The third law of electromagnetic induction tells us about the direction of the induced emf or the induced current. This law was introduced by Lenz and is known as Lenz’s law. The law states that the direction of the induced current is such that it opposes the change that has induced it.
e ∝ – dΦ/dt
e = -K. dΦ/dt
Where K is the proportionality constant.
In the SI system of units K = 1, then
e = – dΦ/dt
Conservation of Energy:
Let a bar magnet, with an N-pole facing a coil, the following figure, be just given a little push towards the coil. As a result, a small current will be induced in the coil. Let us suppose that the direction of the induced current is such that instead of N-pole, S-pole has generated at the end A.
As a result, the magnet will continue to move towards the coil with increasing kinetic energy resulting in the gradual increase of current, in the gradual increase of the magnetic field energy of the coil in a cyclic manner without the expenditure of equivalent energy. This is clearly in violation of the principle of Conservation of Energy