Eddy Current An Electrical Phenomenon Biology Essay

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An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. 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 conductor. These circulating eddies of current create electromagnets with magnetic fields that opposes the change of the magnetic field (see Lenz's law). 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.

Contents

· 1Introduction

o 1.1Strength of eddy currents

· 2Applications

o 2.1Electrical

o 2.2Repulsive effects/levitation

o 2.3Mechanical

§ 2.3.1Structural Testing

o 2.4Side Effects

o 2.5Other Applications

1. Introduction:--

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, 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. 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 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. 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 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. 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.

1.1 Strength of eddy current:--

Some things usually increase the size and effects of eddy currents:

· stronger magnetic fields

· faster changing fields (due to faster relative speeds or otherwise)

· thicker materials

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

Some things reduce the effects

· weaker magnets

· slower changing fields (slower relative speeds)

· thinner materials

· slotted materials so that currents cannot circulate

· laminated materials so that currents cannot circulate

· higher resistance materials (silicon rich iron etc.)

2. Applications

2.1 Electrical

Eddy currents are used to great effect in movement-to-electricity converters such as electrical generators and dynamic microphones.

2.2 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. 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.

2.3 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. Electrical resistance within the plates causes a dragging effect analogous to friction, which dissipates the kinetic energy of the car.

2.3.1 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.

2.4 Side Effects

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

2.5 Other Applications

· Metal detector

· Eddy current adjustable-speed drives

· Eddy-current testing

· Electric meters (Electromechanical Induction Meters)

· Eddy current brakes

· Induction cooker

· Metal detectors

· Proximity sensor (Displacement sensors)

· Traffic detection systems

· Vending machines (detection of coins)

· Coating Thickness Measurements [1]

· Eddy current separator for metal separation [2]

· Mechanical speedometers

· Safety Hazard and defect detection applications

App. Of eddy current--- Circular eddy current brake

Circular eddy current brake on 700 Series Shinkansen

Electromagnetic brakes are similar to electrical motors; non-ferromagnetic metal discs (rotors) are connected to a rotating coil, and a magnetic field between the rotor and the coil creates a resistance used to generate electricity or heat. When electromagnets are used, control of the braking action is made possible by varying the strength of the magnetic field. A braking force is possible when electric current is passed through the electromagnets. The movement of the metal through the magnetic field of the electromagnets creates eddy currents in the discs. These eddy currents generate an opposing magnetic field, which then resists the rotation of the discs, providing braking force. The net result is to convert the motion of the rotors into heat in the rotors.

Japanese Shinkansen trains had employed circular eddy current brake system on trailer cars since 100 Series Shinkansen. However, N700 Series Shinkansen abolished eddy current brake system because it can utilize regenerative brake easily due to 14 electric motor cars out of 16 cars trainset.

A no. of experiment performed to show the phenomenon of eddy currents let us take one of these experiments and shows phenomenon of eddy currents:-

Let us take a electromagent which is connected to the a.c source and a metallic circular plate is placed on the top of electromagent. When a.c is switched on the circular plate is thrown up in to the air. This is because of eddy current developed in the plate.

As currents through the solenoid increases, the magetic flux along the axis of the solenoid increases. Therefore, magetic flux linked with the disc increases . Induced currents or eddy currents developed in the disc and magnetise it. If upper end of the solenoid initially acquires north polarity, the lower face of the disc also acquires north polarity in accordance with Lenz's law. The force of repulsion between the two throws the disc up in air. See the figure 6.1(a)…

Figure 6.1(a)

? How to minimize the eddy currents

To get rid of eddy currents, slits can be cut in metals so that large eddies cannot occur. This is why the metal cores of transformers are often assembled in small laminations with an insulator in between. This prevents AC energy from being lost to eddies generated within the magnetic core (which typically is also conductive because it is a metal like iron). Large resistance between the thin sheets confines the eddy currents to the individual's sheets. Hence the eddy currents are reduced to a large extent.

As dissipation of electric energy into heat varies directly as the square of the strength of electric current, therefore, heat loss is greatly reduced.

NOTE-We can reduce the eddy current, but cannot eliminate them.

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