Electromagnetic Induction Class 12 Notes: CBSE 12th Physics Chapter 6, Download PDF

6.2 THE EXPERIMENTS OF FARADAY AND HENRY

1st Experiment

When a North-pole of a bar magnet is moved towards a coil, an electric current is detected by a galvanometer, causing the pointer to deflect. The deflection continues as long as the magnet is in motion. However, there is no deflection in the galvanometer when the magnet is stationary. When the magnet is pulled away from the coil, the galvanometer shows a deflection in the opposite direction, indicating a reversal of the current's direction. The same observations are made when the South-pole of the magnet is moved towards or away from the coil, but with opposite deflections compared to the North-pole's movements. Furthermore, faster movements of the magnet towards or away from the coil result in larger deflections (and currents). Similarly, when the bar magnet is held still and the coil is moved towards or away from the magnet, the same effects are observed.

It shows that it is the relative motion between the magnet and the coil that is responsible for generation (induction) of electric current in the coil.

2nd Experiment

It is the relative motion between the coils that induces the electric current.

3rd Experiment

Relative motion is not an absolute requirement to induce an electric current.

6.3 MAGNETIC FLUX

In Uniform Magnetic Field

In Non-Uniform Magnetic Field

6.4 FARADAY’S LAW OF INDUCTION

An emf is induced in a coil when magnetic flux through the coil changes with time. 

Faraday’s law of electromagnetic induction

The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit.

The negative sign indicates the direction of emf and hence the direction of current in a closed loop. 

In the case of a closely wound coil of N turns, change of flux associated with each turn, is the same. Therefore, the expression for the total induced emf is given by

6.5 LENZ’S LAW AND CONSERVATION OF ENERGY

Lenz’s Law: The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.

 If the induced current were opposite to the one shown in Fig. 6.6(a), a perpetual-motion machine could be constructed, violating the law of conservation of energy. The magnet would be attracted towards the coil with increasing acceleration, leading to perpetual motion without any energy expenditure. However, this is not possible as it violates the fundamental law of energy conservation but in the correct case depicted in Fig. 6.6(a), the induced current creates a repulsive force on the bar magnet. As a result, work needs to be done by a person to move the magnet. The energy spent by the person is dissipated as Joule heating caused by the induced current. This demonstrates the validity of Lenz's law and the conversion of mechanical energy into heat energy during electromagnetic induction.

6.6 MOTIONAL ELECTROMOTIVE FORCE

When the arm PQ is moved to the left side, thus decreasing the area of the rectangular loop, the movement induces a current I, as shown.

The induced emf Blv is called motional emf. 

The work done in moving the charge from P to Q is,

W = qvBl

Since emf is the work done per unit charge,

EMF ε  = W/q = Blv

In the case of a stationary conductor, the force on its charges is given by F = q (E + v  x B) = qE

6.7 INDUCTANCE

Inductance is the ratio of the flux-linkage to current. 

An electric current can be induced in a coil by flux change produced by another coil in its vicinity or flux change produced by the same coil. 

Inductance is a scalar quantity.

The SI unit of inductance is henry and is denoted by H.

6.7.1 Mutual inductance

6.7.2 Self-inductance

6.8 AC GENERATOR

In an ac generator, mechanical energy is converted to electrical energy by virtue of electromagnetic induction. If coil of N turn and area A is rotated at n revolutions per second in a uniform magnetic field B, then the motional emf produced is 

where we have assumed that at time t = 0 s, the coil is perpendicular to the field.

Download CBSE Class 12 Physics Ch 6 Electromagnetic Induction Notes PDF