inductance
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What is Inductance?
Inductance is a property in which an emf (electro-motive force) is
induced in a circuit due to a change in electric current through that circuit or
through a nearby circuit. This induced emf opposes the change in the current.
The symbol used to denote inductance is L. The unit of inductance is the henry
(H), named after American scientist Joseph Henry. One henry is the
inductance needed to induce one volt by a change of current of
one ampere per second.
In 1831, an English scientist, Michael Faraday discovered that when a
magnetic field is changed in a circuit, a current is induced in a nearby
circuit. Joseph Henry made this discovery at about the same time although Faraday
was the first to publish. The phenomenon of generating
electromotive force and current by changing magnetic field is known as
electromagnetic induction.
Faraday's Law
Faraday's Law of electromagnetic induction gives the relationship between
the changing magnetic flux and induced electromagnetic force. It states:
The electromagnectic force induced in a circuit is
proportional to the rate of change of the magnetic flux that moves through the
circuit.
If
E is the induced electromotive force in volts,
DΦ the change in magnetic force and
Dt is the amount of time in seconds in which the change in magnetic force takes
place, then:
E = - (DΦ/Dt)
It means that the more magnetic field lines cut across the conductor and the faster they
cross, the greater the induced voltage.
Self Induction
The phenomenon in which a changing current in a coil induces an emf in
itself is called self induction. If the flux through one loop of the coil be F, then the total flux through the coil of N
turns would be N.F. As F is
proportional to the magnetic field which is in turn proportional to the current
I, then,
N.Φ = L.I
Here L is the constant of proportionality called the self inductance of
the coil. It depends upon the number of turns of the coil, its area of cross
section and the core material.
If E is the emf induced, using Faraday's Law it can be deduced that,
E = -L.DI/Dt
It shows that the self induced emf in a coil is proportional to the time
rate of change of current in the coil. Self inductance of a coil is defined as
the ratio of emf to the rate of change of current in the coil. The negative sign
indicates that the self induced emf must oppose the change that produced it. For
this reason the self induced emf is also called back emf, which can destroy
semiconductors used to drive relays and other electromagnetic components if they
are not adequately protected.
Mutual Induction
The phenomenon in which a changing current in one coil induces an emf in
another coil is called mutual induction. Let the flux passing through one loop
of the secondary coil be F.
Net flux passing through the coil of N
loops is NF. As this flux is directly proportional to the magnetic
field produced by the current, I, in the primary coil and the magnetic field
itself is proportional to I, then
N.Φ = M.I
where M is the constant of proportionality called the mutual inductance
of the two coils. It depends upon the number of turns of the coils, their area
of cross section, their proximity and the nature of the core material on which
the two coils are wound.
Using Faraday's law, it can be deduced that,
E = -M.DI/Dt
It shows that the emf induced in the secondary coil is proportional to
the time rate of change of current in the primary. Again, the negative sign
indicates that the induced emf opposes the change of current in the primary
coil.