Demonstrative Video
Lenz’s Law
Lenz’s law obeys Newton’s third law of motion and the conservation of energy
Lenz’s law is based on Faraday’s law of induction
Lenz’s law states that when an emf is generated by a change in magnetic flux according to Faraday’s Law, the polarity of the induced emf is such, that it produces an current that’s magnetic field opposes the change which produces it.
- The negative sign used in Faraday’s law of electromagnetic induction, indicates that the induced emf and the change in magnetic flux have opposite signs\[\epsilon=-N\dfrac{\partial\Phi}{\partial t}\]
Explanation of Lenz’s Law:
Case-1:
When north pole of the magnet is approaching towards the coil, the \(\phi\) linking to the coil increases.
According to Faraday’s law of EMI, when \(\phi\) changes, an emf and hence \(I\) is induced in the coil which will create its own magnetic field.
According to Lenz’s law, this magnetic field created will oppose the increase in \(\phi\) through the coil which is possible only if approaching coil side attains north polarity, as we know similar poles repel each other.
Once magnetic polarity of the coil side is known, direction of the induced current can be easily determined by applying right hand rule
In this case, the current flows in anticlockwise direction.
Case-2:
When north pole of the magnet is moving away from the coil, the \(\phi\) linking to the coil decreases.
According to Faraday’s law of EMI, an emf and hence \(I\) is induced in the coil which will create its own magnetic field.
According to Lenz’s law, magnetic field created will oppose the decrease in \(\phi\) through the coil and this is possible only if approaching coil side attains south polarity, as we know dissimilar poles attract each other.
Once we know the magnetic polarity of the coil side, we can easily determine the direction of the induced current by applying right hand rule.
In this case, the current flows in clockwise direction.
For finding the directions of magnetic field or current
Right hand thumb rule i.e if the fingers of the right hand are placed around the wire so that the thumb points in the direction of current flow, then the curling of fingers will show the direction of the magnetic field produced by the wire.
Right Hand