Faraday’s Law – Electromagnetic Induction

In nature Electricity and Magnetism are related to each other. As the magnetic field is induced around a current carrying wire likewise changing magnetic field can also induce electric field. This phenomenon in which electric current is generated by varying magnetic fields is called Electromagnetic Induction (EMI). 

Sir Oersted while performing experiments related to electric current demonstrated that the magnetic field is associated with the current carrying conductor. When a compass is brought near to a current carrying wire it is deflected; this helps to prove that a magnetic field is induced around a current carrying wire.

If electric current can produce magnetic field then the question arises is can magnetic field induce electric field? 

The Answer to the question is yes, changing magnetic fields does induce electric fields. This phenomenon in which electric current is generated by varying magnetic fields is called Electromagnetic Induction (EMI).

Michael Faraday demonstrated that electric currents were induced in closed loops when subjected to changing magnetic fields.

FARADAY’S EXPERIMENT

The discovery of electromagnetic induction is based on experiments carried out by Faraday and Henry.

Experiment consisted of a conducting coil connected to a galvanometer. A bar magnet with one of its poles facing towards the coil was moved towards the coil; a pointer in the galvanometer was observed to be deflected. The magnet was then moved away from the coil, now the pointer in the galvanometer deflected on the opposite side. The deflection of the pointer indicated the flow of current in the coil. 

Likewise, if we kept the bar magnet stationary and moved the coil back and forth within the magnetic field an electric current would be induced in the coil.

This experiment showed that relative motion between the coil and the magnet was responsible for generation (induction) of electric current in the coil.

FARADAY’S LAW

Faraday’s Law is thus stated as, an emf / voltage is induced in a circuit whenever there is a relative motion between a conductor and a magnetic field. The induced emf in turn produces induced current in the circuit. Also the magnitude of induced emf / induced voltage is proportional to the rate of change of magnetic flux.

Mathematical expression of Faraday’s Law:

Induced emf (e) α dΦ/dt …. 

Where Φ is Magnetic Flux

“Induced emf (induced voltage) is proportional to the time rate of change of magnetic flux”. 

• LENZ’S LAW 

Lenz’s deduced a law which gives the polarity of the induced emf.

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

Thus Faraday’s Law along with Lenz law can be expressed mathematically as follows-

Emf (e) = -dΦ /dt

The negative sign indicates that the induced emf opposes the changing magnetic flux.

• Magnetic Flux – It can be defined as the number of magnetic field lines passing through an area. The SI unit of magnetic flux is weber or Tesla metre squared. It is a scalar quantity.

Magnetic flux Φ = B.A … 

Where B= Magnetic field vector, and A= area vector.

Thus for changing magnetic flux any of the following conditions need to be satisfied.

• B(t)

Magnetic field is changing with respect to time. That is the magnetic field is time dependent.

• A(t)

Area changes with respect to time. If the Area of the circuit or the coil loop kept in the magnetic field is changing. 

• ϴ(t)

Theta changes with time. If the coil loop is rotating, that is the angle between the area vector and the magnetic field is changing.

Application of Faraday’s Law – Electromagnetic Induction

Faraday’s law is one of the most basic and important laws of electromagnetism. This law finds its application in electrical machines, industries, etc.

• Power Transformers function based on Faraday’s Law.
• The basic working principle of the electric generator is Faraday’s law.
• Induction cooker works on the basis of Faraday’s law.
• Electric guitar is based on the principle of Faraday’s law.

Conclusion

In nature Electricity and Magnetism are related to each other. The phenomenon in which electric current is generated by varying magnetic fields is called Electromagnetic Induction (EMI).

Electromagnetic Induction is thus a phenomenon which is an interrelation between electricity and magnetism. It is the principle which explains the generation of electric current in a closed loop kept in a changing magnetic field.

Michael Faraday experiments demonstrated the basic principle of generation of induced emf in a closed coil due to the time rate of change of magnetic flux. Thus Faraday’s law states that an induced emf (induced voltage) is proportional to the time rate of change of magnetic flux. On the other hand Lenz stated a law which gives the polarity of induced emf.

Thus Faraday’s Law along with Lenz law can be expressed mathematically as follows-  

Induced emf (e) = – Time rate of change of magnetic flux (dΦ/dt).

• Faraday’s law has its significance in Maxwell’s equation which describes Electromagnetic waves.