Secara umum, bahan magnetik terbuat dari logam




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PENDAHULUAN


Secara umum, bahan magnetik terbuat dari logam
 Banyak peralatan elektroteknik seperti transformator, mesin-mesin listrik memanfaatkan sifat-sifat magnet dari material magnetik
Material magnetik dengan kualitas tinggi sangat dipengaruhi oleh teknologi proses material
Material Magnetik Diamagnetik Paramagnetik Ferromagnetik Anti ferromagnetic ferrimagnetik (ferrit).
FERRIMAGNETIK
Material ferrimagnetik seperti ferrit biasanya non konduktif dan bebas rugi-rugi arus eddy.
Material ini banyak digunakan untuk medan magnetik dengan frekuensi tinggi.
DIAMAGNETIK
Bahan diamagnetik tidak mempunyai dipol magnet parmanen di dalam material namun terdapat momen magnetik induksi yang lemah
Material diamagnetik cenderung menolak medan magnetik luar secara sempurna.
FERROMAGNETIK
 Material ferromagnetik seperti besi mempunyai magnetisasi permanen yang sangat besar meskipun tanpa kehadiran medan magnetik luar.
Sifat magnetik bahan muncul karena struktur elektron dalam atom yang tidak lengkap, artinya ditemukan beberapa elektron yang tidak berpasangan yang akan menyebabkan dipol magnetik.
Sifat ferromagnetik muncul karena atom mempunyai struktur elektron yang tidak berpasangan dalam jumlah yang cukup banyak yang memungkinkan munculnya momen dipol yang cukup besar.
Paramagnetisme
Kehadiran medan magnetik luar H akan mengarahkan momen elektron ke arah tertentu menghasilkan energi potensial sebesar:
Ep = -μm H cos a
Material Magnetik Lunak
Material magnetik yang mudah dimagnetisasi dan didemagnetisasi.
Peran utama material ini adalah untuk meningkatkan pengaruh magnetik yang dihasilkan oleh suatu kumparan yang berarus.
Bahan dipakai untuk tegangan bolak balik,
Mengurangi rugi-rugi yang disebabkan oleh arus pusar (eddy current), yang mana bisa dilakukan dengan menaikkan resistivitas.
Material Magnetik Keras
( Magnet Permanen)
Material magnetik yang sulit di magnetisasi dan di demagnetisasi ditandai dengan :
- Gaya koersif (demagnetisasi) tinggi
- Induksi magnetik sisa tinggi
Sekali dimagnetisasi material magnetik keras akan sulit di demagnetisasi.
Pendekatan Makroskopik
Kehadiran material magnetik akan menimbulkan kerapatan fluks magnetik. Persamaan untuk ini ditunjukkan oleh:
B = mo.mr H
B = fluks magnet
H = medan magnet
mr = permeabilitas relatif
Teori Domain
Bahan Ferromagnetik disusun oleh sejumlah daerah sub mikro yanh disebut domain
Setiap domain terdiri dari momen magnetik yang paralel
Arah momen magnetik masing-masing domain tidak selalu sama
Saat tanpa medan luar , tiap domain mempunyai orientasi sendiri, jadi energi diperlukan paling rendah = 0
Ketika medan magnet luar diberikan, domain magnetik akan mengarah mengikuti medan luar
Susunan domain terjadi kira-kira disebabkan oleh berkurangnya energi untuk semua moment magnetic yang berbaris naik dalam satu arah.
Timbulnya medan magnit yang besar, akan menimbulkan sejumlah energi yang besar yang keluar dari material itu.
Energi ini akan direduksi jika material terpisah ke dalam domain seperti yang terlihat dalam gambar berikut
Aplikasi Material Magnetik
ISOLATOR
Suatu alat yang melewatkan satu arah gelombang elektromagnetik tetapi attenuasinya sangat besar dengan arah yang berlawanan.
SENSOR
 Medan magnet yang dialiri arus dapat digunakan sebagai alat pendeteksi posisi, daya, torsi, kecepatan, putaran, dan akselerasi.
MOTOR-MOTOR LISTRIK
Mengkonversi energi listrik menjadi energi mekanik.
Prinsip kerjanya sama dengan kumparan putar dari sirkit magnetik meter
Kesimpulan
1. Bahan magnetik adalah logam.
 2 Teknologi magnet untuk menghasilkan material magnetik dengan kualitas tinggi sangat dipengaruhi oleh teknologi proses material.
3. Bahan dapat digolongkan menjadi 5 yaitu diamagnetik, paramagnetik, ferromagnetik, anti ferromagnetik dan ferrimagnetik (ferrit).

