnetic material, such as an iron nail, then the net field produced can result in a several hundred- to thousand-fold increase of field strength.

The force needed to separate an electromagnet in contact with a flat soft magnetic plate is greatest when the two surfaces are as flat as possible. This improves their contact, which assists the magnetic field from the electromagnet in going directly to the magnetic plate. In technical terms, this decreases the magnetic circuit's reluctance.

Uses for electromagnets include particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than a simple magnetic dipole; for example, quadrupole magnets are used to focus particle beams.

If enough electric current is passed through the coil of an electromagnet, the attractive magnetic force between adjacent loops of wire can cause the electromagnet to be crushed by its own magnetic field.

[edit] Characteristics of magnets

[edit] Permanent magnets and dipoles

All magnets appear to have at least one north pole (reckoned positive) and at least one south pole (reckoned negative), and the net pole strength of every magnet is zero. Despite their apparent reality, as suggested by the image at the top of the page, where iron filings concentrate in regions of large magnetic field, poles are not physical objects on or in the magnet. They are, rather, a useful concept for describing magnets. Rather than poles being the fundamental unit, it is the magnetic dipole that is the fundamental unit. A magnetic dipole can be thought of as a combination of a positive and a negative pole that are microscopically close to one another and inseparable. This is not a bad description of the magnetic dipole of an electron in a magnetic material.

The effect of aligning many dipoles and placing them head-to-tail in a line is that there appears a north pole at one end and a south pole at the other, with all the intermediate north and south poles cancelling out. The net effect is a very long dipole that appears to have poles only at its ends. Alternatively, aligning many dipoles and placing them on a sheet producing an object whose magnetic field is like that of a wire carrying current around the perimeter of the sheet. Although theories have been developed involving the possibility of north and south magnetic monopoles, no magnetic monopole has yet been found.

[edit] North-south pole designation and the Earth's magnetic field

See also: Earth's magnetic field, North Pole, South Pole, and Magnetic North Pole

A standard naming system for the poles of magnets is important. Historically, the terms north and south reflect awareness of the relationship between magnets and the earth's magnetic field. A freely suspended magnet will eventually orient itself north-to-south, because of its attraction to the north and south magnetic poles of the earth. The end of a magnet that points (approximately) toward the Earth's geographic North Pole is labeled as the north pole of the magnet; correspondingly, the end that points south is the south pole of the magnet. (The actual geographic north pole is in a slightly different location than the corresponding magnetic pole; see Magnetic North Pole.)

The Earth's present geographic north is thus actually its magnetic south. Confounding the situation further, magnetized rocks on the ocean floor show that the Earth's magnetic field has reversed itself in the past, so this system of naming is likely to be incorrect at some time in the future.

Fortunately, by using an electromagnet and the right hand rule relating the electromagnet's current and the magnetic field it produces, the orientation of the field of a magnet can be defined without reference to the Earth's geomagnetic field.

To avoid the confusion between geographic and magnetic north and south poles, the terms positive and negative are sometimes used for the poles of a magnet. The positive pole is that which seeks geographical north.

[edit] Common uses for magnets and electromagnets

• Magnetic recording media: Common VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.

• Credit, debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individual's financial institution and connect with their account(s).

• Common televisions and computer monitors: TV and computer screens using vacuum tube technology employ an electromagnet to guide electrons to the screen, in order to produce an image -- see the article on cathode ray tubes. Plasma screens and LCDs use different technologies.

• Speakers and microphones: Most speakers employ a permanent magnet and a current-carrying coil to convert electric energy (the signal) into mechanical energy (the sound). The coil is wrapped around the speaker cone, and carries the signal, producing a changing magnetic field that interacts with the field of the permanent magnet. The low mass coil feels a magnetic force and in response moves the cone and the neighboring air, thus generating sound. Standard microphones employ the same concept, but in reverse. A microphone has a cone or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is generated in the coil (see Lenz's Law). This voltage drives current in the wire that is characteristic of the original sound.

• Electric motors and generators: Some electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.

• Transformers: Transformers are devices that transfer electric energy between two windings that are electrically isolated but are linked magnetically.

• Chucks: Chucks are used in the metalworking field to hold objects. If these objects can be held securely with a magnet then a permanent or electromagnetic chuck may be used. Magnets are also used in other types of fastening devices, such as the magnetic base, the magnetic clamp and the refrigerator magnet.

• Magic: Naturally magnetic Lodestones as well as iron magnets are used in conjunction with fine iron grains (called "magnetic sand") in the practice of the African-American folk magic known as hoodoo. The stones are symbolically linked to people's names and ritually sprinkled with magnetic sand to reveal the magnetic field. One stone may be utilized to bring desired things to a person; a pair of stones may be manipulated to bring two people closer together in love.

• Art: 1 mm or thicker vinyl magnet sheets may attached to paintings, photographs, and other ornamental articles, allowing them to be stuck to refrigerators and other metal surfaces.

