07(T)%20-%20Electromagnetic%20Induction - 7 ELECTROMAGNETIC...

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7 - ELECTROMAGNETIC INDUCTION Page 1 7.2 Faraday’s Experiments Faraday took a ring of soft iron. On one side of it, an insulated conducting coil was connected with a battery. On the opposite side, another conducting coil was connected with a galvanometer. Faraday observed that passing a steady current through the left coil produced no effect on the galvanometer in the right coil. However, a momentary deflection of galvanometer was noticed whenever the battery was switched on or off. When a steady current is passed magnetic flux produced in the left coil passes through the right coil which does not produce any current in it. Whenever the battery is switched on or off, magnetic flux in the right coil changes from zero to maximum or maximum to zero respectively. This rate of change of magnetic flux in the right coil produces current in it. In another experiment, Faraday arranged two bar magnets in the shape of V. At the open end of V, he kept one soft iron rod with an insulated copper wire wound around it to which galvanometer was connected. On moving the upper magnet up and down, galvanometer showed deflection. Magnetic flux through the coil increased when the magnet touched the iron rod and decreased when it moved away. Faraday concluded from these experiments that ‘To produce electric field in a coil, the change in magnetic flux is important and not the flux itself.’ Faraday also noted that: ( i ) More current is produced when the magnet is moved faster due to faster change of magnetic flux linked with the coil. ( ii ) When a coil carrying electric current is placed above another coil and relative motion produced between the two coils, galvanometer shows deflection in the other coil. ( iii ) If any of the two coils is rotated with respect to the other, then also galvanometer shows deflection.
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7 - ELECTROMAGNETIC INDUCTION Page 2 ( iv ) If the north pole of a bar magnet is moved towards a coil, the galvanometer shows deflection. Now if the magnet is moved away from the coil, the galvanometer shows deflection in the opposite direction. Similar results are obtained with the south pole of the magnet with deflections of galvanometer in opposite direction. Faraday named the current produced as the ‘induced current’, the elf as ‘induced elf’ and the phenomenon as ‘magnetic induction’. 7.3 Lenz’s Law As shown in the above figure, suppose a bar magnet is moved towards a conducting coil with its north pole facing the coil. If this produces current in the coil in the clockwise direction as seen from the side of the magnet, then the side of the coil facing the magnet will act like south pole of a magnet and will attract the magnet. The magnet will have accelerated motion towards the coil which will increase the rate of change of flux and hence the current in the coil. This will increase the force of attraction and the acceleration of the magnet will increase further. Thus the current in the coil will go on increasing. If a resistance
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07(T)%20-%20Electromagnetic%20Induction - 7 ELECTROMAGNETIC...

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