 4. Suatu material magnetik yang sudah dimagnetisasi mempunyai magnetisasi sisa (remanensi), meskipun medan magnet luar sudah dihilangkan.(kurva magnetisasi)


5. Rugi magnet terdiri atas rugi histerisis dan Eddy current
6. Aplikasi material magnetik:
- motor-motor listrik
- trafo
-sensor,dll

Ferromagnetism

Ferromagnetic ordering of microscopic magnets (the magnetic moments of individual particles).



Ferromagnetism is the basic mechanism by which certain materials (such as iron) form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished. Ferromagnetism is the strongest type; it is the only type that can produce forces strong enough to be felt, and is responsible for the common phenomena of magnetism encountered in everyday life. One example is refrigerator magnets. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today," according to a classic text on ferromagnetism.[1]

All permanent magnets (materials that can be magnetized by an external magnetic field and which remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are other materials that are noticeably attracted to them.



Historically, the term ferromagnet was used for any material that could exhibit spontaneous magnetization: a net magnetic moment in the absence of an external magnetic field. This general definition is still in common use. More recently, however, different classes of spontaneous magnetization have been identified[citation needed] when there is more than one magnetic ion per primitive cell of the material, leading to a stricter definition of "ferromagnetism" that is often used to distinguish it from ferrimagnetism.[citation needed] In particular, a material is "ferromagnetic" in this narrower sense only if all of its magnetic ions add a positive contribution to the net magnetization. If some of the magnetic ions subtract from the net magnetization (if they are partially anti-aligned), then the material is "ferrimagnetic".[citation needed] If the moments of the aligned and anti-aligned ions balance completely so as to have zero net magnetization, despite the magnetic ordering, then it is an antiferromagnet. All of these alignment effects only occur at temperatures below a certain critical temperature, called the Curie temperature (for ferromagnets and ferrimagnets) or the Néel temperature (for antiferromagnets).

Among the first investigations of ferromagnetism are the pioneering works of Aleksandr Stoletov on measurement of the magnetic permeability of ferromagnetics, known as the Stoletov curve.



























































Material

Curie
temp. (K)


Co

1388

Fe

1043

FeOFe2O3*

858

NiOFe2O3*

858

CuOFe2O3*

728

MgOFe2O3*

713

MnBi

630

Ni

627

MnSb

587

MnOFe2O3*

573

Y3Fe5O12*

560

CrO2

386

MnAs

318

Gd

292

Dy

88

EuO

69
Ferromagnetic materials

There are a number of crystalline materials that exhibit ferromagnetism (or ferrimagnetism). The table on the right lists a representative selection of them, along with their Curie temperatures, the temperature above which they cease to exhibit spontaneous magnetization (see below).

Ferromagnetism is a property not just of the chemical makeup of a material, but of its crystalline structure and microscopic organization. There are ferromagnetic metal alloys whose constituents are not themselves ferromagnetic, called Heusler alloys, named after Fritz Heusler. Conversely there are nonmagnetic alloys, such as types of stainless steel, composed almost exclusively of ferromagnetic metals.

One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by very rapid quenching (cooling) of a liquid alloy. These have the advantage that their properties are nearly isotropic (not aligned along a crystal axis); this results in low coercivity, low hysteresis loss, high permeability, and high electrical resistivity. A typical such material is a transition metal-metalloid alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a metalloid component (B, C, Si, P, or Al) that lowers the melting point.

A relatively new class of exceptionally strong ferromagnetic materials are the rare-earth magnets. They contain lanthanide elements that are known for their ability to carry large magnetic moments in well-localized f-orbitals.

