MagnetsMagnets AA  magnet (from Greek magnet (from Greek μαγνήτις  λίθος  

magnḗtis  líthos, ", "Magnesian stone") stone") is a is a material or object that produces a material or object that produces a magnetic field.  It attracts ferrous objects like .  It attracts ferrous objects like pieces of iron, steel, nickel and cobalt. This pieces of iron, steel, nickel and cobalt. This magnetic field is invisible but is responsible for magnetic field is invisible but is responsible for the most notable property of a magnet: a force the most notable property of a magnet: a force that pulls on other that pulls on other ferromagnetic materials,  materials, such as such as iron, and attracts or repels other , and attracts or repels other magnets.magnets.

A magnet can be made to stick to objects A magnet can be made to stick to objects which contain magnetic material such as which contain magnetic material such as iron, even if they are not magnets. But a iron, even if they are not magnets. But a magnet cannot be made to stick to magnet cannot be made to stick to materials which are plastic, or cotton, or materials which are plastic, or cotton, or any other material, such as silicate rock, any other material, such as silicate rock, which is not magnetic.which is not magnetic.

TYPES OF MAGNETTYPES OF MAGNET

Permanent magnetsPermanent magnets ELECTROMAGNETSELECTROMAGNETS

Permanent magnetsPermanent magnets Centuries ago, it was discovered that certain types of Centuries ago, it was discovered that certain types of

mineral rock possessed unusual properties of mineral rock possessed unusual properties of attraction to the metal iron. One particular mineral, attraction to the metal iron. One particular mineral, called called lodestonelodestone, or , or magnetite magnetite (Fe(Fe33OO44)), is found , is found mentioned in very old historical records (about 2500 mentioned in very old historical records (about 2500 years ago in Europe, and much earlier in the Far years ago in Europe, and much earlier in the Far East) as a subject of curiosity. Later, it was employed East) as a subject of curiosity. Later, it was employed in the aid of navigation, as it was found that a piece in the aid of navigation, as it was found that a piece of this unusual rock would tend to orient itself in a of this unusual rock would tend to orient itself in a north-south direction if left free to rotate (suspended north-south direction if left free to rotate (suspended on a string or on a float in water). on a string or on a float in water). 

A A permanent magnetpermanent magnet is an object made from a  is an object made from a material that is material that is magnetized and creates its own  and creates its own persistent magnetic field.persistent magnetic field.

Alloys of iron, nickel, cobalt, gadolinium and Alloys of iron, nickel, cobalt, gadolinium and certain ceramic materials can become certain ceramic materials can become "permanent" magnets, such that they retain "permanent" magnets, such that they retain their magnetism for a long time.their magnetism for a long time.

A neodymium magnet (also known as NdFeB, NIB, or Neo magnet), the most widely-used type of rare-earth magnet, is a permanent magnet made from analloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet made. They have replaced other types of magnet in the many applications in modern products that require strong permanent magnets, such as motors in cordless tools, hard disk drives, and magnetic fasteners.

Nickel plated neodymium magnet on a bracket from a hard drive.

Nickel-plated neodymium magnet cubes

Strongest Type of Permanent Magnet

Permanent magnetsPermanent magnets

Permanent Ferrite Magnet Magnets that are commonly used in speakers

Gauss Cannon• There are 3 ball bearings stuck to a magnet in a track. A fourth ball bearing is released on the opposite side of the magnet, and is attracted to it. The ball at the other end shoots off at a much higher velocity.

• A second version of the gauss cannon is below and uses one spherical magnet which looks identical to the ball bearings. The device can first be shown without the magnet, when it acts like Newton's cradle and conserves energy. With the magnet, the end ball shoots off the end of the ramp.

Application of magnets:

In the Barkhausen effect, a large coil of fine wire is connected through an amplifier to a speaker. When an iron rod is placed within the coil and stroked with a magnet, an audible roaring sound will be produced from the sudden realignments of the magnetic domains within the rod. A copper rod, on the other hand, produces no effect.

Barkhausen effectApplication of magnets:

• A good permanent magnet should produce a high magnetic field with a low mass, and should be stable against the influences which would demagnetize it. The desirable properties of such magnets are typically stated in terms of the remanence and coercivity of the magnet materials.

RemanenceRemanence RemanenceRemanence or  or remanent magnetizationremanent magnetization is the  is the 

magnetization left behind in a  left behind in a ferromagnetic material (such as  material (such as iron) after an external) after an externalmagnetic field is removed. It is also the  is removed. It is also the measure of that magnetization. Colloquially, when a magnet is measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence.The remanence of magnetic "magnetized" it has remanence.The remanence of magnetic materials provides the magnetic memory in materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past  devices, and is used as a source of information on the past Earth's magnetic field in Earth's magnetic field in paleomagnetism..

