Table of Contents
Introduction
In 1833, German physicist Heinrich Lenz gave a general law for determining the direction of induced emf and hence that of induced current in a circuit.
Lenz’s law states that the direction of induced current in a circuit is such that it opposes the cause or the change which produces it.
Thus, if the magnetic flux linked with a closed circuit increases, the induced current flows in such a direction so as to create a magnetic flux in the opposite direction of the original magnetic flux.
If the magnetic flux linked with the closed circuit decreases, the induced current flows in such a direction so as to create a magnetic flux in the direction of the original flux.
Illustrations of Lenz’s law:
(i) When the north pole of a bar magnet is moved towards a closed coil, the induced current in the coil flows in the anticlockwise direction, as seen from the magnet side closely. The face of the coil towards the magnet develops north polarity and thus, it opposes the motion of the north pole of the magnet towards the coil which is actually the cause of the induced current in the coil.
In other words, the motion of the magnet increases the flux through the coil. The induced current generates flux in opposite direction, and hence opposes and reduces this flux. (ii) When the north pole of a magnet is taken away from a closed coil, the induced current in the coil flows clockwise, as seen from the magnet side. The face of the coil towards the magnet develops south polarity and attracts the north pole of the magnet, i.e., the motion of the magnet away from the coil is opposed which is really the cause of the induced current.
In other words, the motion of the magnet decreases the flux through the coil. The induced current generates flux in the same direction and hence opposes and increases this flux.
Lenz’s law and law of conservation of energy:
Whether a magnet is moved towards or away from a closed coil, the induced current always opposes the motion of the magnet, as predicted by Lenz’s law.
For example, when the north pole of a magnet is brought closer to a coil [see 1st figure], its face towards the magnet develops north polarity and thus repels north pole of the magnet. Work has to be done in moving the magnet closer to the coil against this force of repulsion.
Similarly, when the north pole of the magnet is moved away from the coil [see 2nd figure], its face towards the magnet develops south polarity and thus attracts the north pole of the magnet. Here work has to be done inmoving the magnet away from the coil against this force of attraction. It is this work done against the force of repulsion or attraction that appears as electric energy in the form of induced current.
Suppose that the Lenz’s law is not valid. Then the induced current flows through the coil in a direction opposite to one dictated by Lenz’s law. The resulting force on the magnet makes it move faster and faster, i.e., the magnet gains speed and hence kinetic energy without expending an equivalent amount of energy. This sets up a perpetual motion machine, violating the law of conservation of energy. Thus, Lenz’s law is valid and is a consequence of the law of conservation of energy.