Actinide ferromagnets

A number of actinide compounds are ferromagnets at room temperature or become ferromagnets below the Curie temperature (TC). PuP is one actinide pnictide that is a paramagnet and has cubic symmetry at room temperature, but upon cooling undergoes a lattice distortion to tetragonal when cooled to below its Tc = 125 K. PuP has an easy axis of <100>,[2] so that



  • (c – a)/a = –(31 ± 1) × 10−4

at 5 K.[3] The lattice distortion is presumably a consequence of strain induced by the magnetoelastic interactions as the magnetic moments aligned parallel within magnetic domains.

In NpFe2 the easy axis is <111>.[4] Above TC ~500 K NpFe2 is also paramagnetic and cubic. Cooling below the Curie temperature produces a rhombohedral distortion wherein the rhombohedral angle changes from 60° (cubic phase) to 60.53°. An alternate discription of this distortion is to consider the length c along the unique trigonal axis (after the distortion has begun) and a as the distance in the plane perpendicular to c. In the cubic phase this reduces to c/a = 1.00. Below the Curie temperature



  • (c – a)/a = –(120 ± 5) × 10−4

which is the largest strain in any actinide compound.[3] NpNi2 undergoes a similar lattice distortion below TC = 32 K, with a strain of (43±5) × 10−4.[3] NpCo2 is a ferrimagnet below 15 K.

Lithium gas

In 2009, a team of MIT physicists demonstrated that a lithium gas cooled to less than one Kelvin can exhibit ferromagnetism. [5] The team cooled fermionic lithium-6 to less than 150 billionths of one Kelvin above absolute zero using infrared laser cooling. This demonstration is the first time that ferromagnetism has been demonstrated in a gas.



Explanation

The property of ferromagnetism is due to the direct influence of two effects from quantum mechanics: spin and the Pauli exclusion principle.[6]



Origin of magnetism

The spin of an electron, combined with its electric charge, results in a magnetic dipole moment and creates a magnetic field. Although an electron can be visualized classically as a spinning ball of charge, spin is actually a quantum mechanical property with differences from the classical picture, such as the fact that it is quantized into discrete up/down states. The spin of the electrons in atoms is the main source of ferromagnetism, although there is also some contribution from the orbital angular momentum of the electron about the nucleus, whose classical analogy is a current loop.

However in many materials (specifically, those with a filled electron shell), the total dipole moment of all the electrons is zero because the spins are in up/down pairs. Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment, so ferromagnetism only occurs in materials with partially filled shells. When these tiny magnetic dipoles are aligned in the same direction, their individual magnetic fields add together to create a measurable macroscopic field.

These unpaired dipoles (often called simply "spins" even though they also generally include angular momentum) tend to align in parallel to an external magnetic field, an effect called paramagnetism. Ferromagnetism involves an additional phenomenon, however: the dipoles tend to align spontaneously, without any applied field. This is a purely quantum-mechanical effect.



Exchange interaction

According to classical electromagnetism, two nearby magnetic dipoles will tend to align in opposite directions, so their magnetic fields will oppose one another and cancel out. However in a few materials, the ferromagnetic ones, they tend to align in the same direction because of a quantum mechanical effect called the exchange interaction. The Pauli exclusion principle says that two electrons with the same spin cannot also have the same "position". Therefore, under certain conditions, when the orbitals of the unpaired outer valence electrons from adjacent atoms overlap, the distribution of their electric charge in space is further apart when the electrons have parallel spins than when they have opposite spins. This reduces the electrostatic energy of the electrons when their spins are parallel compared to their energy when the spins are anti-parallel, so the parallel-spin state is more stable. In simple terms, the electrons, which repel one another, can move "further apart" by aligning their spins, so the spins of these electrons tend to line up. This difference in energy is called the exchange energy.

The exchange interaction is also responsible for the other types of spontaneous ordering of atomic magnetic moments occurring in magnetic solids, antiferromagnetism and ferrimagnetism. In most ferromagnets the exchange interaction is much stronger than the competing dipole-dipole interaction. For instance, in iron (Fe) it is about 1000 times stronger than the dipole interaction. Therefore below the Curie temperature virtually all of the dipoles in a ferromagnetic material will be aligned.