The equivalent term The equivalent term residual magnetizationresidual magnetization is generally used  is generally used in engineering applications. In in engineering applications. In transformers, , electric motors and  and generators a large residual magnetization is desirable . In many  a large residual magnetization is desirable . In many other applications it is an unwanted contamination, for example other applications it is an unwanted contamination, for example a magnetization remaining in an a magnetization remaining in an electromagnet after the current  after the current in the coil is turned off. Where it is unwanted, it can be removed in the coil is turned off. Where it is unwanted, it can be removed bybydegaussing..

Sometimes the term Sometimes the term retentivityretentivity is used for remanence  is used for remanence measured in units of measured in units of magnetic flux density..

Types of remanenceTypes of remanenceSaturation remanenceSaturation remanence The default definition for remanence is the magnetization remaining in zero The default definition for remanence is the magnetization remaining in zero

field after a large magnetic field is applied (enough to achievefield after a large magnetic field is applied (enough to achievesaturation).).[1] A  A magnetic magnetic hysteresis loop is measured using instruments such as a  is measured using instruments such as a vibrating sample magnetometer and the zero-field intercept is a measure of  and the zero-field intercept is a measure of the remanence. In the remanence. In physics this measure is converted to an average  this measure is converted to an average magnetization (the total  (the total magnetic momentdivided by the volume of the divided by the volume of the sample) and denoted in equations as sample) and denoted in equations as MMrr. If it must be distinguished from . If it must be distinguished from other kinds of remanence it is called the other kinds of remanence it is called the saturation saturation remanenceremanence or  or saturation isothermal remanence (SIRM)saturation isothermal remanence (SIRM) and denoted  and denoted by by MMrsrs..

In engineering applications the residual magnetization is often measured In engineering applications the residual magnetization is often measured using a using a B-H Analyzer, which measures the response to an AC magnetic field , which measures the response to an AC magnetic field (as in Fig. 1). This is represented by a (as in Fig. 1). This is represented by a flux density  BBRR. This value of . This value of remanence is one of the most important parameters characterizing remanence is one of the most important parameters characterizing permanent magnets; it measures the strongest magnetic field they can ; it measures the strongest magnetic field they can produce. produce. Neodymium magnets, for example, have a remanence , for example, have a remanence approximately equal to 1.3 approximately equal to 1.3 teslas..

Isothermal remanenceIsothermal remanence Often a single measure of remanence does not provide adequate Often a single measure of remanence does not provide adequate

information on a magnet. For example, magnetic tapes contain a large information on a magnet. For example, magnetic tapes contain a large number of small magnetic particles, and these particles are not number of small magnetic particles, and these particles are not identical. Magnetic minerals in rocks may have a wide range of identical. Magnetic minerals in rocks may have a wide range of magnetic properties. One way to look inside these materials is to add magnetic properties. One way to look inside these materials is to add or subtract small increments of remanence. One way of doing this is or subtract small increments of remanence. One way of doing this is first first demagnetizing the magnet in an AC field, and then applying a  the magnet in an AC field, and then applying a field field HH and removing it. This remanence, denoted by  and removing it. This remanence, denoted by MMrr((HH), depends ), depends on the field.on the field.  It is called the It is called the  initial remanenceinitial remanence or the or the  isothermal isothermal remanent magnetization (IRM)remanent magnetization (IRM)..

Another kind of IRM can be obtained by first giving the magnet a Another kind of IRM can be obtained by first giving the magnet a saturation remanence in one direction and then applying and saturation remanence in one direction and then applying and removing a magnetic field in the opposite direction.removing a magnetic field in the opposite direction. This is This is calledcalleddemagnetization remanencedemagnetization remanence or  or dc demagnetization dc demagnetization remanenceremanence and is denoted by symbols like  and is denoted by symbols like MMdd((HH), where ), where HH is  is the the magnitudemagnitude of the field. of the field.  Yet another kind of remanence can be  Yet another kind of remanence can be obtained by demagnetizing the saturation remanence in an ac field. obtained by demagnetizing the saturation remanence in an ac field. This is called This is called ac demagnetization remanenceac demagnetization remanence or  or alternating field alternating field demagnetization remanencedemagnetization remanence and is denoted by symbols like  and is denoted by symbols like MMafaf((HH).).

If the particles are noninteracting single-domain particles with uniaxial If the particles are noninteracting single-domain particles with uniaxial anisotropy, there are simple linear relations between the remanences.anisotropy, there are simple linear relations between the remanences.

