Lenz’s Law

Lenz’s law is given by a physicist of Germany named Heinrich Friedrich Lenz. He described the direction of electric current relative to the magnetic flux. He deduced that the direction of an induced current in a circuit is such as to oppose the change that causes it.


Applications of Lenz’s Law

  • Braking systems in trains,
  • Metal detectors,
  • Eddy current dynamometers,
  • AC Generators,
  • Card Readers,
  • Microphones, etc.

The process of induction occurs when a change in magnetic flux causes an emf to oppose that change. One of the main reasons for the induction process in motion. We can say, for example, that a magnet moving toward a coil generates an emf, and that a coil moving toward a magnet creates a comparable emf. 
Let x be the distance between the resistance and the rod at any timer t . According to faraday’s law, emf produced in a loop due to change in magnetic flux is, 
ε = – dφ/dT
φt (flux at any time t) = B . A 
                                    = B l x  
d (φ)/dt = d (Blx)/dt 
                          = Bl dx/dt  
                           = Blv  
ε = – dφ/dT 
  = Blv    
Using Ohm’s law: V = IR or I = V/R
Current through the resistance R is, 
I = Blv/R                                                                                                                                                                (in a clockwise direction)
Amount of charge (q) passed through the loop in time ‘t’ = Δφ/R
where Δφ = Total change in flux in time ‘t’.


Let us consider a rectangular loop as shown in the figure above, with the sides PQ, QR, RS and SP. Here, the three sides of the loop are fixed and one of the sides, the side PQ, is free to move. Let r be the resistance of the movable arm under consideration.

loop be taken as ε, then the current in the loop can be given as,

The energy that goes into the motion of the rod in this system is dissipated in the form of heat given by,

Here we see that the value of PJ is equal to the value of P. the magnitude of the induced emf is,

However, as we know that,

And thus, from the above two equations, we can write,

Eddy currents are currents which circulate in conductors like swirling eddies in a stream. They are induced by changing magnetic fields and flow in closed loops, perpendicular to the plane of the magnetic field. So far we have studied the electric currents induced in well defined paths in conductors like circular loops. Even when bulk pieces of conductors are subjected to changing magnetic flux, induced currents are produced in them. However, their flow patterns resemble swirling eddies in water. This effect was discovered by physicist Foucault (1819-1868) and these currents are called eddy currents

The phenomenon of electromagnetic induction has been technologically exploited in many ways. An exceptionally important application is the generation of alternating currents (ac). The modern ac generator with a typical output capacity of 100 MW is a highly evolved machine. In this section, we shall describe the basic principles behind this machine. The Yugoslav inventor Nicola Tesla is credited with the development of the machine.
Working of an AC Generator
When the armature rotates between the poles of the magnet upon an axis perpendicular to the magnetic field, the flux linkage of the armature changes continuously. As a result, an electric current flows through the galvanometer and the slip rings and brushes. The galvanometer swings between positive and negative values. This indicates that there is an alternating current flowing through the galvanometer.
The field consists of coils of conductors that receive a voltage from the source and produce magnetic flux. The magnetic flux in the field cuts the armature to produce a voltage.
The part of an AC generator in which the voltage is produced is known as an armature. This component primarily consists of coils of wire that are large enough to carry the full-load current of the generator.
Prime Mover
The component used to drive the AC generator is known as a prime mover. The prime mover could either be a diesel engine, a steam turbine, or a motor.
The rotating component of the generator is known as a rotor. The generator’s prime mover drives the rotor.
The stator is the stationary part of an AC generator. The stator core comprises a lamination of steel alloys or magnetic iron to minimize the eddy current losses.
Slip Rings
Slip rings are electrical connections used to transfer power to and fro from the rotor of an AC generator. They are typically designed to conduct the flow of current from a stationary device to a rotating one.