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Introduction

Electromagnetic Induction of Class 12

The term electromagnetic induction constitutes two phenomena. The first involves a current that is induced in a conductor moving relative to magnetic field lines. The second involves the generation of an electric field associated with a time-varying magnetic field.

The observations of Michael Faraday are illustrated in the following cases:

(i)Change in Field Strength.

When a bar magnet moves relative to a loop of wire, there is an induced current in the loop (see Fig.4. 1 ) when the magnet and the loop are stationary, nothing happens.

Electromagnetic Induction

Fig.(4.1)(a)When the north pole moves toward the loop a current flows in anticlockwise sense-as seen from the side on which the magnet is located.

(b)When the north pole moves away from the loop, the current flows in the clockwise sense.

(c)When the south pole of the magnet moves toward the loop, the current flows in the clockwise sense.

(d)When the south pole of the magnet moves away from the loop, the current flows in the anticlockwise sense.

These results (shown in Fig 4.1) are unchanged when the magnet is kept at rest and the loop is moved instead. The magnitude and direction of the induced current depends on the relative velocity of the coil and magnet. Let us consider two coils at rest, as shown in Fig. 4.2. The primary coil is connected in series with a battery and a switch, whereas the secondary coil is connected to a galvanometer. When the switch in the primary circuit is closed, the galvanometer in the secondary deflects for an instant. As long as the primary current stays constant (switch remain closed), nothing happens. When the switch is opened, the meter again has a momentary deflection.

Electromagnetic Induction

(ii)Change in Area 

Consider a circular coil made of flexible wire lying with its plane perpendicular to a uniform constant magnetic field B, as shown in Fig. 4.3. When opposite ends of the diameter are suddenly pulled apart, thereby reducing the area enclosed by the loop, an induced current is produced.

Electromagnetic Induction

(iii)Change in Orientation.

Let us consider the case when the magnitude of the field strength and area of the coil remains constant. When the plane of the coil is rotated relative to the direction of the field, an induced current is produced as long as the rotation lasts.

Electromagnetic Induction

 

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