LENZ'S LAW OF ELECTROMAGNETIC INDUCTION

As shown in the figure, suppose a bar magnet is moved towards a conducting coil with its north pole facing the coil. If this produces current in the coil in the clockwise direction as seen from the side of the magnet, then the side of the coil facing the magnet will act like south pole of a magnet and will attract the magnet. The magnet will have accelerated motion towards the coil which will increase the rate of change of flux and hence the current in the coil. This will increase the force of attraction and the acceleration of the magnet will increase further. Thus the current in the coil will go on increasing. If a resistance R is connected in the coil, joule heat I2Rt is produced in it. No mechanical work is done in giving a slight push to the magnet. Thus heat energy is being continuously produced without spending energy. This is contrary to the law of conservation of energy. Thus our assumption about the direction of current induced in the coil being clockwise is incorrect.
If the direction of induced current were counter-clockwise, the end of the coil facing the north pole of the magnet would have become north pole and mechanical work will be required to be done against the force of repulsion which gets converted in the joule heat in the resistance of the coil. This is consistent with the law of conservation of energy.
Thus “induced emf ( or induced current ) is produced in such a direction that the magnetic field produced due to it opposes the very cause ( here motion of the magnet ) that produces it”. This statement is known as Lenz’s law.