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Electromagnetism

Electromagnetism is the physics of the electromagnetic field; a fieldencompassing all ofspace which exerts a force on particles that possess theproperty of electric charge, and is in turn affected by the presence andmotion of those particles.

The magnetic field is produced by the motion of electric charges, i.e.electric current. The magnetic field causes the magnetic force associatedwith magnets.

The term "electromagnetism" comes from the fact that electrical andmagnetic forces are involved simultaneously. A changing magnetic fieldproduces an electric field (this is the phenomenon of electromagneticinduction, which provides for the operation ofelectrical generators, inductionmotors, and transformers). Similarly, a changing electric field generates amagnetic field. Because of this interdependence of the electric and magnetic

fields, it makes sense to consider them as a single coherent entity theelectromagnetic field.

This unification, which was completed by James Clerk Maxwell, andformulated by Oliver Heaviside, is one of the triumphs of 19th centuryphysics. It had far-reaching consequences, one of which was theunderstanding of the nature of light. As it turns out, what is thought of as"light" is actually a propagating oscillatory disturbance in the electromagneticfield, i.e., an electromagnetic wave. Different frequencies of oscillation giverise to the different forms of electromagnetic radiation, from radio waves atthe lowest frequencies, to visible light at intermediate frequencies, to gammarays at the highest frequencies.

The theoretical implications of electromagnetism led to thedevelopment ofspecial relativity by Albert Einstein in 1905.

The Electromagnetic Force

The force that the electromagnetic field exerts on electrically chargedparticles, called the electromagnetic force, is one of the four fundamentalforces. The other fundamental forces are the strong nuclear force (whichholds atomic nuclei together), the weak nuclear force (which causes certainforms ofradioactive decay), and the gravitational force. All other forces areultimately derived from these fundamental forces.

As it turns out, the electromagnetic force is the one responsible forpractically all the phenomena encountered in daily life, with the exception ofgravity. Roughly speaking, all the forces involved in interactions betweenatoms can be traced to the electromagnetic force acting on the electricallycharged protons and electrons inside the atoms. This includes the forces weexperience in "pushing" or "pulling" ordinary material objects, which comefrom the intermolecular forces between the individual molecules in our

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bodies and those in the objects. It also includes all forms of chemicalphenomena, which arise from interactions between electron orbitals.

According to modern electromagnetic theory, electromagnetic forcesare mediated by the transfer ofvirtualphotons.

Classical Electrodynamics

The scientist William Gilbert proposed, in his De Magnete (1600), thatelectricity and magnetism, while both capable of causing attraction andrepulsion of objects, were distinct effects. Mariners had noticed that lightningstrikes had the ability to disturb a compass needle, but the link betweenlightning and electricity was not confirmed until Benjamin Franklin's proposedexperiments in 1752. One of the first to discover and publish a link betweenman-made electric current and magnetism was Romagnosi, who in 1802noticed that connecting a wire across a Voltaic pile deflected a nearbycompass needle. However, the effect did not become widely known until

1820, when rsted performed a similar experiment. rsted's work influencedAmpre to produce a theory of electromagnetism that set the subject on amathematical foundation.

An accurate theory of electromagnetism, known as classicalelectromagnetism, was developed by various physicists over the course ofthe 19th century, culminating in the work ofJames Clerk Maxwell, who unifiedthe preceding developments into a single theory and discovered theelectromagnetic nature of light. In classical electromagnetism, theelectromagnetic field obeys a set of equations known as Maxwell's equations,and the electromagnetic force is given by the Lorentz force law.

One of the peculiarities of classical electromagnetism is that it isdifficult to reconcile with classical mechanics, but it is compatible with specialrelativity. According to Maxwell's equations, the speed of light is a universalconstant, dependent only on the electrical permittivity and magneticpermeability of the vacuum. This violates Galilean invariance, a long-standingcornerstone of classical mechanics. One way to reconcile the two theories isto assume the existence of a luminiferous aether through which the lightpropagates. However, subsequent experimental efforts failed to detect thepresence of the aether. In 1905, Albert Einstein solved the problem with theintroduction of special relativity, which replaces classical kinematics with anew theory of kinematics that is compatible with classical electromagnetism.

