(1) In an ac circuit, the
current
continually
changes
amplitude
and
periodically
reverses
direction.
Any
such
change causes an expansion
or
contraction
of
the
magnetic field. The lines
of force in this changing
field cut the turns of
wire in the coil, inducing
a voltage which opposes
the
applied
ac
voltage
that
is
pushing
the
current
through
the
windings of the coil. The
property of a coil to
develop
an
opposing
voltage is known as self-
inductance.
(2) Direct
application
of
self-inductance
can
be
seen in a radio-frequency
(RF) choke coil. The five
smaller coils in A of
figure 18 are examples of
RF choke coils.
Their
function is to oppose the
flow of RF current, but at
the same time allow the
passage of dc.
They are
widely
used
in
radio
b. Effect of Inductance.
The
magnitude (amount) of counter EMF
produced by the self-inductance of a
coil depends on two factors: the
number of turns in the coil and the
frequency of the applied voltage.
(1) The number of turns in the
coil
determines
the
physical
property
of
inductance.
The greater
the number of turns, the
Figure 21.
Representative capacitor
larger the inductance of
types.
the coil. And, of course,
the larger the inductance,
the more counter EMF is
produced.
(2) Since the counter EMF voltage produced by the self-inductance of a coil
depends upon the rate at which the lines of force cut the turns, it
follows that the higher the cutting rate, the greater will be the
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