Anhysteretic remanenceAnhysteretic remanence Another kind of laboratory remanence is Another kind of laboratory remanence is

''anhysteretic remanenceanhysteretic remanence or  or anhysteretic anhysteretic remanent magnetization (ARM)remanent magnetization (ARM). This is induced . This is induced by exposing a magnet to a large alternating by exposing a magnet to a large alternating field plus a small dc bias field. The amplitude of field plus a small dc bias field. The amplitude of the alternating field is gradually reduced to the alternating field is gradually reduced to zero to get anzero to get ananhysteretic magnetizationanhysteretic magnetization, and , and then the bias field is removed to get the then the bias field is removed to get the remanence. The anhysteretic magnetization remanence. The anhysteretic magnetization curve is often close to an average of the two curve is often close to an average of the two branches of the branches of the hysteresis loop, and is assumed , and is assumed in some models to represent the lowest-energy in some models to represent the lowest-energy state for a given field. ARM has also been state for a given field. ARM has also been studied because of its similarity to the write studied because of its similarity to the write process in some magnetic recording process in some magnetic recording technology and to the acquisition of technology and to the acquisition of natural remanent magnetization in rocks. in rocks.

CoercivityCoercivity In In materials science, the , the coercivitycoercivity, also called the , also called the coercive coercive

fieldfield or  or coercive forcecoercive force, of a, of aferromagnetic material is the  is the intensity of the applied intensity of the applied magnetic field required to reduce  required to reduce the magnetization of that material to zero  of that material to zero afterafter the  the magnetization of the sample has been driven tomagnetization of the sample has been driven tosaturation. . Thus coercivity measures the resistance of a ferromagnetic Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Coercivity is usually material to becoming demagnetized. Coercivity is usually measured in measured in oersted or  or ampere/meter units and is denoted /meter units and is denoted HHCC. It can be measured using a . It can be measured using a B-H Analyzer or  or magnetometer..

Ferromagnetic materials with high coercivity are called Ferromagnetic materials with high coercivity are called magnetically magnetically hardhard materials, and are used to make  materials, and are used to make permanent magnets. Permanent magnets find application in . Permanent magnets find application in electric motors, magnetic recording media (e.g. , magnetic recording media (e.g. hard drives, , floppy disks, or , or magnetic tape) and ) and magnetic separation..

Materials with low coercivity are said to be magnetically Materials with low coercivity are said to be magnetically softsoft. . They are used in They are used in transformer and andinductor  cores, , recording heads. . microwave devices, and  devices, and magnetic shielding..

Experimental determinationExperimental determination Typically the coercivity of a magnetic material is determined by measurement Typically the coercivity of a magnetic material is determined by measurement

of the hysteresis loop, also called the of the hysteresis loop, also called the magnetization curvemagnetization curve, as illustrated in the , as illustrated in the figure. The apparatus used to acquire the data is typically a figure. The apparatus used to acquire the data is typically a vibrating-sample or  or alternating-gradient alternating-gradient magnetometer. The applied field where the data line . The applied field where the data line crosses zero is the coercivity. If an crosses zero is the coercivity. If an antiferromagnet is present in the sample, the  is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a coercivities measured in increasing and decreasing fields may be unequal as a result of the result of the exchange bias effect. effect.

The coercivity of a material depends on the time scale over which a The coercivity of a material depends on the time scale over which a magnetization curve is measured. The magnetization of a material measured at magnetization curve is measured. The magnetization of a material measured at an applied reversed field which is nominally smaller than the coercivity may, an applied reversed field which is nominally smaller than the coercivity may, over a long time scale, slowly over a long time scale, slowly relax to zero. Relaxation occurs when reversal of  to zero. Relaxation occurs when reversal of magnetization by domain wall motion is magnetization by domain wall motion is thermally activated and is dominated  and is dominated by by magnetic viscosity..[2] The increasing value of coercivity at high frequencies is  The increasing value of coercivity at high frequencies is a serious obstacle to the increase ofa serious obstacle to the increase ofdata rates in high- in high-bandwidth magnetic  magnetic recording, compounded by the fact that increased storage density typically recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.requires a higher coercivity in the media.

The coercivity of a material depends on the time scale over which a The coercivity of a material depends on the time scale over which a magnetization curve is measured. The magnetization of a material measured at magnetization curve is measured. The magnetization of a material measured at an applied reversed field which is nominally smaller than the coercivity may, an applied reversed field which is nominally smaller than the coercivity may, over a long time scale, slowly over a long time scale, slowly relax to zero. Relaxation occurs when reversal of  to zero. Relaxation occurs when reversal of magnetization by domain wall motion is magnetization by domain wall motion is thermally activated and is dominated  and is dominated by by magnetic viscosity..[2] The increasing value of coercivity at high frequencies is  The increasing value of coercivity at high frequencies is a serious obstacle to the increase ofa serious obstacle to the increase ofdata rates in high- in high-bandwidth magnetic  magnetic recording, compounded by the fact that increased storage density typically recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.requires a higher coercivity in the media.