In addition, relativity theory shows that in moving frames of referencea magnetic field transforms to a field with a nonzero electric component andvice versa; thus firmly showing that they are two sides of the same coin, andthus the term "electromagnetism".

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The Photoelectric Effect

In another paper published in that same year, Einstein undermined the

very foundations of classical electromagnetism. His theory of thephotoelectric effect (for which he won the Nobel prize for physics) positedthat light could exist in discrete particle-like quantities, which later came tobe known as photons. Einstein's theory of the photoelectric effect extendedthe insights that appeared in the solution of the ultraviolet catastrophepresented by Max Planck in 1900. In his work, Planck showed that hot objectsemit electromagnetic radiation in discrete packets, which leads to a finitetotal energy emitted as black body radiation. Both of these results were indirect contradiction with the classical view of light as a continuous wave.Planck's and Einstein's theories were progenitors of quantum mechanics,which, when formulated in 1925, necessitated the invention of a quantumtheory of electromagnetism. This theory, completed in the 1940s, is known as

quantum electrodynamics (or "QED"), and is one of the most accuratetheories known to physics.

Definition

The term electrodynamics is sometimes used to refer to thecombination of electromagnetism with mechanics, and deals with the effectsof the electromagnetic field on the dynamic behavior of electrically chargedparticles.

Electricity

Electricity (from Greek (electron) "amber") is a general termfor the variety of phenomena resulting from the presence and flow of electriccharge. Together with magnetism, it constitutes the fundamental interactionknown as electromagnetism. It includes many well-known physicalphenomena such as lightning, electromagnetic fields and electric currents,and is put to use in industrial applications such as electronics and electricpower.

In casual usage, the term electricity is applied to several related conceptsthat are better identified by more precise terms:

Electric potential - the capacity of an electric field to do work, typicallymeasured in volts.

Electric current - a movement or flow of electrically charged particles,typically measured in amperes.

Electric field - an effect produced by an electric charge that exerts aforce on charged objects in its vicinity.

Electrical energy - the energy made available by the flow of electriccharge through an electrical conductor.

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Electric power - the rate at which electric energy is converted to orfrom another energy form, such as light, heat, or mechanical energy.

Electric charge - a fundamental conserved property of some subatomicparticles, which determines their electromagnetic interactions.Electrically charged matter is influenced by, and produces,electromagnetic fields.

History of electricity

The ancient Greek and Parthian civilizations knew of static electricityfrom rubbing objects against fur. The Parthians may have had someknowledge ofelectroplating, based on the discovery of the Baghdad Battery,which resembles a Galvanic cell.

Benjamin Franklin conducted extensive research in electricity. Histheories on the relationship between lightning and static electricity, includinghis famous kite-flying experiment, sparked the interest of later scientists

whose work provided the basis for modern electrical technology. Mostnotably these include Luigi Galvani (17371798), Alessandro Volta (1745-1827), Michael Faraday (17911867), Andr-Marie Ampre (17751836), andGeorg Simon Ohm (1789-1854). The late 19th and early 20th centuryproduced such giants of electrical engineering as Nikola Tesla, Samuel Morse,Antonio Meucci,Thomas Edison, George Westinghouse, Werner von Siemens,Charles Steinmetz, and Alexander Graham Bell.

Electric charge

Electric charge is a property of certain subatomic particles (e.g.,electrons and protons) which interacts with electromagnetic fields and causes

attractive and repulsive forces between them. Electric charge gives rise toone of the four fundamental forces of nature, and is a conserved property ofmatter that can be quantified. In this sense, the phrase "quantity ofelectricity" is used interchangeably with the phrases "charge of electricity"and "quantity of charge". There are two types of charge: we call one kind ofcharge positive and the other negative. Through experimentation, we findthat like-charged objects repel and opposite-charged objects attract oneanother. The magnitude of the force of attraction or repulsion is given byCoulomb's law.

Electric field

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Michael Faraday

The concept of electric fields was introduced by Michael Faraday. Theelectrical field force acts between two charges, in the same way that the

gravitational field force acts between two masses. However, the electric fieldis a little bit different. Gravitational force depends on the masses of twobodies, whereas electric force depends on the electric charges of two bodies.While gravity can only pull masses together, the electric force can be anattractive or repulsive force. If both charges are of same sign (e.g. bothpositive), there will be a repulsive force between the two. If the charges areopposite, there will be an attractive force between the two bodies. Themagnitude of the force varies inversely with the square of the distancebetween the two bodies, and is also proportional to the product of theunsigned magnitudes of the two charges.

