Magnets A magnet (from Greek μαγνήτις λίθος magnḗtis líthos, " Magnesian stone") is a material or object that produces a magnetic field. It attracts ferrous objects like pieces of iron, steel, nickel and cobalt. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets. A magnet can be made to stick to objects which contain magnetic material such as iron, even if they are not magnets. But a magnet cannot be made to stick to materials which are plastic, or cotton, or any other material, such as silicate rock, which is not magnetic.
TYPES OF MAGNET Permanent magnets ELECTROMAGNETS
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Strongest Type of Permanent MagnetA 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 Nickel-plated neodymium magnet cubes on a bracket from a hard drive.
Permanent Ferrite Magnet Magnets that are commonly used in speakers
Application of magnets: G a us s C a nnon • 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 Newtons cradle and conserves energy. With the magnet, the end ball shoots off the end of the ramp.
Application of magnets: Barkhausen effect 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.
• 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.
R e ma ne nc e Remanence or remanent ma gnetization is the magnetization left behind in a ferromagnetic material (such as iron) after an externalmagnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence.The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earths magnetic field in paleomagnetism. The equivalent term residual ma gnetization is generally used in engineering applications. In transformers, electric motors and generators a large residual magnetization is desirable . In many other applications it is an unwanted contamination, for example a magnetization remaining in an electromagnet after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing. Sometimes the term retentivity is used for remanence measured in units of magnetic flux density.
Types of remanenceSaturation remanence The default definition for remanence is the magnetization remaining in zero field after a large magnetic field is applied (enough to achievesaturation). A magnetic hysteresis loop is measured using instruments such as a vibrating sample magnetometer and the zero-field intercept is a measure of the remanence. In physics this measure is converted to an average magnetization (the total magnetic momentdivided by the volume of the sample) and denoted in equations as Mr. If it must be distinguished from other kinds of remanence it is called the saturation remanence or saturation isothermal remanence (SIRM) and denoted by Mrs. In engineering applications the residual magnetization is often measured using a B-H Analyzer, which measures the response to an AC magnetic field (as in Fig. 1). This is represented by a flux density BR. This value of remanence is one of the most important parameters characterizing permanent magnets; it measures the strongest magnetic field they can produce. Neodymium magnets , for example, have a remanence approximately equal to 1.3 teslas.
Is o t h e r m a l r e m a n e n c e Often a single measure of remanence does not provide adequate information on a magnet. For example, magnetic tapes contain a large number of small magnetic particles, and these particles are not identical. Magnetic minerals in rocks may have a wide range of magnetic properties. One way to look inside these materials is to add or subtract small increments of remanence. One way of doing this is first demagnetizing the magnet in an AC field, and then applying a field H and removing it. This remanence, denoted by Mr(H), depends on the field. It is called the initial remanence or the isothermal remanent magnetization (IRM) . Another kind of IRM can be obtained by first giving the magnet a saturation remanence in one direction and then applying and removing a magnetic field in the opposite direction. This is calleddemagnetization remanence or dc demagnetization remanence and is denoted by symbols like Md(H), where H is the magnitude of the field. Yet another kind of remanence can be obtained by demagnetizing the saturation remanence in an ac field. This is called ac demagnetization remanence or alternating field demagnetization remanence and is denoted by symbols like Maf(H). If the particles are noninteracting single-domain particles with uniaxial anisotropy, there are simple linear relations between the remanences.
Anhysteretic remanence Another kind of laboratory remanence is anhysteretic remanence or anhysteretic remanent magnetization (ARM). This is induced by exposing a magnet to a large alternating field plus a small dc bias field. The amplitude of the alternating field is gradually reduced to zero to get ananhysteretic magnetization, and then the bias field is removed to get the remanence. The anhysteretic magnetization curve is often close to an average of the two branches of the hysteresis loop, and is assumed in some models to represent the lowest-energy state for a given field. ARM has also been studied because of its similarity to the write process in some magnetic recording technology and to the acquisition of natural remanent magnetization in rocks.
Coercivity In materials science, the coercivity, also called the coercive field or coercive force, of aferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven tosaturation. Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC. It can be measured using a B-H Analyzer or magnetometer. Ferromagnetic materials with high coercivity are called magnetically hard materials, and are used to make permanent magnets. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation . Materials with low coercivity are said to be magnetically soft. They are used in transformer andinductor cores, recording heads. microwave devices, and magnetic shielding.
Experimental determination Typically the coercivity of a magnetic material is determined by measurement of the hysteresis loop, also called the magnetization curve, as illustrated in the figure. The apparatus used to acquire the data is typically a vibrating-sample or alternating-gradient magnetometer. The applied field where the data line crosses zero is the coercivity. If an antiferromagnet is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of the exchange bias effect. The coercivity of a material depends on the time scale over which a magnetization curve is measured. The magnetization of a material measured at an applied reversed field which is nominally smaller than the coercivity may, over a long time scale, slowly relax to zero. Relaxation occurs when reversal of magnetization by domain wall motion is thermally activated and is dominated by magnetic viscosity. The increasing value of coercivity at high frequencies is a serious obstacle to the increase ofdata rates in high-bandwidth magnetic recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media. The coercivity of a material depends on the time scale over which a magnetization curve is measured. The magnetization of a material measured at an applied reversed field which is nominally smaller than the coercivity may, over a long time scale, slowly relax to zero. Relaxation occurs when reversal of magnetization by domain wall motion is thermally activated and is dominated by magnetic viscosity. The increasing value of coercivity at high frequencies is a serious obstacle to the increase ofdata rates in high-bandwidth magnetic recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.
Material Coercivity [Oe][.1Mn:]6Fe:27Ni:Mo, Supermalloy 0.002Fe:4Ni, Permalloy 0.01–1.9995 iron–filings 0.05–47011Fe:Si, silicon iron 0.4–0.9Raw iron 2 (1896).99 Nickel 0.7–290Zn , xFeNi1-xO3 15–200ferrite for magnetron2Fe:Co, Iron pole 240>.99 cobalt 10–90066Al:18Fe:8Co:Cu:6Ni–3Ti:8Al:20Fe:20Co:2Cu:8Ni, 640–2000alnico 5–9, fridge magnet and strongerCr:Co:Pt, 1700disk drive recording media2Nd:14Fe:B, neodymium-iron-boron 10,000–12,00012Fe:13Pt, Fe48Pt52 12,300+ ?(Dy,Nb,Ga,Co):2Nd:14Fe:B 25,600–26,3002Sm:17Fe:3N, samarium-iron-nitrogen <500–35,000 (10 K)
Anelectromagnet A soft iron rod has no magnetic field When current flows in the wire the soft iron becomes magnetized so a magnetic field is detected by the plotting compasses. The magnetic field disappears when the current is turned off.
Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a form of energy that exhibits wave-like behavior as it travels through space. EMR has both electric and magnetic fieldcomponents, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation.
Uses of Electromagnets in the Medical Field Electromagnets are also widely used in the medical field. They are mainly used in removing embedded magnetic metal particles from inside the eyes, usually deposited during an accident. One of the most important uses of electromagnet in hospital is in magnetic resonance imaging (MRI), which is used for getting a detailed image of the inside of the body to diagnose a number of diseases.
Technological Uses of Electromagnets The main technological uses of electromagnets are in storing information and moving things. They are used in many electrical devices like electrical balls, loudspeakers, magnetic locks and various magnetic recording devices such as computer disks, tape recorders, VCR, etc. Televisions also uses electromagnets to power the cathode ray tube to regulate the direction of the beam of electrons, used to illuminate the screen. Electromagnets are also used in telephones, mobile phones and doorbells. Moving metals and picking up cars in junkyards are some of the common everyday uses of electromagnets. Spacecraft also use electromagnets in the propulsion system to generate power. Electromagnets are also used for dumping garbage in recycling plants. Some studies are being carried out to discover the potential of using electromagnets in developing electric cars. The possibility of using electromagnetism in developing more environmental friendly or less polluting energy storage systems, 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 Fields The region where the magnetic forces act is called the “magnetic field”
• A compass table with a hundred or so tiny compass needles displays the magnetic field of a bar magnet, or two attracting or repeling magnets, for overhead projection.