Material Coercivity [Oe]

[.1Mn:]6Fe:27Ni:Mo, Supermalloy 0.002

Fe:4Ni, Permalloy 0.01–1

.9995 iron–filings 0.05–470

11Fe:Si, silicon iron 0.4–0.9

Raw iron 2 (1896)

.99 Nickel 0.7–290

ZnxFeNi1-xO3,

ferrite for magnetron15–200

2Fe:Co, Iron pole 240

>.99 cobalt 10–900

6Al:18Fe:8Co:Cu:6Ni–3Ti:8Al:20Fe:20Co:2Cu:8Ni,alnico 5–9, fridge magnet and stronger

640–2000

Cr:Co:Pt,disk drive recording media

1700

2Nd:14Fe:B, neodymium-iron-boron 10,000–12,000

12Fe:13Pt, Fe48Pt52 12,300+[1]

 ?(Dy,Nb,Ga,Co):2Nd:14Fe:B 25,600–26,300

2Sm:17Fe:3N, samarium-iron-nitrogen <500–35,000 (10 K)

Sm:5Co, samarium-cobalt 40,000

ELECTROMAGNELECTROMAGNETSETS

An An electromaelectroma

gnetgnet A soft iron rod has A soft iron rod has

no magnetic fieldno magnetic field When current flows When current flows

in the wire the soft in the wire the soft iron becomes iron becomes magnetized so a magnetized so a magnetic field is magnetic field is detected by the detected by the plotting compasses.plotting compasses.

The magnetic field The magnetic field disappears when the disappears when the current is turned current is turned off.off.

Electromagnetic radiationElectromagnetic radiation (often abbreviated  (often abbreviated E-E-M radiationM radiation or  or EMREMR) ) is a form of is a form of energy that  that exhibits wave-like behavior as it travels through exhibits wave-like behavior as it travels through space. EMR has both space. EMR has both electric and  and magnetic fieldcomponents, which components, which oscillate in phase perpendicular  in phase perpendicular to each other and perpendicular to the direction of to each other and perpendicular to the direction of energy energy propagation..

Uses of Electromagnets in the Uses of Electromagnets in the Medical FieldMedical Field

Electromagnets are also widely used in the Electromagnets are also widely used in the medical field. They are mainly used in medical field. They are mainly used in removing embedded magnetic metal removing embedded magnetic metal particles from inside the eyes, usually particles from inside the eyes, usually deposited during an accident. One of the deposited during an accident. One of the most important uses of electromagnet in most important uses of electromagnet in hospital is in magnetic resonance imaging hospital is in magnetic resonance imaging (MRI), which is used for getting a detailed (MRI), which is used for getting a detailed image of the inside of the body to diagnose a image of the inside of the body to diagnose a number of diseases.number of diseases.

magnetic resonance imaging (MRI)

Technological Uses of Technological Uses of ElectromagnetsElectromagnets

The main technological uses of electromagnets are in storing The main technological uses of electromagnets are in storing information and moving things. They are used in many electrical information and moving things. They are used in many electrical devices like electrical balls, loudspeakers, magnetic locks and devices like electrical balls, loudspeakers, magnetic locks and various magnetic recording devices such as computer disks, various magnetic recording devices such as computer disks, tape recorders, VCR, etc. Televisions also uses electromagnets tape recorders, VCR, etc. Televisions also uses electromagnets to power the cathode ray tube to regulate the direction of the to power the cathode ray tube to regulate the direction of the beam of electrons, used to illuminate the screen. beam of electrons, used to illuminate the screen. Electromagnets are also used in telephones, mobile phones and Electromagnets are also used in telephones, mobile phones and doorbells. Moving metals and picking up cars in junkyards are doorbells. Moving metals and picking up cars in junkyards are some of the common everyday uses of electromagnets. some of the common everyday uses of electromagnets. Spacecraft also use electromagnets in the propulsion system to Spacecraft also use electromagnets in the propulsion system to generate power.generate power.

Electromagnets are also used for dumping garbage in recycling Electromagnets are also used for dumping garbage in recycling plants. Some studies are being carried out to discover the plants. Some studies are being carried out to discover the potential of using electromagnets in developing electric cars. potential of using electromagnets in developing electric cars. The possibility of using electromagnetism in developing more The possibility of using electromagnetism in developing more environmental friendly or less polluting energy storage systems, environmental friendly or less polluting energy storage systems, is also a subject of many studies. is also a subject of many studies. 

Common Uses Of Magnets:Magnetic Recording

Data Common T.V. and Computer Monitors

Credit, Debit and ATM cards

Magnetic FieldsMagnetic Fields

The region where the magnetic forces act is The region where the magnetic forces act is called the “magnetic field”called the “magnetic field”

• A compass table with a hundred or so A compass table with a hundred or so tiny compass needles displays the tiny compass needles displays the magnetic field of a bar magnet, or two magnetic field of a bar magnet, or two attracting or repeling magnets, for attracting or repeling magnets, for overhead projection. overhead projection.

