Faraday’s Law

Experiments of Faraday and Henry:

Experiment 1:

· Coil is connected to a galvanometer G. When the North-pole of a bar magnet is pushed towards the coil, the pointer in the galvanometer deflects, indicating the presence of electric current in the coil. The deflection lasts as long as the bar magnet is in motion.

· The galvanometer does not show any deflection when the magnet is held stationary.

· When the magnet is pulled away from the coil, the galvanometer shows deflection in the opposite direction, which indicates reversal of the current’s direction.

· Moreover, when the South-pole of the bar magnet is moved towards or away from the coil, the deflections in the galvanometer are opposite to that observed with the North-pole for similar movements.

· Further, the deflection (and hence current) is found to be larger when the magnet is pushed towards or pulled away from the coil faster.

· Instead, when the bar magnet is held fixed and the coil C₁ 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.

Experiment 2:

· In this experiment, the bar magnet is replaced by a second coil connected to a battery. The steady current in the coil produces a steady magnetic field.

· As coil is moved towards the coil, the galvanometer shows a deflection. This indicates that electric current is induced in coil.

· When is moved away, the galvanometer shows a deflection again, but this time in the opposite direction. The deflection lasts as long as coil is in motion.

· When the coil is held fixed and is moved, the same effects are observed. Again, it is the relative motion between the coils that induces the electric current.

Experiment 3:

· In this experiment, it is observed that the galvanometer shows a momentary deflection when the tapping key K is pressed. The pointer in the galvanometer returns to zero immediately.

· If the key is held pressed continuously, there is no deflection in the galvanometer.

· When the key is released, a momentary deflection is observed again, but in the opposite direction.

· It is also observed that the deflection increases dramatically when an iron rod is inserted into the coils along their axis.

Parameters	Experiment 1	Experiment 2	Experiment 3
Contains	A Bar magnet, coil, galvanometer	Two coils, galvanometer	Two coils, galvanometer, key
Observation	It’s the relative motion between the magnet and the coil	It’s the relative motion between the coils	Momentary deflection when iron rod is inserted
No Deflection in Galvanometer	When magnet is held stationary	When coil is held stationary	When key is pressed continuously

Faraday’s Law of Electromagnetic Induction:

Based on his studies on the phenomenon of electromagnetic induction, Faraday proposed the following two laws.

First law

Whenever the amount of magnetic flux linked with a closed circuit changes, an emf is induced in the circuit. The induced emf lasts so long as the change in magnetic flux continues.

Second law

The magnitude of emf induced in a closed circuit is directly proportional to the rate of change of magnetic flux linked with the circuit.

Let be the magnetic flux linked with the coil initially and be the magnetic flux linked with the coil after a time t. Then

Rate of change of magnetic flux =

According to Faraday’s second law, the magnitude of induced emf is, . If is the change in magnetic flux in a time dt, then the above equation can be written as