Magnetic domains

The above would seem to suggest that every piece of ferromagnetic material should have a strong magnetic field, yet iron and other ferromagnets are often found in an "unmagnetized" state.





Weiss domains microstructure

The reason for this is that a bulk piece of ferromagnetic material is divided into many tiny magnetic domains (also known as Weiss domains). Within each domain, the spins are aligned, but (if the bulk material is in its lowest energy configuration, i.e. "unmagnetized"), the spins of separate domains point in different directions and their magnetic fields cancel out, so the object has no net large scale magnetic field.

Ferromagnetic materials spontaneously divide into magnetic domains because this is a lower energy configuration. At long distances (after many thousands of ions), the exchange energy advantage is overtaken by the classical tendency of dipoles to anti-align. The boundary between two domains, where the magnetization flips, is called a domain wall (i.e., a Bloch/Néel wall, depending upon whether the magnetization rotates parallel/perpendicular to the domain interface) and is a gradual transition on the atomic scale (covering a distance of about 300 ions for iron).

Thus, an ordinary piece of iron generally has little or no net magnetic moment. However, if it is placed in a strong enough external magnetic field, the domains will re-orient in parallel with that field, and will remain re-oriented when the field is turned off, thus creating a "permanent" magnet. The domains don't go back to their original minimum energy configuration when the field is turned off because the domain walls tend to become 'pinned' or 'snagged' on defects in the crystal lattice, preserving their parallel orientation.

This is shown by the Barkhausen effect: as the magnetizing field is changed, the magnetization changes in thousands of tiny discontinuous jumps as the domain walls suddenly "snap" past defects. This magnetization as a function of the external field is described by a hysteresis curve. Although this state of aligned domains is not a minimal-energy configuration, it is extremely stable and has been observed to persist for millions of years in seafloor magnetite aligned by the Earth's magnetic field (whose poles can thereby be seen to flip at long intervals).

Alloys used for the strongest permanent magnets are "hard" alloys made with many defects in their crystal structure where the domain walls "catch" and stabilize. The net magnetization can be destroyed by heating and then cooling (annealing) the material without an external field, however. The thermal motion allows the domain boundaries to move, releasing them from any defects, to return to their low-energy unaligned state.

Curie temperature

As the temperature increases, thermal motion, or entropy, competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the Curie temperature, there is a second-order phase transition and the system can no longer maintain a spontaneous magnetization, although it still responds paramagnetically to an external field. Below that temperature, there is a spontaneous symmetry breaking and random domains form (in the absence of an external field). The Curie temperature itself is a critical point, where the magnetic susceptibility is theoretically infinite and, although there is no net magnetization, domain-like spin correlations fluctuate at all length scales.

The study of ferromagnetic phase transitions, especially via the simplified Ising spin model, had an important impact on the development of statistical physics. There, it was first clearly shown that mean field theory approaches failed to predict the correct behavior at the critical point (which was found to fall under a universality class that includes many other systems, such as liquid-gas transitions), and had to be replaced by renormalization group theory.

Antiferromagnetism



Antiferromagnetic ordering



  • In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism. Generally, antiferromagnetic order may exist at sufficiently low temperatures, vanishing at and above a certain temperature, the Néel temperature (named after Louis Néel, who had first identified this type of magnetic ordering).[1] Above the Néel temperature, the material is typically paramagnetic.

Physical Origin

An anti-ferromagnetic interaction acts to anti-align neighboring spins. If the energy is expressed as the sum of all pairs, i, j, over an interaction term J(i,j), times the spin of atom i times the spin of atom j, J < 0 is a ferromagnetic interaction and J > 0 is an antiferromagnetic interaction. The combination of both can lead to spin glass behavior.

When no external field is applied, the antiferromagnetic structure corresponds to a vanishing total magnetization. In a field, a kind of ferrimagnetic behavior may be displayed in the antiferromagnetic phase, with the absolute value of one of the sublattice magnetizations differing from that of the other sublattice, resulting in a nonzero net magnetization.

The magnetic susceptibility of an antiferromagnetic material typically shows a maximum at the Néel temperature. In contrast, at the transition between the ferromagnetic to the paramagnetic phases the susceptibility will diverge. In the antiferromagnetic case, a divergence is observed in the staggered susceptibility.

