Electrical Engineering ⇒ Topic : Statically Induced E.M.F.

Statically Induced E.M.F.

When the conductor is stationary and the field is moving or changing, the e.m.f. induced in the conductor is called statically induced e.m.f. A statically induced e.m.f. can be further sub-divided into :

1. Self-induced e.m.f.

2. Mutually induced e.m.f.

1. Self-induced e.m.f. The e.m.f induced in a coil due to the change of its own flux linked with it is called self-induced e.m.f.

When a coil is carrying current (See Fig. a), a magnetic field is established through the coil. If current in the coil changes, then the flux linking the coil also changes. Hence an e.m.f. (= N dΦ/dt) is induced in the coil.This is known as self-induced e.m.f. The direction of this e.m.f. (by Lenz's law) is such so as to oppose the cause producing it, namely the change of current (and hence field) in the coil. The self-induced e.m.f. will persist so long as the current in the coil is changing.

                                                                                                figure (a)

The following points are worth noting : 

(i) When current in a coil changes, the self-induced e.m.f. opposes the change of current in the coil. This property of the coil is known as its self-inductance or inductance

(ii) The self-induced e.m.f. (and hence inductance) does not prevent the current from changing ;it serves only to delay the change. Thus after the switch is closed (See Fig. a),the current will rise from zero ampere to its final steady value in some time (a fraction of a second). This delay is due to the self-induced e.m.f. of the coil.

2. Mutually induced e.m.f. The e.ml induced in a coil due to the changing current in the neighbouring coil is called mutually induced e.m.f.

Consider two coils A and B placed adjacent to each other as shown in Fig. (b) A part of the magnetic flux produced by coil A passes through or links with coil B. This flux which is common to both the coils A and B is called mutual flux (Φ_m). If current in coil A is varied, the mutual flux also varies and hence e.m.f. is induced in both the coils. The e.m.f. induced in coilA is called self-induced e.m.f. as already discussed. The e.m.f. induced in coil B is known as mutually induced e.m.f.

                                                                                          figure (a)

The magnitude of mutually induced e.m.f. is given by Faraday's laws i.e. e_m= N_B dΦ_m/dt where NB is the number of turns of coil B and dΦ_m/dt is the rate of change of mutual flux  i.e. flux common to both the coils. The direction of mutually induced e.m.f. (by Lenz's law) is always such so as tooppose the very cause producing it. The cause producing the mutually induced e.m.f. in coil B is the changing mutual flux produced by coil A. Hence the direction of induced current (when the circuit is completed) in coil B will be such that the flux set up by it will oppose the changing mutual flux produced by coil A.

The following points may be noted carefully

(1) The mutually induced e.m.f. in coil B persists so long as the current in coil A is changing. If current in coilA becomes steady, the mutual flux also becomes steady and mutually induced e.m.f. drops to zero.

(2) The property of two neighbouring coils to induce voltage in one coil due to the change of current in the other is called mutual inductance.