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www.GOALias.blogspot.com218PhysicsFIGURE 6.13 Eddy currents aregenerated in the copper plate,while enteringand leaving the region ofmagnetic field.FIGURE 6.14 Cutting slotsin the copper plate reducesthe effect of eddy currents.6.8 EDDY CURRENTSSo far we have studied the electric currents induced in well defined pathsin conductors like circular loops. Even when bulk pieces of conductorsare subjected to changing magnetic flux, induced currentsare produced in them. However, their flow patterns resembleswirling eddies in water. This effect was discovered by physicistFoucault (1819-1868) and these currents are called eddycurrents.Consider the apparatus shown in Fig. 6.13. A copper plateis allowed to swing like a simple pendulum between the polepieces of a strong magnet. It is found that the motion is dampedand in a little while the plate comes to a halt in the magneticfield. We can explain this phenomenon on the basis ofelectromagnetic induction. Magnetic flux associated with theplate keeps on changing as the plate moves in and out of theregion between magnetic poles. The flux change induces eddycurrents in the plate. Directions of eddy currents are oppositewhen the plate swings into the region between the poles andwhen it swings out of the region.If rectangular slots are made in the copper plate as shownin Fig. 6.14, area available to the flow of eddy currents is less.Thus, the pendulum plate with holes or slots reduceselectromagnetic damping and the plate swings more freely.Note that magnetic moments of the induced currents (whichoppose the motion) depend upon the area enclosed by thecurrents (recall equation m = I A in Chapter 4).This fact is helpful in reducing eddy currents in the metalliccores of transformers, electric motors and other such devices inwhich a coil is to be wound over metallic core. Eddy currents areundesirable since they heat up the core and dissipate electricalenergy in the form of heat. Eddy currents are minimised by usinglaminations of metal to make a metal core. The laminations areseparated by an insulating material like lacquer. The plane of thelaminations must be arranged parallel to the magnetic field, sothat they cut across the eddy current paths. This arrangementreduces the strength of the eddy currents. Since the dissipationof electrical energy into heat depends on the square of the strengthof electric current, heat loss is substantially reduced.Eddy currents are used to advantage in certain applications like:(i) Magnetic braking in trains: Strong electromagnets are situatedabove the rails in some electrically powered trains. When theelectromagnets are activated, the eddy currents induced in therails oppose the motion of the train. As there are no mechanicallinkages, the braking effect is smooth.(ii) Electromagnetic damping: Certain galvanometers have a fixedcore made of nonmagnetic metallic material. When the coiloscillates, the eddy currents generated in the core oppose themotion and bring the coil to rest quickly.