Various microscopic (exchange) interactions between the magnetic moments or spins may lead to antiferromagnetic structures. In the simplest case, one may consider an Ising model on an bipartite lattice, e.g. the simple cubic lattice, with couplings between spins at nearest neighbor sites. Depending on the sign of that interaction, ferromagnetic or antiferromagnetic order will result. Geometrical frustration or competing ferro- and antiferromagnetic interactions may lead to different and, perhaps, more complicated magnetic structures.

Geometric Frustration

Unlike ferromagnetism, anti-ferromagnetic interactions can lead to multiple optimal states (ground states—states of minimal energy). In one dimension, the anti-ferromagnetic ground state is an alternating series of spins: up, down, up, down, etc. Yet in two dimensions, multiple ground states can occur.

Consider an equilateral triangle with three spins, one on each vertex. If each spin can take on only two values (up or down), there are 23 = 8 possible states of the system, six of which are ground states! The two situations which are not ground states are when all three spins are up or are all down. In any of the other six states, there will be two favorable interactions and one unfavorable one. This illustrates frustration: the inability of the system to find a single ground state. This type of magnetic behavior has been found in minerals that have a crystal stacking structure such as a Kagome lattice or hexagonal lattice.

Antiferromagnetic Materials

Antiferromagnetic materials occur commonly among transition metal compounds, especially oxides. An example is the heavy-fermion superconductor URu2Si2. Better known examples include hematite, metals such as chromium, alloys such as iron manganese (FeMn), and oxides such as nickel oxide (NiO). There are also numerous examples among high nuclearity metal clusters. Organic molecules can also exhibit antiferromagnetic coupling under rare circumstances, as seen in radicals such as 5-dehydro-m-xylylene.

Antiferromagnets can couple to ferromagnets, for instance, through a mechanism known as exchange bias, in which the ferromagnetic film is either grown upon the antiferromagnet or annealed in an aligning magnetic field, causing the surface atoms of the ferromagnet to align with the surface atoms of the antiferromagnet. This provides the ability to "pin" the orientation of a ferromagnetic film, which provides one of the main uses in so-called spin valves, which are the basis of magnetic sensors including modern hard drive read heads. The temperature at or above which an antiferromagnetic layer loses its ability to "pin" the magnetization direction of an adjacent ferromagnetic layer is called the blocking temperature of that layer and is usually lower than the Néel temperature.

Other properties

Antiferromagnetism plays a crucial role in giant magnetoresistance, as had been discovered in 1988 by the Nobel prize winners Albert Fert and Peter Grünberg.

There are also examples of disordered materials (such as iron phosphate glasses) that become antiferromagnetic below their Néel temperature. These disordered networks 'frustrate' the antiparallelism of adjacent spins; i.e. it is not possible to construct a network where each spin is surrounded by opposite neighbour spins. It can only be determined that the average correlation of neighbour spins is antiferromagnetic. This type of magnetism is sometimes called speromagnetism.

Eddy current

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Electromagnetism









This article is about the electrical phenomon. For the Ted McKeever comic, see Eddy Current (comics).

Eddy currents are induced currents in a conductor which arise to oppose the change in flux that generated them.[citation needed] It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body of the conductor. These circulating eddies of current create induced magnetic fields that oppose the change of the original magnetic field due to Lenz's law, causing repulsive or drag forces between the conductor and the magnet. The stronger the applied magnetic field, or the greater the electrical conductivity of the conductor, or the faster the field that the conductor is exposed to changes, then the greater the currents that are developed and the greater the opposing field.

The term eddy current comes from analogous currents seen in water when dragging an oar breadthwise: localised areas of turbulence known as eddies give rise to persistent vortices.

Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. The heat can be harnessed for induction heating. The electromagnetic forces can be used for levitation, creating movement, or to give a strong braking effect. Eddy currents can often be minimised with thin plates, by lamination of conductors or other details of conductor shape.

Explanation

As the circular plate moves down through a small region of constant magnetic field directed into the page, eddy currents are induced in the plate. The direction of those currents is given by Lenz's law.

