Page 156 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Comparison between electrical and magnetic quantities

Electric Circuit Magnetic Circuit

e.m.f. m.m.f.
current I (A) flux Φ (Wb).

resistance R (Ω) reluctance S (H−1)

R= (ρl) / A S = l / μ0μrA

I = E / R Φ = mmf / S

Electro magnetic induction
Whenever an electric current flows through the conductor, a magnetic field is set up around the conductor in the
shape of concentric circle. It can be said that when electrons are in motion, they produce magnetic field. The
convers of this is also true ie, when a magnetic field linking a conductor moves relative to the conductor, it produce
a flow of electron in the conductor. This phenomenon where by an emf and hence current (ie, flow of electron) is
induced in any conductor which is cut across or by a magnetic flux is known as electromagnetic induction.
Faradays laws of electro magnetic induction

Historical back ground
After the discovery of (by Oersted) that electric current produces a magnetic field, scientist began to search for the
converse phenomenon from about 1821 on words. The problem is how to convert magnetism in to electricity. It is
recorded the Michel faraday was in the habit of walking about with magnet in his pocket so as to constantly remind
him of the problem. After nine years of continuous research and experimentation, he succeeded in producing
electricity by converting magnetism in 1831.He formulated basic laws under laying the phenomenon of electro
magnetism (known after his name), ie

Faradays summed up the above fact in two laws.
First law
Whenever the magnetic flux linked with a circuit changes, an emf is induced in it.
Eg: transformer action
OR

Whenever a conductor cuts magnetic flux, an emf is induced in that conductor.
Eg: Electric generator
Second law
The magnitude of the induced emf is equal to the rate of change of flux linkages or cutting.
ie, induced emf = rate of change of flux linkages. E Change in

Explanation of faradays law Time
Suppose a coil has N number of turns and flux that it changes from initial value of ∅1Webbers to the final value
of ∅ 2 Webbers in time T second.

Then, flux linkages are meant product of no. of turns and flux linked with the coil.
We have,
Initial flux linkages =NØ1 Final flux linkages =NØ2
Therefore, induced emf, e = NØ2 - NØ1 wb/s or
t

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CITS : Power - Electrician & Wireman - Lesson 26-29