Electromagnetic Induction

- Learn important concepts of the chapter

Faraday's Experiment

A galvanometer is connected to an insulated copper coil.When magnet is moved towards or away fromt he coil, the following things happen:
1. If the magnet and coil is at rest then no deflection is observed in galvanometer G.
2. When the N pole of magnet is moved towards coil, then the galvanometer gives deflection in one direction fig b
3. when magnet is stopped deflection becomes zero
4. When N pole is moved away from the coil, the galvanometer gives deflection in opposite direction.
5. If magnet is moved fast deflection increases
6. If number of turns is increased then deflection also increases.
7. The induced current is produced only by relative motion between magnet and coil.


Magnetic flux through an area

Magnetic flux is a measure of the quantity of magnetism, being the total number of magnetic lines of force passing through a specified area in a magnetic field. Magnetic flux through a plane of area A placed in a uniform magnetic field B can be written as 

Faraday's Law of Electromagnetic Induction

According to Faraday's Law of electromagnetic induction:

1. Whenever there is a change in the magnetic flux linked with a coil, an e.m.f. is induced. 
2. The magnitude of e.m.f. induced is directly proportional to the rate of change of magnetic flux linked with the coil.
Note: Magnetic flux through a coil, where, and are perpendicular to each other.

Lenz's Law

According to Lenz's Law, if an induced current flows in a coil due to electromagnetic induction, its direction is always such that it will oppose the change which produced it. Hence, the magnetic field produced by the current in the coil is opposite to the direction of external magnetic field. It is shown by a negative sign in the Faraday's law.

Motional emf

When a metal rod of length is placed normal to a uniform magnetic field and moved with a velocity perpendicular to the field, the induced emf (called motional emf) across its ends is

Power required to move a conductor in a magnetic field

where B is the magnetic field,
l is the length of the conductor
v is the velocity of the conductor
R is the resistance

Eddy currents

Currents induced in the conductor due to changing magnetic flux are called eddy currents. They flow in closed loops in plane perpendicular to the magnetic field. The value of eddy currents can be found using faraday's law of electromagnetic induction.

Mutual inductance

If two coils of wire are brought into close proximity with each other so the magnetic field from one links with the other, a voltage will be generated in the second coil as a result. This is called mutual inductance when voltage impressed upon one coil induces a voltage in another.

Self inductance

Self inductance is defined as the induction of a voltage in a current-carrying wire when the current in the wire itself is changing. In the case of self-inductance, the magnetic field created by a changing current in the circuit itself induces a voltage in the same circuit. Therefore, the voltage is self-induced.The self-inductance of the coil depends on its geometry and on the permeability of the medium.

AC Generator

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Principle : AC generator works on the principle of electromagnetic induction in which electric current is induced in the coil placed in magnetic field.


An electric generator, as shown in Fig., consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings are made insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in Fig. By applying Flemings right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1. After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator.