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Electromagnetic Induction — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsElectromagnetic Induction4 Marks
Question
Define eddy currents. How are they produced? In what sense are these currents undesirable in a transformer and how are these reduced in a device ?
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Answer

Eddy Currents : In 1824 Gambey discovered that the oscillations of a magnet are rapidly damped if a copper sheet is placed under it and close to it. In 1895 Facault observed that when a metal piece is moved in a constant magnetic field or a varying magnetic field is subjected to the metal piece then the magnetic flux linked with the metal piece changes due to which induced current is generated in the whole volume of the piece. According to Lenz's law these currents oppose the change in magnetic flux. These currents look like eddies or whirlpool's in a fluid and are called eddy currents. These are also called Facault current after the name of their discoverer.
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In adjoining figure, a copper sheet in the plane of the paper and a magnetic field is subjected is placed to it in the perpendicular direction to the plane of the paper directed inward represented by cross signs. If we withdraw the sheet out of the field, there is a decrease in its area A within the field and so the magnetic flux $(\Phi= BA )$ linked with the sheet also decreases. Because of this flux change, current loops are induced in the sheet coming out of the field and according to Lenz's law direction of currents is such that the magnetic field produced due to them is in the same direction as the original magnetic field. Thus, eddy currents oppose the decrease in magnetic flux i.e. the drawing out of the sheet. Similarly, if we insert the sheet into the magnetic field the eddy currents are generated in opposite direction to the previous one and oppose the entry of the sheet into the field.
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Eddy Currents undesirable in a device (transformer) and means of their reduction : Eddy currents produce a large amount of heat. This heating effect of eddy currents is undesirable in a number of cases like dynamos, transformers, etc. where the coil is wound on iron core. On the other hand, the heating effect of eddy currents is used to make induction furnaces. Similarly, the braking effect of eddy currents is also undesirable in a number of cases. On the other hand, the braking effect of eddy currents is used in electric brakes and ballistic galvanometers. Let us now see how the breaking and heating effects of eddy currents can be minimised.
The solid iron core [Fig. (a)] is divided into a number of thin sheets as shown in Fig. (b). These thin sheets are electrically insulated from each other. Moreover, these sheets are so placed that the path of the induced eddy currents is broken by the insulating material between the sheets. In this way, the eddy currents are considerably reduced. Such cores are called laminated cores as used in transformer.

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