Electric potential

The electric potential difference between two points is defined as the workdone (against electrical forces) per unit of charge in moving a positive pointcharge slowly between two points. If one of the points is taken to be areference point with zero potential, then the electric potential at any pointcan be defined in terms of the work done per unit charge in moving a positivepoint charge from that reference point to the point at which the potential is tobe determined. For isolated charges, the reference point is usually taken tobe infinity. The potential is measured in volts. (1 volt = 1joule/coulomb) Theelectric potential is analogous to temperature: there is a differenttemperature at every point in space, and the temperature gradient indicates

the direction and magnitude of the driving force behind heat flow. Similarly,there is an electric potential at every point in space, and its gradientindicates the direction and magnitude of the driving force behind chargemovement.

Electric current

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Nikola Tesla

An electric current is a flow of electric charge, and its intensity ismeasured in amperes. Examples of electric currents include metallic

conduction, where electrons flow through a conductor or conductors such asa metal wire, and electrolysis, where ions (charged atoms) flow throughliquids. The particles themselves often move quite slowly, while theelectric field that drives them propagates at close to the speed of light. Seeelectrical conduction for more information.

Devices that use charge flow principles in materials are calledelectronic devices.

A direct current (DC) is a unidirectional flow, while an alternatingcurrent (AC) reverses direction repeatedly. The time average of analternating current is zero, but its energy capability (RMS value) is not zero.

Ohm's Law is an important relationship describing the behaviour of electriccurrents, relating them to voltage.

For historical reasons, electric current is said to flow from the mostpositive part of a circuit to the most negative part. The electric current thusdefined is called conventional current. It is now known that, depending on theconditions, an electric current can consist of a flow of charged particles ineither direction, or even in both directions at once. The positive-to-negativeconvention is widely used to simplify this situation. If another definition isused - for example, "electron current" - it should be explicitly stated.

Electrical energy

Electrical energy is energy stored in an electric field or transported byan electric current. Energy is defined as the ability to do work, and electricalenergy is simply one of the many types of energy. Examples of electricalenergy include:

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the energy that is constantly stored in the Earth's atmosphere, and ispartly released during a thunderstorm in the form oflightning

the energy that is stored in the coils of an electrical generator in apower station, and is then transmitted by wires to the consumer; theconsumer then pays for each unit of energy received

the energy that is stored in a capacitor, and can be released to drive a

current through an electrical circuit

Electric power

Electric power is the rate at which electrical energy is produced orconsumed, and is measured in watts (symbol is: W).

A nuclear power station.

A fossil-fuel, solar-thermal, nuclear or biomasspower station convertsheat to electrical energy, and the faster the station burns fuel, assuming

positively-sloped efficiency of conversion, the higher its power output. Theoutput of a power station is usually specified in megawatts (millions of watts).

The electrical energy is then sent over transmission lines to reach theconsumers.

Every consumer uses appliances that convert the electrical energy toother forms of energy, such as heat (in electric arc furnaces and electricheaters), light (in light bulbs and fluorescent lamps), or motion, i.e. kineticenergy (in electric motors). Like the power station, each appliance is alsorated in watts, depending on the rate at which it converts electrical energyinto another form. The power station must produce electrical energy at thesame rate as all the connected appliances consume it.

In electrical engineering, the concepts of apparent power and reactivepower are also used. Apparent power is the product of RMS voltage and RMScurrent, and is measured in volt-amperes (VA). Reactive power is measuredin volt-amperes-reactive (VAr).

Non-nuclear electric power is categorized as either green or brown electricity.

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Green power is a cleaner alternative energy source in comparison totraditional sources, and is derived from renewable energy resources that donot produce any nuclear waste; examples include energy produced fromwind, water, solar, thermal, hydro, combustible renewables and waste. Some,however, argue that nuclear energy is also a form of "clean" energy, and isone of the many ways future generations will supply themselves with energy.

Modern day nuclear power techniques have been able to greatly minimizenuclear waste output from nuclear plants.