A piece of lodestone picks up small iron objects, or can be suspended so as to point north.

EARTH’S MAGNETISMThe Earth itself has a huge magnetic field - as if it had a huge bar magnet embedded at its centre. The Earth’s magnetic field lines emerge from near the geographical north pole and re-enter it at the south pole. The nature of the field around the Earth varies in both strength and direction. The Earth’s magnetic field is strongest at the magnetic poles and weakest near the Equator.

Why does the Earth have a magnetic field?

The Earth has, at its centre, a dense liquid core, of about half the radius of the Earth, with a solid inner core. This core is though to be mostly made of molten iron and nickel perhaps mixed with some lighter elements. Circulating ions of iron and nickel in highly conducting liquid region of earth’s core might be forming current loops and producing earth’s magnetism.

The Earth’s magnetic field

Magnetic ElementsMagnetic Elements

Magnetic Declination Magnetic Inclination or Magneti

c Dip

Magnetic DeclinationMagnetic Declination

The small angle The small angle between magnetic between magnetic axis and axis and geographic axis at geographic axis at a place is defined a place is defined as the magnetic as the magnetic declinationdeclination. .

Magnetic Inclination or Magnetic Inclination or Magnetic DipMagnetic Dip

The angle which The angle which the direction of the direction of total strength of total strength of earth’s earth’s magnetic field magnetic field makes with a makes with a horizontal line in horizontal line in magnetic magnetic meridian.meridian.

Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

These areas of atoms are called “domains”

Groups of atoms join so that their magnetic fields are all going in the same direction

Magnetic Properties of Atoms

When an unmagnetized substance is placed in a magneticWhen an unmagnetized substance is placed in a magneticfield, the substance can become magnetized.field, the substance can become magnetized.

This happens when the spinning electrons line up in theThis happens when the spinning electrons line up in thesame direction.same direction.

The metals affected by The metals affected by magnetism consist of tiny magnetism consist of tiny regions called 'Domains' regions called 'Domains' which behave like tiny which behave like tiny magnets. Normally they are magnets. Normally they are arranged in the magnetic arranged in the magnetic material all pointing in material all pointing in different directions in a different directions in a completely random fashion completely random fashion and so their magnetic and so their magnetic effects cancel each other effects cancel each other out. If an object is out. If an object is magnetized it is because magnetized it is because the domains are all made the domains are all made to point in the same to point in the same direction. This can be done direction. This can be done by stroking the magnetic by stroking the magnetic material with a magnet (or material with a magnet (or magnets) as shown in the magnets) as shown in the diagram. When aligned the diagram. When aligned the domains reinforce one domains reinforce one another and create north another and create north and south poles at either and south poles at either end.end.

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Classification of Classification of magnetic materialsmagnetic materials

Materials respond differently to the force of Materials respond differently to the force of a magnetic field. There are three main a magnetic field. There are three main classifications of magnetic materials. A classifications of magnetic materials. A magnet will strongly attract ferromagnetic magnet will strongly attract ferromagnetic materials, weakly attract paramagnetic materials, weakly attract paramagnetic materials and weakly repel diamagnetic materials and weakly repel diamagnetic materials.materials.

Classification of Classification of magnetic materialsmagnetic materials

Diamagnetic SubstancesDiamagnetic Substances Paramagnetic substancesParamagnetic substances Ferromagnetic substancesFerromagnetic substances

Diamagnetic substancesDiamagnetic substances

• The diamagnetic substances are The diamagnetic substances are those in which the individual atoms those in which the individual atoms or ions do not possess any net or ions do not possess any net magnetic moment on their own.magnetic moment on their own.

• When such substances are placed in When such substances are placed in an external magnetizing field, they an external magnetizing field, they get feebly magnetized in a direction get feebly magnetized in a direction opposite to a magnetizing field. opposite to a magnetizing field.

• Certain materials are diamagnetic, which Certain materials are diamagnetic, which means that when they are exposed to a means that when they are exposed to a strong magnetic field, they induce a weak strong magnetic field, they induce a weak magnetic field in the opposite direction. In magnetic field in the opposite direction. In other words, other words, they weakly repel a strong they weakly repel a strong magnetmagnet. .

• Diamagnetic materials have a weak, Diamagnetic materials have a weak, negative susceptibility to magnetic fields. negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled Diamagnetic materials are slightly repelled by a magnetic field and the material does by a magnetic field and the material does not retain the magnetic properties when the not retain the magnetic properties when the external field is removed. In diamagnetic external field is removed. In diamagnetic materials all the electron are paired so materials all the electron are paired so there is no permanent net magnetic moment there is no permanent net magnetic moment per atom. Diamagnetic properties arise from per atom. Diamagnetic properties arise from the realignment of the electron paths under the realignment of the electron paths under the influence of an external magnetic field. the influence of an external magnetic field. Most elements in the periodic table, Most elements in the periodic table, including copper, silver, and gold, are including copper, silver, and gold, are diamagnetic.diamagnetic.