When a conductor moves relative to the field generated by a source, electromotive forces (EMFs) can be generated around loops within the conductor. These EMFs acting on the resistivity of the material generate a current around the loop, in accordance with Faraday's law of induction. These currents dissipate energy, and create a magnetic field that tends to oppose the changes in the field.

Eddy currents are created when a moving conductor experiences changes in the magnetic field generated by a stationary object, as well as when a stationary conductor encounters a varying magnetic field. Both effects are present when a conductor moves through a varying magnetic field, as is the case at the top and bottom edges of the magnetized region shown in the diagram. Eddy currents will be generated wherever a conducting object experiences a change in the intensity or direction of the magnetic field at any point within it, and not just at the boundaries.

The swirling current set up in the conductor is due to electrons experiencing a Lorentz force that is perpendicular to their motion. Hence, they veer to their right, or left, depending on the direction of the applied field and whether the strength of the field is increasing or declining. The resistivity of the conductor acts to damp the amplitude of the eddy currents, as well as straighten their paths. Lenz's law encapsulates the fact that the current swirls in such a way as to create an induced magnetic field that opposes the phenomenon that created it. In the case of a varying applied field, the induced field will always be in the opposite direction to that applied. The same will be true when a varying external field is increasing in strength. However, when a varying field is falling in strength, the induced field will be in the same direction as that originally applied, in order to oppose the decline.

An object or part of an object experiences steady field intensity and direction where there is still relative motion of the field and the object (for example in the center of the field in the diagram), or unsteady fields where the currents cannot circulate due to the geometry of the conductor. In these situations charges collect on or within the object and these charges then produce static electric potentials that oppose any further current. Currents may be initially associated with the creation of static potentials, but these may be transitory and small.

Eddy currents generate resistive losses that transform some forms of energy, such as kinetic energy, into heat. In many devices, this Joule heating reduces efficiency of iron-core transformers and electric motors and other devices that use changing magnetic fields. Eddy currents are minimized in these devices by selecting magnetic core materials that have low electrical conductivity (e.g., ferrites) or by using thin sheets of magnetic material, known as laminations. Electrons cannot cross the insulating gap between the laminations and so are unable to circulate on wide arcs. Charges gather at the lamination boundaries, in a process analogous to the Hall effect, producing electric fields that oppose any further accumulation of charge and hence suppressing the eddy currents. The shorter the distance between adjacent laminations (i.e., the greater the number of laminations per unit area, perpendicular to the applied field), the greater the suppression of eddy currents.

The conversion of input energy to heat is not always undesirable, however, as there are some practical applications. One is in the brakes of some trains known as eddy current brakes. During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The eddy currents meet resistance as charges flow through the metal, thus dissipating energy as heat, and this acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.



Strength of eddy currents

Under certain assumptions (uniform material, uniform magnetic field, no skin effect, etc.) the power lost due to eddy currents can be calculated from the following equations:

For thin sheets:

For thin wires:

where: Bp - peak flux density (T), d - thickness of the sheet or diameter of the wire (m), ρ - resistivity (Ωm), D - penetration depth or skin depth (m).[1]

Therefore, the following things usually increase the size and effects of eddy currents:



  • stronger magnetic fields - increases flux density B

  • faster changing fields (due to faster relative speeds or otherwise) - increases the frequency f

  • thicker materials - increases the thickness d

  • lower resistivity materials (aluminium, copper, silver etc.)

Some things reduce the effects

  • weaker magnets - lower B

  • slower changing fields (slower relative speeds) - lower f

  • thinner materials - lower d

  • slotted materials so that currents cannot circulate - reduced d or coefficient in the denominator (6, 12, etc.)

  • laminated materials so that currents cannot circulate - reduced d

  • higher resistance materials (silicon rich iron etc.)

  • very fast changing fields - due to skin effect the above equations are not valid because the magnetic field does not penetrate the material uniformly.

Applications

Repulsive effects and levitation

In a fast varying magnetic field the induced currents, in good conductors, particularly copper and aluminium, exhibit diamagnetic-like repulsion effects on the magnetic field, and hence on the magnet and can create repulsive effects and even stable levitation, albeit with reasonably high power dissipation due to the high currents this entails.