Electricity from coal, oil, and natural gas is known as traditional power or"brown" electricity.

Electrical phenomena in nature

Matter: since atoms and molecules are held together by electricforces.

Lightning: electrical discharges in the atmosphere. The Earth's magnetic field created by electric currents circulating in

the planet's core. Sometimes due to solar flares, a phenomenon known as a power surge

can be created. Piezoelectricity: the ability of certain crystals to generate a voltage in

response to applied mechanical stress. Triboelectricity: electric charge taken on by contact or friction between

two different materials. Bioelectromagnetism: electrical phenomena within living organisms.

o Bioelectricity Many animals are sensitive to electric fields,some (e.g., sharks) more than others (e.g., people). Most alsogenerate their own electric fields.

Gymnotiformes, such as the electric eel, deliberately

generate strong fields to detect or stun their prey. Neurons in the nervous system transmit information by

electrical impulses known as action potentials.

Magnetism

In physics, magnetism is one of the phenomena by which materialsexert an attractive or repulsive force on other materials. Some well knownmaterials that exhibit easily detectable magnetic properties are iron, somesteels, and the mineral lodestone; however, all materials are influenced togreater or lesser degree by the presence of a magnetic field.

History

In China, the earliest literary reference to magnetism lies in a 4thcentury BC book called Book of the Devil Valley Master ( ): "Thelodestone makes iron come or it attracts it."[1] The earliest mention of theattraction of a needle appears in a work composed between 20 and 100 AD(Louen-heng): "A lodestone attracts a needle."[2] By the 12th century theChinese were known to use the lodestone compass for navigation. Far earlier

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Magnetotactic bacteria had evolved to build miniature magnets insidethemselves and use them to establish their orientation relative to the Earth'smagnetic field [1].

Physics of magnetism

Magnetic forces are forces that arise from the movement of electricalcharge. Maxwell's equations and the Biot-Savart law describe the origin andbehavior of the fields that govern these forces. Thus, magnetism is seenwhenever electrically charged particles are in motion. This can arise eitherfrom movement of electrons in an electric current, resulting in"electromagnetism", or from the quantum-mechanical spin and orbital motionof electrons, resulting in what are known as "permanent magnets". Electronspin is the dominant effect within atoms. The so-called 'orbital motion' ofelectrons around the nucleus is a secondary effect that slightly modifies themagnetic field created by spin.

The magnetic force is actually due

[3]

to the finite speed (the speed oflight) of a disturbance of the electric field which gives rise to forces thatappear to be acting along a line at right angles to the charges. In effect, themagnetic force is the portion of the electric force directed to where thecharge used to be. For this reason magnetism can be considered to bebasically an electric force that is a direct consequence ofrelativity.

Charged particle in a magnetic field

When a charged particle moves through a magnetic field B, it feels aforce F given by the cross product:

where is the electric charge of the particle, is the velocity vector of the

particle, and is the magnetic field.

Because this is a cross product, the force is perpendicular to both themotion of the particle and the magnetic field. It follows that the magneticforce does no work on the particle; it may change the direction of theparticle's movement, but it cannot cause it to speed up or slow down.

One tool for determining the direction of the velocity vector of a

moving charge, the magnetic field, and the force exerted is labeling the indexfinger "V", the middle finger "B", and the thumb "F" with your right hand.When making a gun-like configuration (with the middle finger crossing underthe index finger), the fingers represent the velocity vector, magnetic fieldvector, and force vector, respectively. See also right hand rule.

Magnetic dipoles

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Normally, magnetic fields are seen as dipoles, having a "South pole"and a "North pole"; terms dating back to the use of magnets as compasses,interacting with the Earth's magnetic field to indicate North and South on theglobe. Since opposite ends of magnets are attracted, the 'north' magneticpole of the earth must be magnetically 'south'.

A magnetic field contains energy, and physical systems stabilize intothe configuration with the lowest energy. Therefore, when placed in amagnetic field, a magnetic dipole tends to align itself in opposed polarity tothat field, thereby canceling the net field strength as much as possible andlowering the energy stored in that field to a minimum. For instance, twoidentical bar magnets normally line up North to South resulting in no netmagnetic field, and resist any attempts to reorient them to point in the samedirection. The energy required to reorient them in that configuration is thenstored in the resulting magnetic field, which is double the strength of the fieldof each individual magnet. (This is, of course, why a magnet used as acompass interacts with the Earth's magnetic field to indicate North andSouth).