Strongest Diamagnetic materialsStrongest Diamagnetic materials• Bismuth and carbon graphite Bismuth and carbon graphite are the are the

strongest diamagnetic materials. strongest diamagnetic materials. They are about eight times stronger They are about eight times stronger than mercury and silver. Other than mercury and silver. Other weaker diamagnetic materials weaker diamagnetic materials include water, diamonds, wood and include water, diamonds, wood and living tissue. Note that the last living tissue. Note that the last three items are carbon-based.three items are carbon-based.

• The electrons in a diamagnetic The electrons in a diamagnetic material rearrange their orbits material rearrange their orbits slightly creating small persistent slightly creating small persistent currents, which oppose the external currents, which oppose the external magnetic field.magnetic field.

Uses of Diamagnetic materialsUses of Diamagnetic materials• Although the forces created by diamagnetism Although the forces created by diamagnetism

are extremely weak—millions of times smaller are extremely weak—millions of times smaller than the forces between magnets and than the forces between magnets and ferromagnetic materials like iron, there are ferromagnetic materials like iron, there are some interesting uses of those materials.some interesting uses of those materials.

Curving waterCurving water• A thin layer of water laying on the top surface of A thin layer of water laying on the top surface of

a very power magnet will be slightly repelled by a very power magnet will be slightly repelled by the magnetic field. This can be verified by the magnetic field. This can be verified by viewing the reflection off the water surface and viewing the reflection off the water surface and seeing a slight dimple on the surface.seeing a slight dimple on the surface.

Used in levitationUsed in levitation• The most popular application of diamagnetic The most popular application of diamagnetic

materials is magnetic levitation, where an materials is magnetic levitation, where an object will be made to float in are above a object will be made to float in are above a strong magnet. Although most experiments use strong magnet. Although most experiments use inert objects, researchers as the University of inert objects, researchers as the University of Nijmegen in the Netherlands demonstrated Nijmegen in the Netherlands demonstrated levitating a small frog in a powerful magnetic levitating a small frog in a powerful magnetic field.field.

Levitating pyrolytic Levitating pyrolytic graphitegraphite

Paramagnetic SubstancesParamagnetic Substances

Paramagnetic substances are those Paramagnetic substances are those in which each individual atom or in which each individual atom or molecule or ion has a net non zero molecule or ion has a net non zero magnetic moment of its own.magnetic moment of its own.

When such substances are placed in When such substances are placed in an external magnetic field, they get an external magnetic field, they get feebly magnetized in the direction of feebly magnetized in the direction of the magnetizing field. the magnetizing field.

Paramagnetic materials are metals Paramagnetic materials are metals that are weakly attracted to magnets. that are weakly attracted to magnets. Aluminum and copper are such metals. Aluminum and copper are such metals. These materials can become very weak These materials can become very weak magnets, but their attractive force can magnets, but their attractive force can only be measured with sensitive only be measured with sensitive instruments.instruments.

TemperatureTemperature can affect the magnetic can affect the magnetic properties of a material. Paramagnetic properties of a material. Paramagnetic materials like aluminum, uranium and materials like aluminum, uranium and platinum platinum become more magnetic become more magnetic when when they are they are very coldvery cold..

The force of a ferromagnetic magnet is The force of a ferromagnetic magnet is about a million times that of a magnet about a million times that of a magnet made with a paramagnetic material. made with a paramagnetic material. Since the attractive force is so small, Since the attractive force is so small, paramagnetic materials are typically paramagnetic materials are typically considered nonmagnetic.considered nonmagnetic.

Paramagnetic materials have a small, Paramagnetic materials have a small, positive susceptibility to magnetic positive susceptibility to magnetic fields. These materials are slightly fields. These materials are slightly attracted by a magnetic field and the attracted by a magnetic field and the material does not retain the magnetic material does not retain the magnetic properties when the external field is properties when the external field is removed. Paramagnetic properties are removed. Paramagnetic properties are due to the presence of some unpaired due to the presence of some unpaired electrons, and from the realignment of electrons, and from the realignment of the electron paths caused by the the electron paths caused by the external magnetic field. Paramagnetic external magnetic field. Paramagnetic materials materials include magnesium, include magnesium, molybdenum, lithium, and tantalum.molybdenum, lithium, and tantalum.