• Science Projects: Many topic questions are often based on magnets. For example; how is the strength of a magnet affected by glass, plastic, and cardboard?

• Magnets can be used to make jewelry. Necklaces and bracelets can have a magnetic clasp. Necklaces and bracelets can be made from small but strong, cylindrical magnets and slightly larger iron or steel balls connected in a pattern that is repeated until it is long enough to fit on the wrist or neck. These accessories may be fragile enough to accidentally come apart, but they also can be disassembled and reassembled with a different design. When connected as a necklace or a bracelet, magnets lose their attraction to other pieces of iron steel because they are already attached to their own iron and steel balls. Magnetic lip-rings and earrings are sometimes employed to avoid piercing.

• Most children enjoy playing with magnets; they usually try to attach the magnets to metallic objects to see if the objects are magnetic. The more children play with magnets, the more they learn.

• Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold.

• A recently developed use of magnetism is to connect portable computer power cables. Such a connection will occasionally break by accidentally pushing against the cable, but the computer battery prevents interruption of service, and the easy disconnection protects the cable from serious jerks or from being stepped on.

[edit] Magnetization of materials

Ferromagnetic materials can be magnetized in the following ways:

• Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the earth's magnetic field and which are subject to vibration (e.g. frame of a conveyor) have been shown to acquire significant residual magnetism.
• Placing the item in a solenoid with a direct current passing through it.
• Stroking - An existing magnet is moved from one end of the item to the other repeatedly in the same direction.
• Placing a steel bar in a magnetic field, then heating it to a high temperature and then finally hammering it as it cools. This can be done by laying the magnet in a North-South direction in the Earth's magnetic field. In this case, the magnet is not very strong but the effect is permanent.

[edit] Demagnetizing materials

Permanent magnets can be demagnetized in the following ways:

• Heating a magnet past its Curie point will destroy the long range ordering.
• Contact through stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.
• Hammering or jarring will destroy the long range ordering within the magnet.
• A magnet being placed in a solenoid which has an alternating current being passed through it will have its long range ordering disrupted, in much the same way that direct current can cause ordering.

In an electromagnet which uses a soft iron core, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis. Magnet can be characterized either by

[edit] Types of permanent magnets

• Rare Earth types:
  • Neodymium magnets - The second generation of rare-earth magnets are made from sintered neodymium, iron and small amounts of boron. These magnets have the highest energy product of any permanent magnetic material.
  • Samarium-cobalt magnets - A sintered rare-earth magnetic material made of samarium and cobalt. These magnets are corrosion and temperature resistant.
  • Ceramic magnets - A low cost sintered magnet made from a composite of iron oxide and barium/strontium carbonate.
  • Alnico magnets - t made from aluminum, nickel and cobalt.
  • Injection Molded/Bonded - A magnet made by the combination of resins and magnetic powder to form a soft and flexible magnetic material.
  • Plastic magnets

[edit] Magnetic forces

Magnetized items interact with other items in very specific ways.

[edit] Magnets and ferromagnetic materials

If a magnet is brought close enough to a ferromagnetic material (which is not magnetized itself), the magnet will strongly attract the ferromagnetic material regardless of orientation. Both the north and south pole of the magnet will attract the other item with equal strength.

[edit] Magnets and diamagnetic materials

By definition, diamagnetic materials weakly repel a magnetic field. This occurs regardless of the north/south orientation of the field.

[edit] Magnets and paramagnetic materials

By definition, paramagnetic materials are weakly attracted to a magnetic field. This occurs regardless of the north/south orientation of the field.

[edit] Units in Magnetism

Two sets of units are commonly employed in magnetism. How we write the laws of magnetism depends on which set of units we employ. We consider first the units known as SI (Système Internationale). The other set of units is actually two sets -- Gaussian and cgs-emu -- but these are the same for magnetic properties. Magnetic units is the subject of much frustration in the area of magnetism, as the SI units replace the older cgs-emu units; some magneticians have grown up on one set of units and some on the other.

In all units it is convenient to employ two types of magnetic field, B and H, as well as the magnetization M, defined as the magnetic moment per unit volume.

(1) The magnetic induction field B is given in SI units of T (tesla). B is the true magnetic field, whose time-variation produces, by Faraday's Law, circulating electric fields (which the power companies sell). B also produces a deflection force on moving charged particles (as in TV tubes). The tesla is equivalent to the magnetic flux (in webers) per unit area (in meters squared), thus giving B the unit of a flux density. In cgs-emu the unit of B is G (gauss). One T equals 10⁴ G.

(2) The magnetic field H is given in SI units of ampere-turns/meter (A-turn/m). The "turns" appears because when H is produced by a current-carrying wire, its value is proportional to the number of turns of that wire. In cgs-emu the unit of H is Oe (oersted). One A-turn/m equals 4π x 10^-3 Oe.