Magnetic monopoles

The modern understanding of magnetism posits that all magneticeffects are actually due to relativistic effects [4] caused by relative motionbetween the observer and the charged particles. Since all magnetism iscaused by moving charges, all magnets are in fact electromagnets.

Even atoms have a tiny field. In the planetary model of an atom, theelectrons orbit the nucleus and thus have a change in motion giving rise to amagnetic field. Permanent magnets have measurable magnetic fields

because the atoms (and molecules) are arranged in a way that theirindividual tiny fields align and add up.

In this model, the lack of a single pole makes intuitive sense; cutting abar magnet in half does nothing to the arrangement of the molecules within,and you end up with two bars with the same arrangement, and thus the samefield. This also explains how heating or simply hitting a magnet made from asoft material will degauss it, as the molecules within are moved about.

Since all known forms of magnetic phenomena involve the motion ofelectrically charged particles, and since no theory suggests that "pole" is, inthat context, a thing rather than a convenient fiction, it may well be that

nothing that could be called a magnetic monopole exists or ever did or could.

Contrary to normal experience, some theoretical physics modelspredict the existence of magnetic monopoles. Paul Dirac observed in 1931that, because electricity and magnetism show a certain symmetry, just asquantum theory predicts that individual positive or negative electric chargescan be observed without the opposing charge, isolated South or Northmagnetic poles should be observable. In practice, however, although charged

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particles like protons and electrons can be easily isolated as individualelectrical charges, magnetic south and north poles have never been found inisolation. Using quantum theory Dirac showed that if magnetic monopolesexist, then one could explain why the observed elementary particles carrycharges that are multiples of the charge of the electron.

In modern elementary particle theory, the quantization of charge isrealized in a spontaneous breakdown of a non-abelian gauge symmetry.Monopoles predicted in certain grand unified theories differ from the oneoriginally thought of by Dirac. These monopoles, unlike elementary particles,are solitons, which are localized energy packets. If they exist at all, theycontradict cosmological observations. A solution to this monopole problem incosmology gave rise to the currently-interesting idea ofinflation.

Atomic magnetic dipoles

The physical cause of the magnetism of objects, as distinct fromelectrical currents, is the atomic magnetic dipole. Magnetic dipoles, ormagnetic moments, result on the atomic scale from the two kinds ofmovement of electrons. The first is the orbital motion of the electron aroundthe nucleus; this motion can be considered as a current loop, resulting in anorbital dipole magnetic moment along the axis of the nucleus. The second,much stronger, source of electronic magnetic moment is due to a quantummechanical property called the spin dipole magnetic moment (althoughcurrent quantum mechanical theory states that electrons neither physicallyspin, nor orbit the nucleus).

Dipole moment of a bar magnet.

The overall magnetic moment of the atom is the net sum of all of themagnetic moments of the individual electrons. Because of the tendency ofmagnetic dipoles to oppose each other to reduce the net energy, in an atomthe opposing magnetic moments of some pairs of electrons cancel eachother, both in orbital motion and in spin magnetic moments. Thus, in the case

of an atom with a completely filled electron shell or subshell, the magneticmoments normally completely cancel each other out and only atoms withpartially-filled electron shells have a magnetic moment, whose strengthdepends on the number of unpaired electrons.

The differences in configuration of the electrons in various elementsthus determine the nature and magnitude of the atomic magnetic moments,which in turn determine the differing magnetic properties of various

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materials. Several forms of magnetic behavior have been observed indifferent materials, including:

Diamagnetism Paramagnetism

o Molecular magnet Ferromagnetism

o Antiferromagnetismo Ferrimagnetismo Metamagnetism

Spin glass Superparamagnetism

Magnetars, stars with extremely powerful magnetic fields, are also known toexist.

Types of magnets

Electromagnets

Electromagnets are useful in cases where a magnet must be switchedon or off; for instance, large cranes to lift junked automobiles.

