Electromagnetism

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An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. This article is about the country For a topic outline on this subject see List of basic France topics. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Jean Bernard Léon Foucault (ʒɑ̃ bɛʁnaʁ leɔ̃ fu'ko ( 18 September 1819 &ndash 11 February 1868) was a French physicist 1851 ( MDCCCLI) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian Calendar (or a Common year 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. In Science and engineering, a conductor is a material which contains movable Electric charges. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges This can cause a circulating flow of electrons, or current, within the conductor. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. These circulating eddies of current create electromagnets with magnetic fields that opposes the change of the magnetic field (see Lenz's law). In Fluid dynamics, an eddy is the swirling of a Fluid and the reverse current created when the fluid flows past an obstacle An electromagnet is a type of Magnet in which the Magnetic field is produced by the flow of an electric current. Lenz's law (ˈlɛntsɨz ˌlɔː gives the direction of the induced Electromotive force (emf and current resulting from Electromagnetic induction. The stronger the applied magnetic field, or greater the electrical conductivity of the conductor, or greater the relative velocity of motion, the greater the currents developed and the greater the opposing field. Electrical conductivity or specific conductivity is a measure of a material's ability to conduct an Electric current.

Eddy currents can generate a lot of heat, and also can create a strong repulsive force between the conductor and the field source which can be used for levitation or creating movement, or to give a strong braking effect.

Introduction

When a conductor moves relative to the field generated by a source, then electro-magnetic fields (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. Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of 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. In Physics, the Lorentz force is the Force on a Point charge due to Electromagnetic fields It is given by the following equation 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. Lenz's law (ˈlɛntsɨz ˌlɔː gives the direction of the induced Electromotive force (emf and current resulting from Electromagnetic induction. 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.

Sometimes 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 flow of current. Currents may be initially associated with the creation of static potentials, but these may be transitory and small.

Eddy currents transform useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. In many devices, this Joule heating reduces efficiency of iron-core transformers and electric motors and other devices that use changing magnetic fields. Joule heating is the process by which the passage of an Electric current through a conductor releases Heat. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors An electric motor uses Electrical energy to produce Mechanical energy. Eddy currents are minimized in these devices by selecting magnetic core materials that have low electrical conductivity (e. The magnetic core is a key component in electrical and electromechanical devices such as Electromagnets Transformers and Inductors A magnetic core is a g. , ferrites) or by using thin sheets of magnetic material, known as laminations. A laminate is a material constructed by uniting two or more layers of material together 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 flow of eddy currents. The Hall effect refers to the Potential difference ( Hall voltage) on the opposite sides of an Electrical conductor through which there is an Electric 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 loss of useful energy is not always undesirable, however, as there are some practical applications. One is in the brakes of some trains known as an eddy current brake. An eddy current brake, like a conventional Friction brake is responsible for slowing an object such as a train or a roller coaster 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 they 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.

The term eddy current comes from analogous currents seen in water when dragging an oar: localised areas of turbulence known as eddies give rise to persistent vortices. Water is a common Chemical substance that is essential for the survival of all known forms of Life. An oar is an implement used for water-borne propulsion. Oars have a flat blade at one end

Applications

Electrical

Eddy currents are used to great effect in movement-to-electricity converters such as electrical generators and dynamic microphones. In Electricity generation, an electrical generator is a device that converts Mechanical energy to Electrical energy, generally using Electromagnetic

Repulsive effects/levitation

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. Superconductivity is a phenomenon occurring in certain Materials generally at very low Temperatures characterized by exactly zero electrical resistance Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than Magnetic fields The For the same reason, the magnetic field inside a superconducting medium will be exactly zero, regardless of the external applied field.

In addition, 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. A beverage can (or drinks can) is most often an Aluminum Can manufactured to hold a single serving of a Beverage.

Mechanical

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. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 Electrical resistance within the plates causes a dragging effect analogous to friction, which dissipates the kinetic energy of the car. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is

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. Nondestructive testing ( NDT) also called nondestructive examination ( NDE) and nondestructive inspection ( NDI) is testing that does A heat exchanger is a device built for efficient Heat transfer from one medium to another whether the media are separated by a solid wall so that they never mix or the media

A new Eddy Currents technique for nondestructive testing was developed by Dr. Nondestructive testing ( NDT) also called nondestructive examination ( NDE) and nondestructive inspection ( NDI) is testing that does Abdollah Abtahi using Fourier transform method in 1981 * to calculate small flaws detection in large area much faster than any other techniques. This method predicts very accurate results for small flaws in large area. The method can be applied for testing any flaws in airplane body or large satellite dishes. This research was supported with a grant by EPRII,

Side Effects

Eddy currents are the root cause of the skin effect in conductors carrying AC current. The skin effect is the tendency of an alternating electric current (AC to distribute itself within a conductor so that the current density near the surface of the

Other Applications

• Eddy current adjustable-speed drives
• Eddy-current testing
• Electric meters (Electromechanical Induction Meters)
• Electromagnetic brakes
• Induction cooker
• Metal detectors
• Proximity sensor (Displacement sensors)
• Traffic detection systems
• Vending machines (detection of coins)

• Eddy current separator for metal separation ^[1]

• Mechanical speedometers
• Safety Hazard and defect detection applications

References

• Fitzgerald, A. Adjustable speed drive (ASD or variable-speed drive (VSD describes equipment used to control the speed of machinery Eddy-current testing uses Electromagnetic induction to detect flaws in Conductive materials On Trams and Trains an electromagnetic Brake is a track brake where the braking element is pressed by magnetic force to the rail An induction cooker uses Induction heating for Cooking. A conducting pot is placed above an Induction coil for the heating process to take place Metal detectors use Electromagnetic induction to detect Metal. A proximity sensor is a Sensor able to detect the presence of nearby objects without any physical contact A vending machine is a machine that provides various snacks beverages and other products to consumers A speedometer is a device that measures the instantaneous Speed of a land vehicle E. ; Kingsley, Charles Jr. and Umans, Stephen D. (1983). Electric Machinery, 4th ed. , Mc-Graw-Hill, Inc. , page 20. ISBN 0-07-021145-0.  
• Sears, Francis Weston; Zemansky, Mark W. (1955). University Physics, 2nd ed. , Reading, MA: Addison-Wesley, pages 616-618.