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Faraday’s Law of induction states that “The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.”
i.e if there is relative motion (or change in flux) between a magnet and a conductor, then an electromotive force (emf) is induced.
Lenz’s Law says that if an induced current flows, its direction is always such that it will oppose the change which produced it.
The importance of Lenz’s law
Set up an experiment where the north pole of bar magnet is moving into a coil. As there is relative motion between the magnet and the conductor (the coils), an emf is induced by Faraday’s Law. Lenz’s law tells us that the induced emf will flow to oppose the change in flux that induced it, thus it would flow anticlockwise as viewed from the magnet. Now let us consider if the induced current flowed clockwise!
IF the induced current flowed clockwise then the coil would form a south pole (at the end the magnet is entering), and thus attracts the bar magnet, accelerating it. As the bar magnet increases in velocity, so does the magnitude of induced emf, and again it accelerates the bar magnet further. This clearly violates the law of conservation of energy as kinetic energy and electrical energy is being created. That is why induced emf MUST oppose the change in flux that induced it, otherwise it would violate one of the most fundamental laws of physics!
From the diagram, we can see a magnet moving towards a coil. By Faraday’s Law as there is a change in flux and relative motion between a magnet and a conductor, emf is induced. As it is a closed circuit, current is formed. This current will flow to oppose the change in flux that induced it by Lenz’s Law. Thus by using right hand grip rule, the current will flow anticlockwise as viewed from the magnet.
As the current will flow to oppose the change in flux that induced it, the current will induce a north pole at X, to repel the magnet. Using right hand grip rule, our thumbs point north and our fingers indicate the direction of current flow.