For the case of electric current moving through a wire, the resultingfield is directed according to the "right hand rule." If the right hand is used asa model, and the thumb of the right hand points along the wire from positivetowards the negative side ("conventional current", the reverse of thedirection of actual movement of electrons), then the magnetic field will wraparound the wire in the direction indicated by the fingers of the right hand. Ascan be seen geometrically, if a loop or helix of wire is formed such that thecurrent is traveling in a circle, then all of the field lines in the center of theloop are directed in the same direction, resulting in a magnetic dipole whosestrength depends on the current around the loop, or the current in the helixmultiplied by the number of turns of wire. In the case of such a loop, if thefingers of the right hand are directed in the direction of conventional currentflow (i.e., positive to negative, the opposite direction to the actual flow ofelectrons), the thumb will point in the direction corresponding to the Northpole of the dipole.

Permanent and temporary magnets

Permanent and temporary magnets are alike in that they do notrequire another influence to create their magnetic field, they rely onmagnetic poles. There are always two poles, a north and a south. Even bycutting a magnet in numerous pieces you will not get a magnetic monopole,

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you will get many abated magnets. A helpful way to think of it is to think of aline of pencils, all facing the same way. each has a sharp end and an eraserend, or a north and south end. If you diverse that line into two lines each linewill still have a sharp side and eraser side. A permanent magnet differs froma temporary magnet in that a temprary magnet is simply temporary. Strokinga metal onto a magnetized material such as magnetite (a naturally

magnetized mineral) would turn the metal into a ferromagnetic material.Permanent magnets that contain other materials such as those in strongmagnets, are difficult to magnetize, but tend to keep its magnetism for agreater period. Permanent magnets may be metals such as steel, and iron,natural minerals such as magnetite, or even plastic magnets. Although theyare called permanent they are not completely permanent, if dropped, heated,or struck against a hard object at a fast speed the magnetic domains withinthe magnet may shift out of alignment causing the magnet to becomedebilitated. There are two types of rare-earth magnets:

Neodymium magnets - made from sintered neodymium, iron and smallamounts of boron; the most powerful and affordable.

Samarium-cobalt magnets (SmCo5) are less common than neodymiummagnets, are not as strong, and are more expensive, but they have ahigher curie point, making them more applicable for situations whenthey will be under intense heat.

Magnetic metallic elements

Many materials have unpaired electron spins, but the majority of these

materials are paramagnetic. When the spins interact with each other in sucha way that the spins align spontaneously, the materials are calledferromagnetic (what is often loosely termed as "magnetic"). Due to the waytheir regular crystallineatomic structure causes their spins to interact, somemetals are (ferro)magnetic when found in their natural states, as ores. Theseinclude iron ore (magnetite or lodestone), cobalt, zinc and nickel, as well therare earth metals gadolinium and dysprosium (when at a very lowtemperature). Such naturally occurring (ferro)magnets were used in the firstexperiments with magnetism. Technology has since expanded the availabilityof magnetic materials to include various manmade products, all based,however, on naturally magnetic elements.

Ceramic or ferrite

Ceramic, or ferrite, magnets are made of a sintered composite ofpowdered iron oxide and barium/strontium carbonate ceramic. Due to the lowcost of the materials and manufacturing methods, inexpensive magnets (ornonmagnetized ferromagnetic cores, for use in electronic component such asradio antennas, for example) of various shapes can be easily mass produced.

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The resulting magnets are noncorroding, but brittle and must be treated likeother ceramics.

Alnico

Alnico magnets are made by casting or sintering a combination ofaluminium, nickel and cobalt with iron and small amounts of other elementsadded to enhance the properties of the magnet. Sintering offers superiormechanical characteristics, whereas casting delivers higher magnetic fieldsand allows for the design of intricate shapes. Alnico magnets resist corrosionand have physical properties more forgiving than ferrite, but not quite asdesirable as a metal.

Injection molded

Injection molded magnets are a composite of various types of resinand magnetic powders, allowing parts of complex shapes to be manufactured

by injection molding. The physical and magnetic properties of the productdepend on the raw materials, but are generally lower in magnetic strengthand resemble plastics in their physical properties.

Flexible

Flexible magnets are similar to injection molded magnets, using aflexible resin or binder such as vinyl, and produced in flat strips or sheets.

These magnets are lower in magnetic strength but can be very flexible,depending on the binder used.

