Introduction
Electricity and magnetism were initially separate subjects. Electricity was related to the charges and magnetism was related to the magnetic materials only. However in the 19th century a Danish physicist, Oersted demonstrated that there is a relation between these two. He found that electric current can create a magnetic field around them.
Michael Faraday the British experimental scientist reported one interesting phenomenon. He found that if we move a magnet around a coil connected to a galvanometer, we see deflection in that.
The presence of the current in the circuit in such cases implies that an electromotive force gets induced in the circuit when we change the magnetic field around it. This phenomenon is called electromagnetic induction. The amount of induced electromotive force is given by faraday’s law. It says that the rate of change of magnetic flux is equal to the induced electromotive force in the circuit. However, if we want to know the direction of the current induced in the circuit due to electromagnetic induction, we use another law. It is called Lenz’s law.
What is Lenz’s Law?
In, 1834 Emil Lenz a Russian physicist gave a law that helps us to get the direction of induced current. It is known as Lenz’s law.
It says – If a current is induced inside a conductor it will also create a magnetic field around it. The direction of the current will be such that the magnetic field created by it will oppose the change that induced it. For better understanding, let us take an example.
MikeRun, Lenzs-law-cylindrical-magnet-leaving-ring, CC BY-SA 4.0
Fig:1 Illustration of Lenz’s law
Suppose there is a circular conductor as shown in the figure above. A bar magnetic is approaching it from the left. The moving bar magnet will induce a current in the conductor. We can see that the current creates a magnetic field that is clockwise and the original field due to the bar magnet was anticlockwise. Hence the magnetic field due to the induced current is opposing the source of the induced current.
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The Formula for Lenz’s Law
Lenz’s law says that the current or emf will, will oppose the change in the magnetic flux since the change in the magnetic flux is the reason for this induced emf. If the magnetic field is given by B, and magnetic flux is ϕBϕB. Then
e=−NdϕBdte=−NdϕBdt
N = no. of loops
dt = change in time
Here negative sign is crucial since it tells us about the direction. The word “oppose” is contained in the sign. If we want to only the magnitude of the induced emf, then we can write it like this
|e|∝dϕBdt|e|∝dϕBdt
What is Induced EMF?
When the magnetic flux of a conductor changes, an emf gets induced in it. It is known as induced emf. Suppose there is a coil with N loops, and it is slowly rotating in the magnetic field, then an emf will be induced in it. The amount of induced emf can be given by Faraday’s law. If the magnetic field is B, and the flux dphi is changing for time dt. Then induced emf
e=−NdϕBdte=−NdϕBdt
Experiment with Lenz’s law
Lenz formulated his law theoretically. His theory was proved by the following three experiments.
Keministi, Lenz law demonstration, CC0 1.0
First Experiment
In the first experiment, when the current flows in the circuit, a magnetic field gets created around it. As we increase the current, the magnetic flux increases since the magnetic field depends upon the current. The current opposes any change in the magnetic flux.
Second Experiment
In the second experiment, he found that, if we take a current carrying wire and wound it over an iron rod, then the one end of it behaves like the North pole and it gets attracted toward the south pole of the magnet. Hence an induced current gets generated.
Third Experiment
In the third experiment, when the coil is dragged in the direction of the magnetic flux, the area of the coil inside the field decreases. Lenz’s law tells us that if we apply the induced current in the same direction the motion of the coil is opposed.
For this, the magnetic field exerts a force on the coil. In opposition, a magnetic force is exerted by the current of the coil on the magnetic field.
Applications
It is used in the following areas.
- Card readers, microphones, and AC generators.
- It tells us the magnetic energy stored in the inductors.
- This concept is used in metal detectors and baking systems of trains
- It gives the physical understanding of the minus sign present in faraday’s formulation.
- It is used in eddy current dynamometers and balances.
Conclusion
Electromagnetic induction is a phenomenon when a change in flux creates an emf in a conductor. The amount of induced emf can be given by faraday’s law. The direction of induced emf or current is given by Lenz’s law. It says the induced emf has a direction such that it opposes any change arising in the flux. It is useful in many technological applications. Its prominence can be seen almost in each place where the electromagnetic induction phenomenon is used.
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