Lenz’s Law of Electromagnetic Induction
Lenz’s law is named after the German scientist H. F. E. Lenz in 1834. Lenz’s law obeys Newton’s third law of motion (i.e. to every action there is always an equal and opposite reaction) and the conservation of energy (i.e. energy may neither be created nor destroyed and therefore the sum of all the energies in the system is a constant).
Lenz’s Law: It 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 (ε) and the change in magnetic flux (δφB) have opposite signs.
\(\varepsilon \,=\,-\,N\frac{\partial \phi }{\partial t}\),
Where,
ε = Induced emf,
δφB = Change in Magnetic Flux,
N = Number of turns in coil.
Explanation of Lenz’s Law: For understanding Lenz’s Law, consider two cases:
Case – 1: When a magnet is moving towards the coil.
When the north pole of the magnet is approaching towards the coil, the magnetic flux linking to the coil increases. According to Faraday’s law of electromagnetic induction, when there is change in flux, an emf and hence current is induced in the coil and this current will create its own magnetic field. Now according to Lenz’s law, this magnetic field created will oppose its own or we can say opposes the increase in flux through the coil and this is possible only if approaching coil side attains north polarity, as we know similar poles repel 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 anticlockwise direction.
Case – 2: When a magnet is moving away from the coil.
When the north pole of the magnet is moving away from the coil, the magnetic flux linking to the coil decreases. According to Faraday’s law of electromagnetic induction, an emf and hence current is induced in the coil and this current will create its own magnetic field. Now according to Lenz’s law, this magnetic field created will oppose its own or we can say opposes the decrease in flux 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.
Application of Lenz’s Law:
- Lenz’s law can be used to understand the concept of stored magnetic energy in an inductor. When a source of emf is connected across an inductor, a current starts flowing through it. The back emf will oppose this increase in current through the inductor. In order to establish the flow of current, the external source of emf has to do some work to overcome this opposition. This work can be done by the emf is stored in the inductor and it can be recovered after removing the external source of emf from the circuit
- This law indicates that the induced emf and the change in flux have opposite signs which provide a physical interpretation of the choice of sign in Faraday’s law of induction.
- Lenz’s law is also applied to electric generators. When a current is induced in a generator, the direction of this induced current is such that it opposes and causes rotation of generator (as in accordance to Lenz’s law) and hence the generator requires more mechanical energy. It also provides back emf in case of electric motors.
- Lenz’s law is also used in electromagnetic braking and induction cook tops.