A piece of lodestone picks up small iron objects, orcan be suspended so as to point north.
EARTH’SMAGNETISMThe 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.
Magnetic eleMents Magnetic Declination Magnetic Inclination or Magnetic Dip
Magnetic Declination The small angle between magnetic axis and geographic axis at a place is defined as the magnetic declination.
Magnetic Inclination orMagnetic Dip The angle which the direction of total strength of earth’s magnetic field makes with a horizontal line in magnetic meridian.
M a g n e t ic P r o p e r t ie s o f A to ms Atoms themselves have magnetic properties due to the spin of the atom’s electrons. Groups of atoms join so that their magnetic fields are all going in the same direction These areas of atoms are called “domains”
When an unmag netized s ubs tance is placed in a mag netic field, the s ubs tance can become mag netized.This happens when the s pinning electrons line up in the s ame direction.
The metals affected by magnetism consist of tiny regions called Domains(.wav) which behave like tiny magnets. Normally they are arranged in the magnetic material all pointing in different directions in a completely random fashion and so their magnetic effects cancel each other out. If an object is magnetized it is because the domains are all made to point in the same direction. This can be done by stroking the magnetic material with a magnet (or magnets) as shown in the diagram. When aligned the domains reinforce one another and create north and south poles at either end.
Classification of magnetic materials Materials respond differently to the force of a magnetic field. There are three main classifications of magnetic materials. A magnet will strongly attract ferromagnetic materials, weakly attract paramagnetic materials and weakly repel diamagnetic materials.
Classification of magnetic materials Diamagnetic Substances Paramagnetic substances Ferromagnetic substances
Diamagnetic substances• The diamagnetic substances are those in which the individual atoms or ions do not possess any net magnetic moment on their own.• When such substances are placed in an external magnetizing field, they get feebly magnetized in a direction opposite to a magnetizing field.
• Certain materials are diamagnetic, which means that when they are exposed to a strong magnetic field, they induce a weak magnetic field in the opposite direction. In other words, they weakly repel a strong magnet.• Diamagnetic materials have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. In diamagnetic materials all the electron are paired so there is no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron paths under the influence of an external magnetic field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.
Strongest Diamagnetic materials• Bismuth and carbon graphite are the strongest diamagnetic materials. They are about eight times stronger than mercury and silver. Other weaker diamagnetic materials include water, diamonds, wood and living tissue. Note that the last three items are carbon- based.• The electrons in a diamagnetic material rearrange their orbits slightly creating small persistent currents, which oppose the external magnetic field.
Uses of Diamagnetic materials• Although the forces created by diamagnetism are extremely weak—millions of times smaller than the forces between magnets and ferromagnetic materials like iron, there are some interesting uses of those materials.Curving water• A thin layer of water laying on the top surface of a very power magnet will be slightly repelled by the magnetic field. This can be verified by viewing the reflection off the water surface and seeing a slight dimple on the surface.Used in levitation• The most popular application of diamagnetic materials is magnetic levitation, where an object will be made to float in are above a strong magnet. Although most experiments use inert objects, researchers as the University of Nijmegen in the Netherlands demonstrated levitating a small frog in a powerful magnetic field.
Paramagnetic Substances Paramagnetic substances are those in which each individual atom or molecule or ion has a net non zero magnetic moment of its own. When such substances are placed in an external magnetic field, they get feebly magnetized in the direction of the magnetizing field.
Paramagnetic materials are metals that are weakly attracted to magnets. Aluminum and copper are such metals. These materials can become very weak magnets, but their attractive force can only be measured with sensitive instruments. Temperature can affect the magnetic properties of a material. Paramagnetic materials like aluminum, uranium and platinum become more magnetic when they are very cold. The force of a ferromagnetic magnet is about a million times that of a magnet made with a paramagnetic material. Since the attractive force is so small, paramagnetic materials are typically considered nonmagnetic.
Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.
Ferromagnetic materialsFerromagneticDomains inMaterials Ferromagnetic material are those in which each individual atom or molecule has a non zero magnetic moment
• Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atoms moments (1012 to 1015) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnified state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. Components with these materials are commonly inspected using the magnetic particle method.
Ferromagnets• A ferromagnetic material is one that has magnetic properties similar to those of iron. In other words, you can make a magnet out of it. Ferromagnetic materials are strongly attracted by a magnetic force. The elements iron (Fe), nickel (Ni), cobalt (Co) and gadolinium (Gd) are such materials.• Magnetic fields come from currents. This is true even in ferromagnetic materials; their magnetic properties come from the motion of electrons in the atoms. Each electron has a spin. This is a quantum mechanical phenomenon that is difficult to make a comparison to, but can be thought of as similar to the rotation of the Earth about its axis.
Iron and steel• Iron is the most common element associated with being attracted to to a magnet. Steel is also a ferromagnetic material. It is an alloy or combination of iron and several other metals, giving it greater hardness than iron, as well as other specialized properties. Because of its hardness, steel retains magnetism longer than iron.
TWO FORMS OF IRON :• Soft Iron , If you were hit on the head with a soft iron bar, it would still feel very hard; soft is simply a term describing the magnetic properties. In hard iron, the domains will not shift back to their starting points when the field is taken away. In soft iron, the domains return to being randomly aligned when the field is removed.• Hard iron is used in permanent magnets. To make a permanent magnet, a piece of hard iron is placed in a magnetic field. The domains align with the field, and retain a good deal of that alignment when the field is removed, resulting in a magnet.
blade with soft ironsoft iron wire Soft Iron, Stainless Steel
CURIE TEMPERATURE The Curie temperature (Tc) is the critical temperaturebeyond which a previously ferromagnetic material becomesparamagnetic. On the atomic level, below the Curietemperature the magnetic moments, contributed mainly bythe electrons, are alligned in their respective domains andeven a weak external field results in a net magnetization. Asthe temperature increases to Tc and above however,fluctuations due to the increase in thermal energy destroythat allignment. Tc for nickel is 631K, while that for iron is1043K.
CURIE TEMPERATURE: Curie temperature in ferromagnetic and ferrimagnetic materials. Substance Curie temp °CIron (Fe) 770Cobalt (Co) 1130Nickel (Ni) 358Iron Oxide 622(Fe2O3)
Magnetizing ferromagnets Ferromagnetic materials can be magnetized in the following ways:• Heating the object above its Curie temperature, allowing it to cool in a magnetic field and hammering it as it cools. This is the most effective method and is similar to the industrial processes used to create permanent magnets.• 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 Earths magnetic field that are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant residual magnetism.• Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction.
Demagnetizing ferromagnets Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:• Heating a magnet past its Curie temperature; the molecular motion destroys the alignment of the magnetic domains. This always removes all magnetization.• Placing the magnet in an alternating magnetic field with an intensity above the materials coercivity and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools and erase credit cards and hard disks and degaussing coils used to demagnetize CRTs.• Some demagnetization or reverse magnetization will occur if any part of the magnet is subjected to a reverse field above the magnetic materials coercivity.• Demagnetisation progressively occurs if the magnet is subjected to cyclic 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 demagnetisation curve).• Hammering or jarring: the mechanical disturbance tends to randomize the magnetic domains. Will leave some residual magnetization.
Hysteresis curveMagnetic Saturation The relationship between magnetic field strength (H) and magnetic flux density (B) will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation till point (a).
R e te nt it y• the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or Retentity as shown in graph at point (b)• The thickness of the middle, describes the amount of hysteresis, related to the coercivity of the material as from (c) to (f)
Hysteresis curve of soft Iron and steel The retentivity of soft iron > retentivity of steel Soft iron is more strongly magnetized than steel Coercivity of soft iron < Coercivity of steel Hence, soft iron loses its magnetism more rapidly than steel does.