They can thus be used to induce a magnetic field in aluminum cans, which allows them to be separated easily from other recyclables. With a very strong handheld magnet, such as those made from neodymium, one can easily observe a very similar effect by rapidly sweeping the magnet over a coin with only a small separation. Depending on the strength of the magnet, identity of the coin, and separation between the magnet and coin, one may induce the coin to be pushed slightly ahead of the magnet - even if the coin contains no magnetic elements, such as the US penny.

Superconductors allow perfect, lossless conduction, which creates perpetually circulating eddy currents that are equal and opposite to the external magnetic field, thus allowing magnetic levitation. For the same reason, the magnetic field inside a superconducting medium will be exactly zero, regardless of the external applied field.

Identification of metals

In coin operated vending machines, eddy currents are used to detect counterfeit coins, or slugs. The coin rolls past a stationary magnet, and eddy currents slow its speed. The strength of the eddy currents, and thus the amount of slowing, depends on the conductivity of the coin's metal. Slugs are slowed to a different degree than genuine coins, and this is used to send them into the rejection slot.



Vibration | Position Sensing

Eddy currents are used in certain types of proximity sensors to observe the vibration and position of rotating shafts within their bearings. This technology was originally pioneered in the 1930s by researchers at General Electric using vacuum tube circuitry. In the late 1950s, solid-state versions were developed by Donald E. Bently at Bently Nevada Corporation. These sensors are extremely sensitive to very small displacements making them well suited to observe the minute vibrations (on the order of several thousandths of an inch) in modern turbomachinery. A typical proximity sensor used for vibration monitoring has a scale factor of 200 mV/mil. Widespread use of such sensors in turbomachinery has led to development of industry standards that prescribe their use and application. Examples of such standards are American Petroleum Institute (API) Standard 670 and ISO 7919.



Electromagnetic braking

Main article: Eddy current brake

Eddy currents are used for braking at the end of some roller coasters. This mechanism has no mechanical wear and produces a very precise braking force. Typically, heavy copper plates extending from the car are moved between pairs of very strong permanent magnets. Electrical resistance within the plates causes a dragging effect analogous to friction, which dissipates the kinetic energy of the car. The same technique is used in electromagnetic brakes in railroad cars and to quickly stop the blades in power tools such as circular saws.

Structural testing

Eddy current techniques are commonly used for the nondestructive examination (NDE) and condition monitoring of a large variety of metallic structures, including heat exchanger tubes, aircraft fuselage, and aircraft structural components.



Side effects

Eddy currents are the root cause of the skin effect in conductors carrying AC current.

Similarly, in magnetic materials of finite conductivity eddy currents cause the confinement of magnetic fields to only a couple skin depths of the surface of the material. This effect limits the flux linkage in inductors and transformers having magnetic cores.

Other applications


  • Metal detectors

  • Eddy current adjustable-speed drives

  • Eddy-current testing

  • Electric meters (Electromechanical Induction Meters)

  • Eddy current brakes

  • Induction heating

  • Proximity sensor (Displacement sensors)

  • Traffic detection systems

  • Vending machines (detection of coins)

  • Coating Thickness Measurements [2]

  • Sheet Resistance Measurement [3]

  • Eddy current separator for metal separation [4]

  • Mechanical speedometers

  • Safety Hazard and defect detection applications

Diffusion Equation

The derivation of a useful equation for modeling the effect of eddy currents in a material starts with the differential, magnetostatic form of Ampère's Law[5], providing an expression for the magnetic field H surrounding a current density J,



.

The curl is taken on both sides of the equation,



,

and using a common vector calculus identity for the curl of the curl results in



.

From Gauss's law for magnetism, , which drops a term from the expression and gives



.

Using Ohm's law, , which relates current density J to electric field Ε in terms of a material's conductivity σ, and assuming isotropic conductivity, the equation can be written as



.

The differential form of Faraday's law, , provides an equivalence for the change in magnetic flux B in place of the curl of the electric field, so that the equation can be simplified to



.

By definition, , where M is the magnetization of a material, and the diffusion equation finally appears as





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