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“The phenomenon whereby an e.m.f. and hence current is induced in any conductor which is cut across or is cut by a magnetic flux is known as electromagnetic induction”.

Faraday’s First Law. It states as follows:

“Whenever the magnetic flux linked with a circuit changes, an e.m.f. is always induced in it”.


“Whenever a conductor cuts magnetic flux, an e.m.f. is induced in that conductor”. 

[Usually, a minus sign is given to the right-hand side expression to signify the fact that the induced e.m.f. sets up current in such a direction that magnetic effect produced by it opposes the very cause producing it.]

Direction of induced e.m.f. and current:

The direction of the induced current may be found easily by applying either Fleming’s right-hand Rule (Fig. 31) or Lenz’s law. Fleming’s rule is used where induced e.m.f. is due to flux-cutting (i.e., dynamically induced e.m.f.) and Lenz’s law when it is due to change by flux-linkages (i.e., statically induced e.m.f.).

-         Len’s law. Fig.32 shows induction of an e.m.f. in a simple circuit. The direction of the induced e.m.f. is determined by Lenz’s law, which states that the current produced by the induced e.m.f. opposes the change of flux.

Lenz’s law may also be stated as follows:

In all cases of electromagnetic induction, an induced voltage will cause a current to flow in a closed circuit in such a direction that the magnetic field which is caused by that current will oppose the change that produced the current”.

Example 24. A coil has 480 turns and flux in it charges from 0.24 m Wb to 1.2 m Wb in 0.24 second. Calculate the voltage induced in the coil. 

Example 25. The field coils of a six-poles D. C. generator each having 600 turns, are connected in series. On the excitation of the field, there is a magnetic flux of 0.024 Wb/pole. If the field circuit is opened in 0.24 second and residual magnetism is 0.0024 Wb/pole, calculate the average voltage which is induced across field terminals. In which direction is this voltage directed relative to the direction of the current.

Solution. Total number of turns = 6 × 600 = 3600

Total initial flux = 6 × 0.024 = 0.144 Wb

Total residual flux = 6 × 0.0024 = 0.0144 Wb

Change in flux, dɸ = 0.144 – 0.0144 = 0.1296 Wb

Time of opening the circuit, dt = 0.024 second

The direction of this is induced e.m.f. is the same as the initial direction of exciting current.  (Ans.)