Lenz's law : The direction of the induced current is such as to oppose the change that produces it.
The change that induces a current may be (i) the motion of a conductor in a magnetic field or (ii) the change of the magnetic flux through a stationary circuit.
Explanation : Consider Faraday's magnet-and-coil experimant. If the bar magnet is moved towards the coil with its N-pole facing the coil, as in Fig. the magnet's lines of magnetic induction (pointing to the left) through the coil increases. The induced current in the coil sets up a magnetic field of iis own pointing to the right (as given by Amperes right-hand rule) to oppose the growing flux due to the magnet. Hence, work must to be done move to magnet towards the coil against this repulsive flux of the induced current. The work done shows up as electric energy in the coil.
When the magnet is withdrawn, with its N-pole still facing the coil, its lines of induction (pointing left) through the coil decrease. The induced current reverses its direction to supplement the decreasing flux with its own, as shown in Fig. Facing the coil along the magnet, the induced current is in the clockwise sence. The electric energy in the coil comes from the work done to withdrow the magnet, now against an attractive force. Thus, we see that Lenz's law is a consequence of the law of conservation of energy.