Rare earth magnets

'Rare earth' (lanthanoid) elements have a partially occupied felectronshell (which can accommodate up to 14 electrons.) The spin of theseelectrons can be aligned, resulting in very strong magnetic fields, andtherefore these elements are used in compact high-strength magnets wheretheir higher price is not a factor.

Samarium-cobalt

Samarium-cobalt magnets are highly resistant to oxidation, with highermagnetic strength and temperature resistance than alnico or ceramicmaterials. Sintered samarium-cobalt magnets are brittle and prone tochipping and cracking and may fracture when subjected to thermal shock.

Neodymium-iron-boron (NIB)

Neodymium magnets, more formally referred to as neodymium-iron-boron (NdFeB) magnets, have the highest magnetic field strength, but are

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inferior to samarium cobalt in resistance to oxidation and temperature. Thistype of magnet has traditionally been expensive, due to both the cost of rawmaterials and licensing of the patents involved. This high cost limited theiruse to applications where such high strengths from a compact magnet arecritical. Use of protective surface treatments such as gold, nickel, zinc and tinplating and epoxy resin coating can provide corrosion protection where

required. Beginning in the 1980s, NIB magnets have increasingly become lessexpensive and more popular in other applications such as controversialchildren's magnetic building toys. Even tiny neodymium magnets are verypowerful and have important safety considerations.[5]

Single-molecule magnets (SMMs) and single-chain magnets (SCMs)

In the 1990s it was discovered that certain molecules containingparamagnetic metal ions are capable of storing a magnetic moment at verylow temperatures. These are very different from conventional magnets thatstore information at a "domain" level and theoretically could provide a fardenser storage medium than conventional magnets. In this direction researchon monolayers of SMMs is currently under way. Very briefly, the two mainattributes of an SMM are:

1. a large ground state spin value (S), which is provided by ferromagneticor ferrimagnetic coupling between the paramagnetic metal centres.

2. a negative value of the anisotropy of the zero field splitting (D)

Most SMM's contain manganese, but can also be found with vanadium,iron, nickel and cobalt clusters. More recently it has been found that somechain systems can also display a magnetization which persists for long timesat relatively higher temperatures. These systems have been called single-

chain magnets.

Nano-structured magnets

Some nano-structured materials exhibit energy waves called magnonsthat coalesce into a common ground state in the manner of a Bose-Einsteincondensate.

Electromagnetic induction

Electromagnetic induction is the production of voltage across aconductor situated in a changing magnetic field or a conductor moving

through a stationary magnetic field.

Discovery

Michael Faraday is generally credited with having discovered the inductionphenomenon in 1831 though it may have been anticipated by the work ofFrancesco Zantedeschi in 1829. Around 1830 [1] to 1832 [2]Joseph Henrymade a similar discovery, but did not publish his findings until later.

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Findings

Faraday found that the electromotive force (EMF) produced around aclosed path is proportional to the rate of change of the magnetic flux throughany surface bounded by that path.

In practice, this means that an electrical current will be induced in anyclosed circuit when the magnetic flux through a surface bounded by theconductor changes. This applies whether the field itself changes in strengthor the conductor is moved through it.

Electromagnetic induction underlies the operation of generators,induction motors, transformers, and most other electrical machines.

Faraday's law of electromagnetic induction states that:

,

where

is the electromotive force (emf) in voltsB is the magnetic flux in webers

For the common but special case of a coil of wire, comprised of N loops withthe same area, Faraday's law of electromagnetic induction states that

where

is the electromotive force (emf) in voltsN is the number of turns of wire (per metre)B is the magnetic flux in webers through a single loop.

Further, Lenz's law gives the direction of the induced emf, thus:

The emf induced in an electric circuit always acts in such a direction

that the current it drives around the circuit opposes the change inmagnetic flux which produces the emf.

Lenz's law is therefore responsible for the minus sign in the above equation.

Applications

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The principles of electromagnetic induction are applied in many devices andsystems, including:

Induction Sealing Induction motors Electrical generators Transformers Contactless charging ofrechargeable batteries Induction cookers Induction welding Inductors Electromagnetic forming Magnetic flow meters Transcranial magnetic stimulation Faraday Flashlight Graphics tablet

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