Faraday Law of Electro-Magnetic Induction

Whenever magnetic lines of forces are cut by a closed circuit, an induced current flows in the circuit. The current lasts only so long as the flux in changing. The emf which produces this current, is called induced emf. This phenomenon is called electromagnetic induction.

Expt. 1. Coil connected in series with a galvanometer.

(i)    When the magnet is moved towards the coil, with its north pole facing the coil, the galvanometer shows a deflection in one direction.
(ii)    When the magnet is moved away from the coil, the galvanometer shows a deflection in the opposite direction.
(iii)    If the experiment is repeated with South Pole of the magnet facing the coil, the deflections in the galvanometer are reversed.
(iv)    When the magnet is stationary, there is no deflection in the galvanometer.
(v)    It is further observed that the deflections increase with the velocity of the magnet relative  to the coil
(vi)    The same results are obtained if the magnet is kept fixed and coil moved.

Expt. 2. A primary coil P connected to a battery and tap key K, and a secondary coil connected to a galvanometer.

(i)    When a battery circuit is closed by pressing K and then broken, the galvanometer shows a deflection first in one direction and then in the other direction.
(ii)    If the current in the primary flows continuously, no deflection is produced in the galvanometer. The deflection is produced only at the time of make and break.

Similar effects are observed while increasing of decreasing the primary current of changing the relative position of the coils.
From his experimental results, Faraday gave two laws:
(i)    Whenever the magnetic flux through a conductor is changed, an emf is induced in the conductor. The magnetic of the induced emf is equal to the rate of change of magnetic flux through the circuit.

If φ is the magnetic flux linked with the circuit at any instant t and ε is the induced e.m.f., then
        ε ∞ dφ/dt
(ii)    The direction of the induced emf, or current, is such as to oppose the change that produced it.
This is also known as Lenz’s law
Combining both laws, ε = - dφ/dt