Laws of Electromagnetic Induction

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There are two types of laws which govern the phenomenon of electromagnetic induction:

A. Faraday’s laws which give us the magnitude of induced emf.

B. Lenz’s law which gives us the direction of induced emf.

A. Faraday’s laws of electromagnetic induction:

These can be stated as follows:

First Law

Whenever the magnetic flux linked with a closed-circuit change, an emf (and hence a current) is induced in it which lasts only so long as the change in flux is taking place. This phenomenon is called electromagnetic induction.

Second law:

The magnitude of the induced emf is equal to the rate of change of magnetic flux linked with the closed circuit. Mathematically,

|ε| = dø/dt

B. Third Law or Lenz’s law:

This law states that the direction of induced current is such that it opposes the cause which produces it, i.e., it opposes the change in magnetic flux.

Mathematical form of the laws of electromagnetic induction:

Expression for induced emf. According to the Faraday’s flux rule,

Magnitude of induced emf

= Rate of change of magnetic flux

|ε| = dø/dt

Taking into account Lenz’s rule for the direction of induced emf, Faraday’s law takes the form:

ε = – dø/dt

The negative sign indicates that the direction of induced emf is such that it opposes the change in magnetic flux.

If the coil consists of N tightly wound turns, then the emfs developed in all these turns will be equal and in the same direction and hence get added up. Total induced emf will be

ε = – N. (dø/dt)

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