Transcript Self Induction By Nadra Parveen

Presented To:
Sir Tariq Bhatti
Presented By:
Nadra Parveen
(07-30)3rd
Topic:
Self Induction
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Self Induction
The phenomenon in which a changing current in a coil induces an
emf in itself is called self induction
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Explanation:
Consider a coil is connected in series with a battery and a
rheostat as shown in fig.
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Cont…
Magnetic flux is produced through the coil due to
current in it.
If the current is changed by changing the
resistance of the rheostat.
The magnetic flux through the coil changes that
causes an induced emf in the causes an induced
emf in the coil.
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Such an emf is called as self induced emf.
Let the flux passing through one loop of the coil be Φ ,
then the total flux passing through the coil of N turns
will be NΦ.
As this flux is proportional to the magnetic field produced
by the current I , then
N Φα I
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or
or
NΦ = LI …..(1)
L = NΦ ⁄ I …..(2)
where L is constant of proportionality
called the self inductance of the coil.
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It depends upon the following factors.
Number of turns of the coil.
Area of cross-section of the coil.
Core material upon which the coil is wound.
According to faraday’s law, emf induced in the coil is given
by
ε
L =
A ΔΦ/ Δt
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ε
L = - Δ (AΦ)/ Δt
Putting NΦ =LI from equation(1)
into equation(3).
ε
L = - Δ (LI)/ Δt
ε
L = - Δ LI/ Δt
……..(3)
………(4)
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The negative sign shows that the self induced emf must
oppose the change that produced it.
The self induced emf is also called back emf.
Self Inductance:
In equation(4), negative sign is ignored, then
L = ε/(ΔI/ Δt )
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Its defined as ”the ratio of average emf in the coil to the
time rate of change of current in the same coil.”
Unit of Self Inductance:
The SI unit for self inductance is VS/A
which is called Henry.
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