Ferromagnetic materialsFerromagnetic materials Ferromagnetic Ferromagnetic

Domains in MaterialsDomains in Materials

Ferromagnetic Ferromagnetic material are material are those in which those in which each individual each individual atom or atom or molecule has a molecule has a non zero non zero magnetic magnetic momentmoment

• Ferromagnetic materials have a large, positive Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after are able to retain their magnetic properties after the external field has been removed. the external field has been removed. Ferromagnetic materials have some unpaired Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic electrons so their atoms have a net magnetic moment. They get their strong magnetic moment. They get their strong magnetic properties due to the presence of magnetic properties due to the presence of magnetic domains. In these domains, large numbers of domains. In these domains, large numbers of atom's moments (10atom's moments (101212 to 10 to 101515) are aligned ) are aligned parallel so that the magnetic force within the parallel so that the magnetic force within the domain is strong. When a ferromagnetic material domain is strong. When a ferromagnetic material is in the unmagnified state, the domains are is in the unmagnified state, the domains are nearly randomly organized and the net magnetic nearly randomly organized and the net magnetic field for the part as a whole is zero. When a field for the part as a whole is zero. When a magnetizing force is applied, the domains magnetizing force is applied, the domains become aligned to produce a strong magnetic become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. examples of ferromagnetic materials. Components with these materials are commonly Components with these materials are commonly inspected using the magnetic particle method.inspected using the magnetic particle method.

FerromagnetsFerromagnets• A ferromagnetic material is one that has A ferromagnetic material is one that has

magnetic properties similar to those of magnetic properties similar to those of iron. In other words, you can make a iron. In other words, you can make a magnet out of it. Ferromagnetic magnet out of it. Ferromagnetic materials are strongly attracted by a materials are strongly attracted by a magnetic force. The elements magnetic force. The elements iron (Fe)iron (Fe), , nickel (Ni), cobalt (Co) nickel (Ni), cobalt (Co) and and gadolinium gadolinium (Gd) (Gd) are such materials.are such materials.

• Magnetic fields come from currents. This Magnetic fields come from currents. This is true even in ferromagnetic materials; is true even in ferromagnetic materials; their magnetic properties come from the their magnetic properties come from the motion of electrons in the atoms. Each motion of electrons in the atoms. Each electron has a spin. This is a quantum electron has a spin. This is a quantum mechanical phenomenon that is difficult mechanical phenomenon that is difficult to make a comparison to, but can be to make a comparison to, but can be thought of as similar to the rotation of thought of as similar to the rotation of the Earth about its axis.the Earth about its axis.

Iron and steelIron and steel• Iron is the most common element Iron is the most common element

associated with being attracted to to a associated with being attracted to to a magnet. Steel is also a ferromagnetic magnet. Steel is also a ferromagnetic material. It is an alloy or combination of material. It is an alloy or combination of iron and several other metals, giving it iron and several other metals, giving it greater hardness than iron, as well as other greater hardness than iron, as well as other specialized properties. Because of its specialized properties. Because of its hardness, steel retains magnetism longer hardness, steel retains magnetism longer than iron.than iron.

• Soft IronSoft Iron , If you were hit on the head , If you were hit on the head with a soft iron bar, it would still feel with a soft iron bar, it would still feel very hard; soft is simply a term very hard; soft is simply a term describing the magnetic properties. In describing the magnetic properties. In hard iron, the domains will not shift hard iron, the domains will not shift back to their starting points when the back to their starting points when the field is taken away. In soft iron, the field is taken away. In soft iron, the domains return to being randomly domains return to being randomly aligned when the field is removed.aligned when the field is removed.

• Hard ironHard iron is used in permanent is used in permanent magnets. To make a permanent magnet, magnets. To make a permanent magnet, a piece of hard iron is placed in a a piece of hard iron is placed in a magnetic field. The domains align with magnetic field. The domains align with the field, and retain a good deal of that the field, and retain a good deal of that alignment when the field is removed, alignment when the field is removed, resulting in a magnet.resulting in a magnet.

TWO FORMS OF IRON :TWO FORMS OF IRON :

soft iron wiresoft iron wire Soft Iron, Stainless Steel

blade with soft iron

CURIE TEMPERATURECURIE TEMPERATURE

The Curie temperature (Tc) is the critical The Curie temperature (Tc) is the critical temperature beyond which a previously temperature beyond which a previously ferromagnetic material becomes paramagnetic. On ferromagnetic material becomes paramagnetic. On the atomic level, below the Curie temperature the the atomic level, below the Curie temperature the magnetic moments, contributed mainly by the magnetic moments, contributed mainly by the electrons, are alligned in their respective domains electrons, are alligned in their respective domains and even a weak external field results in a net and even a weak external field results in a net magnetization. As the temperature increases to Tc magnetization. As the temperature increases to Tc and above however, fluctuations due to the increase and above however, fluctuations due to the increase in thermal energy destroy that allignment. Tc for in thermal energy destroy that allignment. Tc for nickel is 631K, while that for iron is 1043K.nickel is 631K, while that for iron is 1043K.

