Q. What is Lenz law?

Lenz’s Law is based on Farday’s law of electromagnetic induction.

Faraday’s Law of electromagnetic induction:

“The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit.”

Mathematically,         

  

       The negative sign used in Faraday's law of electromagnetic induction, indicates that the induced emf (ε) and the change in magnetic flux (∂Φ_B) have opposite signs.

Lenz’s law states that:

“The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.”

Reason for Opposing, Cause of Induced current in Lenz's Law?

·         Lenz's law obeys the law of conservation of energy and if the direction of the magnetic field that creates the current and the magnetic field of the current in a conductor are in same direction, then these two magnetic fields would add up and produce the current of twice the magnitude and this would in turn create more magnetic field, which will cause more current and this process continuing on and on leads to violation of the law of conservation of energy.

·         If the induced current creates a magnetic field which is equal and opposite to the direction of magnetic field that creates it, then only it can resist the change in the magnetic field in the area, which is in accordance to the Newton's third law of motion.

The Lenz law can be summarized as under:

·         If the magnetic flux Ф linking a coil increases, the direction of current in the coil will be such that it will oppose the increase in flux and hence the induced current will produce its flux in a direction as shown below (using right hand thumb rule).

• If magnetic flux Ф linking a coil is decreasing, the flux produced by the current in the coil is such, that it will aid the main flux and hence the direction of current is as shown below,

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 an 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.