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Electromagnetic Induction and Alternating Current

Electromagnetic Induction and Alternating Current Notes for Class 11 - 12

Learning Outcomes

  • Electromagnetic Induction
  • Generating Electricity with Magnet
  • Induced Electromotive Force
  • Faraday's Laws of Electromagnetic Induction
  • Lenz's Law
  • Lenz's Law and Conservation of Energy
  • Self Induction and Mutual Induction
  • Generating Alternating Current
  • Root Mean Square Value, Peak Value, and Current

Electromagnetic Induction

The process of inducing temporary electromotive force or electricity by means of a moving magnet or current-conducting coil in a closed circuit is called electromagnetic induction.

Magnetic flux

The product of the area of a surface and the perpendicular component of a magnetic field on the surface is called magnetic flux.

Magnetic flux density

The magnetic flux density refers to the flux per unit area.

Faraday's Laws of Electromagnetic Induction

In 1831, A famous scientist Faraday discovered the laws of electromagnetic induction. These laws are named Faraday's laws of electromagnetic induction.

First Law: Whenever any change in magnetic flux takes place in a closed coil, emf is temporarily induced across the circuit. Induced emf or current flow last, as long as the magnetic flux is changed. The direction in which the current flows with the increasing of magnetic flux is exactly opposite to the direction of the current flow with decreasing magnetic flux.

Second Law: Magnetic of the induced electromotive force of a closed circuit is inversely proportional to the negative value of the rate of change of magnetic flux across the circuit.

Lenz's Law

The direction of the induced current or emf is such obstructing to the change of magnetic flux as causes the production of inducing current in the circuit. 

Self Induction and Mutual Induction


A magnetic flux associated with a coil changes due to a change in electric flow through the coil or due to a change in motion of the coil. The electromagnetic induction taking place due to change is called self-induction.

Co-efficient of self-induction

If one unit of current flows in a coil, the total amount of magnetic flux associated with the coil is called the co-efficient of self-induction.

Unit co-efficient of self-induction

In S.I. system, the unit co-efficient of self-induction is henry. The units is named after the American physicist Joseph Henry.

Mutual induction

The phenomenon of electromagnetic induction produced in a secondary coil due to the non-uniform flow of current in the primary coil is called mutual induction.

Co-efficient of mutual induction

The number of magnetic flux associated with the secondary coil due to the flow of one unit of current in the primary coil is called mutual induction.

Alternating current

In a current, if the current flow changes its direction after a certain period and gains its maximum and minimum values after a certain time interval, this current flow is called alternating current.

A.C Generator or A.C dynamo

A machine by which mechanical energy is transformed into electrical energy is called generator or dynamo. Alternating current can be produced by this machine.

An A.C dynamo has the following components:

Magnet NS: This is a permanent electric magnet.

Armature or revolving coil: This is an insulated and rectangular iron bar ABCD that contains a lot of coils of wires situated at the middle of magnet.

Two slip rings or wheels: These are two special rings E and F made of metal. The rings are kept away from one another but are attached along their two ends of revolving coil. The slip rings are strongly attached with the axis around which the coil rotates. So, the rings rotate at the time of rotating of coil.

Brush: Two brushes I and J are made of carbon. These two brushes are pressed down the slip rings along a spring. An external R is connected to these two brushes.


A machine with which the high voltage. A.C is converted into low voltage or low voltage A.C is converted into high voltage is called transformer.

There are two types of transformer: step up and step down.

Step up: A transformer which changes low potential to high potential is called step up transformer. 

Step down: a transformer which changes high potential to low potential is called step down transformer.

Use of transformer

Transformer is used in TV, Radio, Telephone, Telegraph, etc. A strong step up transformer is used is power station to increase voltage from 25000 volts to 4,00,000 volts. Again, step down transformer is used to distribute electricity at low voltage in different places.

220V line is required for household use and 440V line is for air cooler.

Electric motor

An electric motor is an electrical machine that converts electrical energy into mechanical energy. There are two kinds of electrical motors: A.C motor and D.C motor. Electric motor is used to run electric fan and pump.

Root Mean Square Value, Peak Value and Current

Maximum value of alternating current or electromotive force is called peak value.

Full cycle: A complete change of current or Pd from a particular value and to the same value in the same direction is known as a full cycle.

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