Page 15 - 10-phy-15 Electromagnetism

15. Electromagnetism eLearn.Punjab

When the magnet is moved towards the solenoid, the needle of galvanometer deflects towards
right, indicating that a current is being induced in the solenoid (Fig.15.14-a). When the magnet is
pulled away from the solenoid, the galvanometer deflects towards left, indicating that the induced
current in the solenoid is in the opposite direction (Fig.15.14-b).
From the above experiments, we conclude that an e.m.f. is induced in the coil when there is a
relative motion between the coil and the magnet. This phenomenon in which an e.m.f. is induced
due to the relative motion between the coil and the magnet is called electromagnetic induction.

The value of induced e.m.f. in a circuit is directly proportional
to the rate of change of number of magnetic lines of force
through it.

This is called Faraday’s law of electromagnetic induction.

Factors Affecting Induced e.m.f

The magnitude of induced e.m.f. in a circuit depends on the following factors:
1. Speed of relative motion of the coil and the magnet
2. Number of turns of the coil

15.6 DIRECTION OF INDUCED E.M.F. – LENZ'S LAW

Lenz devised a rule to find out the direction of a current induced in a circuit. It is explained from the

following activity:

Activity 15.4: If we bring a north pole of a bar magnet near a solenoid, an e.m.f. will be induced

in the solenoid by electromagnetic induction (Fig. 15.15-a). The direction of the induced current in

the solenoid by the induced e.m.f. will be such that it will repel the north pole of the magnet. This is

only possible if the right end of the solenoid becomes a north pole. Hence, according to right hand

grip rule, the direction of the induced current in the solenoid will be clockwise. Similarly, when we

move the north pole of the magnet away from the solenoid, the direction of the induced current will

be anticlockwise (Fig. 15.15-b). The direction of an induced current in a circuit is always
In this case left end of solenoid such that it opposes the cause that produces it.
becomes south pole.

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