Chapter 12 Electromagnetic Induction
215
zero. This decrease in circuit current causes
the magnetic fi eld around the coil to collapse.
The collapsing magnetic fi eld again induces a
voltage in the coil. The induced emf produces
a current that fl ows until the magnetic fi eld
has completely collapsed. In other words, the
current of a circuit with a coil in it takes longer
to drop to zero after the switch has been
opened than the same circuit without a coil.
When the magnetic fi eld either expands or
collapses, it induces a current in a coil.
Figure 12-7 shows the waveforms of a dc
circuit without a coil and one with a coil. Notice
that when the circuit does not have a coil,
the wave takes the shape of a square wave
when the switches are closed and opened,
Figure 12-7A. When the dc circuit has a coil
in it, the wave does not immediately reach
its peak. Instead, the climb to its peak ampli-
tude is slower. The wave is a curve on a scope
before and after it levels out. The induced
emf tends to oppose the change in current,
Figure 12-7B.
Most of the time, a coil has little or no
effect on a dc circuit. It affects a dc circuit
when the switch is opened or closed because
those are the only times the circuit current is
changing. However, a coil has a large effect on
an ac circuit because the circuit current in an
ac circuit is always changing.
Inductors
Any part of a circuit that has the property
of inductance is called an inductor. The most
common form of an inductor is a coil of wire.
There are several symbols for inductors. The
symbol for a basic inductor and the symbols
for more complex inductors are shown in
Figure 12-8. A circuit board containing induc-
tors is shown in Figure 12-9.
Occasionally, an inductor is called a choke.
Figure 12-10 shows a radio frequency (RF)
choke for an automobile. An RF choke is used
to reduce circuit current or to stop ac from
getting into a dc circuit. The core of a choke can
be either air or adjustable ferrite (soft iron).
Figure 12-7. Graphs of current waveforms in a
dc circuit. A—Current waveform in a dc circuit
without a coil. B—Current waveform in a dc
circuit with a coil.
I
Switch
opened
Circuit without Coil
Circuit with Coil
A
B
Switch
closed
time
time Switch
opened
Switch
closed
I
Figure 12-8. These are the symbols commonly
used to indicate the use of different types of
inductors in electrical circuits.
Basic Inductor
or
Magnetic Core
Adjustable
Adjustable