Lenz’s law is a rule to determine the direction of induced current due to induced emf. This rule was given by German scientist H.F. Emil Lenz. It states that the direction of induced current is such that it opposes the cause which produces it.
The cause of the current may be the motion of a conductor in a magnetic field or it may be the change of magnetic flux through a stationary circuit. Lenz quoted this law as “this law seems that a son revolts with his father“.
Explanation of Lenz’s Law
Consider a coil placed near a bar magnet such that the north pole is pointing towards the bar magnet. If the magnet is moved towards the coil, then the magnetic lines of force passing through the coil increase and then consequently induces current in the coil. According to Lenz’s law, the direction of induced current opposes the motion of the magnet. It is possible only when current is in anticlockwise direction so that the face of the coil facing the magnet behaves like a north pole thus tending to oppose (repel) the approaching magnet.
As the magnet is moved away from the coil, the induced emf again produces current but in opposite direction as before so that the bar magnet will be attracted by the magnetic field due to induced current. So, the current will now be in clockwise direction so that the face of the coil behaves as a south pole tending to attract the magnet. Thus in both cases, the induced current produces magnetic field such that it opposes the motion of the magnet which is the cause producing the current.
Lenz’s law is in accordance with the principle of conservation of energy
According to the principle of conservation of energy, energy can neither be created nor can be destroyed, it can be only converted from one form to another. Lenz’s law obeys this law of conservation of energy.
When the bar magnet is moved towards the coil, an emf is produced in it and a current flows in the coil. Due to this current, a magnetic field is produced in the coil which opposes the approaching magnet. Hence the coil exerts a force of repulsion on the magnet. Due to this force, the magnet will be stopped.
To continue the motion of approaching magnet, an external agent must do work to move the magnet towards the coil and therefore, mechanical energy is applied to move the magnet. This mechanical energy is converted into electrical energy in the coil.
Similarly, if the magnet is moved away from the coil, the induced current in the coil produces a magnetic field which attracts the bar magnet. To continue the motion of magnet, work has to be done which is converted into electrical energy in the coil.
If the direction of induced current in the coil does not oppose the motion of approaching magnet, then electrical energy will be obtained without doing any work which violates the law of conservation of energy. Hence, Lenz’s law is in accordance with the law of conservation of energy.