(3) The magnetization M is given in SI units of ampere/meter (A/m). In cgs-emu the unit of M is emu. One A/m equals 10^-3 emu. A good permanent magnet can have a magnetization as large as a million A/m. Magnetic fields produced by current-carrying wires would require comparably huge currents per unit length, one reason we employ permanent magnets and electromagnets.

(4) In SI units, the relation B=μ₀(H+M) holds, where μ₀ is the permeability of space, which equals 4π x 10^-7 tesla∙meter/ampere. In cgs-emu it is written as B=H+4πM.

Materials that are not permanent magnets usually satisfy the relation M=χH in SI, where χ is the (dimensionless) magnetic susceptibility. Most non-magnetic materials have a relatively small χ (on the order of a millionth), but soft magnets can have χ's on the order of hundreds or thousands. For materials satisfying M=χH, we can also write B=μ₀(1+χ)H=μ₀μ_rH=μH, where μ_r=1+χ is the (dimensionless) relative permeability and μ = μ₀μ_r is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops, which give either B vs H or M vs H. In cgs-emu M=χH, but χ(SI) = 4πχ(cgs − emu), and μ = μ_r.

Caution: In part because there are not enough Roman and Greek symbols, there is no commonly agreed upon symbol for magnetic pole strength and magnetic moment. The symbol m has been used for both pole strength (unit = A-m, where here "m is for meter") and for magnetic moment (unit = A-m²). The symbol μ has been used in some texts for magnetic permeability and in other texts for magnetic moment. We will use μ for magnetic permeability and m for magnetic moment. For pole strength we will employ q_m. For a bar magnet of cross-section A with uniform magnetization M along its axis, the pole strength is given by q_m=MA, so that M can be thought of as a pole strength per unit area.

[edit] Calculating the magnetic force

Calculating the attractive or repulsive force between two magnets is, in the general case, an extremely complex operation, as it depends on the shape, magnetization, orientation and separation of the magnets.

[edit] Force between two magnetic poles

The force between two magnetic poles is given by:

[1]

where

F is force (SI unit: newton)

q_m1 and q_m2 are the pole strengths (SI unit: ampere-meter)

μ is the permeability of the intervening medium (SI unit: tesla meter per ampere or henry per meter)

r is the separation (SI unit: meter).

The pole description is useful to practicing magneticians who design real-world magnets, but real magnets have a pole distribution more complex than a single north and south. Therefore, implementation of the pole idea is not simple. In some cases, one of the more complex formulae given below will be more useful.

[edit] Force between two nearby attracting surfaces of area A and equal but opposite magnetizations M

[2]

where

A is the area of each surface, in m²

M is their magnetization, in ampere/m².

μ₀ is the permeability of space, which equals 4π x 10^-7 tesla∙meter/ampere

[edit] Force between two bar magnets

The force between two identical cylindrical bar magnets placed end-to-end is given by:

[3]

where

B₀ is the magnetic flux density very close to each pole, in T,

A is the area of each pole, in m²,

L is the length of each magnet, in m,

R is the radius of each magnet, in m, and

x is the separation between the two magnets, in m

B₀=M relates the flux density at the pole to the magnetization of the magnet.

[edit] See also

• B-H Analyzer
• electromagnet
• electromagnetism
• electromagnetic field
• neodymium magnet
• diamagnetism
• magnetic dipole
• magnetic monopole
• magnetism
• molecular magnet
• paramagnetism
• single-molecule magnet
• magnet therapy
• supermagnets

[edit] Online references

• HyperPhysics E/M, good complete tree diagram of electromagnetic relationships with magnets
• Maxwell's Equations and some history...
• Detailed Theory on Designing a Solenoid or a Coil Gun

[edit] Printed references

1. "positive pole n." The Concise Oxford English Dictionary. Ed. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University Press.

2. Wayne M. Saslow, "Electricity, Magnetism, and Light", Academic (2002). ISBN 0-12-619455-6. Chapter 9 discusses magnets and their magnetic fields using the concept of magnetic poles, but it also gives evidence that magnetic poles don't really exist in ordinary matter. Chapters 10 and 11, following what appears to be a 19th century approach, use the pole concept to obtain the laws describing the magnetism of electric currents.

[edit] External links

• Joseph J. Stupak Jr., "Methods of Magnetizing Permanent Magnets"PDF (223 KiB). Oersted Technology at the EMCW Coil Winding Show, 2000.
• Floating Magnet
• Magnets and Electromagnets - Video
• Magnet Technology Center Commercial source of information on permanent magnetic materials.
• Magnet Design Guide Commercial compilation on permanent magnet design.
• Research Papers on Magnetics Commercial company compilation of research papers related to permanent magnets.
• Answers to several questions from curious kids about magnets
• Magnetic units are discussed here