CURIE TEMPERATURE:CURIE TEMPERATURE:

Curie temperature in ferromagnetic and Curie temperature in ferromagnetic and ferrimagnetic materials.ferrimagnetic materials.

Substance Curie temp °C

Iron (Fe) 770

Cobalt (Co) 1130

Nickel (Ni) 358

Iron Oxide (Fe2O3)

622

Magnetizing ferromagnetsMagnetizing ferromagnets Ferromagnetic materials can be magnetized in the  materials can be magnetized in the

following ways:following ways:• Heating the object above its Curie temperature, allowing Heating the object above its Curie temperature, allowing

it to cool in a magnetic field and hammering it as it it to cool in a magnetic field and hammering it as it cools. This is the most effective method and is similar to cools. This is the most effective method and is similar to the industrial processes used to create permanent the industrial processes used to create permanent magnets.magnets.

• Placing the item in an external magnetic field will result Placing the item in an external magnetic field will result in the item retaining some of the magnetism on in the item retaining some of the magnetism on removal. Vibration has been shown to increase the removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the Earth's effect. Ferrous materials aligned with the Earth's magnetic field that are subject to vibration (e.g., frame magnetic field that are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant of a conveyor) have been shown to acquire significant residual magnetism.residual magnetism.

• Stroking: An existing magnet is moved from one end of Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction.the item to the other repeatedly in the same direction.

Demagnetizing ferromagnetsDemagnetizing ferromagnets

Magnetized ferromagnetic materials can be demagnetized (or degaussed) Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:in the following ways:

• Heating a magnet past its Curie temperature; the molecular motion Heating a magnet past its Curie temperature; the molecular motion destroys the alignment of the magnetic domains. This always removes all destroys the alignment of the magnetic domains. This always removes all magnetization.magnetization.

• Placing the magnet in an alternating magnetic field with an intensity Placing the magnet in an alternating magnetic field with an intensity above the material's coercivity and then either slowly drawing the magnet above the material's coercivity and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools and erase credit used in commercial demagnetizers to demagnetize tools and erase credit cards and hard disks and degaussing coils used to demagnetize CRTs.cards and hard disks and degaussing coils used to demagnetize CRTs.

• Some demagnetization or reverse magnetization will occur if any part of Some demagnetization or reverse magnetization will occur if any part of the magnet is subjected to a reverse field above the magnetic the magnet is subjected to a reverse field above the magnetic material's coercivity.material's coercivity.

• Demagnetisation progressively occurs if the magnet is subjected to cyclic Demagnetisation progressively occurs if the magnet is subjected to cyclic fields sufficient to move the magnet away from the linear part on the fields sufficient to move the magnet away from the linear part on the second quadrant of the B-H curve of the magnetic material (the second quadrant of the B-H curve of the magnetic material (the demagnetisation curve).demagnetisation curve).

• Hammering or jarring: the mechanical disturbance tends to randomize the Hammering or jarring: the mechanical disturbance tends to randomize the magnetic domains. Will leave some residual magnetization.magnetic domains. Will leave some residual magnetization.

Hysteresis CurveHysteresis Curve

The relationship between magnetic field The relationship between magnetic field strength (H) and magnetic flux density (B) will strength (H) and magnetic flux density (B) will follow a curve up to a point where further follow a curve up to a point where further increases in magnetic field strength will result increases in magnetic field strength will result in no further change in flux density. This in no further change in flux density. This condition is called magnetic saturation till condition is called magnetic saturation till point (a).point (a).

Magnetic Saturation

• the plotted relationship will follow a the plotted relationship will follow a different curve back towards zero field different curve back towards zero field strength at which point it will be offset strength at which point it will be offset from the original curve by an amount from the original curve by an amount called the called the remanent flux densityremanent flux density or or Retentity as shown in graph at point (b) Retentity as shown in graph at point (b)

• The 'thickness' of the middle, describes The 'thickness' of the middle, describes the amount of hysteresis, related to the the amount of hysteresis, related to the coercivity of the material as from (c) to coercivity of the material as from (c) to (f)(f)

Retentity

Hysteresis curve of soft Iron and steelHysteresis curve of soft Iron and steel

The retentivity of soft The retentivity of soft iron > retentivity of steeliron > retentivity of steel

Soft iron is more strongly Soft iron is more strongly magnetized than steelmagnetized than steel

Coercivity of soft iron < Coercivity of soft iron < Coercivity of steelCoercivity of steel

Hence, soft iron loses its Hence, soft iron loses its magnetism more rapidly magnetism more rapidly than steel does.than steel does.

THE ENDTHE END

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BS EE – IIIBS EE – III

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