at these principles of magnetism and how
they work together to create inductance. It also
looks at the characteristics of inductors and
transformers.
Induced Voltage
In Chapter 10, you performed a simple
experiment with a coil of wire, a galvanom-
eter, and a magnet. When you pushed the
magnet into the coil and quickly removed it,
the galvanometer detected a slight current,
Figure 12-1. The current was present only
when the magnet was moving through the
coil. Current can be created in a conductor
when there is a change in the magnetic fi eld
around it. The changing magnetic fi eld induces
voltage in the coil, which creates current in
the coil. This voltage is called induced emf.
There is always an opposition to change
in electricity. As current starts to fl ow, a force
opposes the fl ow. The opposing force is greatly
reduced once the fl ow of current reaches its
maximum. When the fl ow of current is shut
off, a force opposes the fl ow until the current
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Electricity and Basic Electronics
reaches zero. You have seen examples of this
outside of the electrical fi eld, such as the
following:
• Lower gas mileage that results while a
car is increasing speed from 0 miles per
hour to 40 miles per hour.
• Time to bring a pressure washer or saw
blade up to speed.
• Effort required to pedal a bike from a
standing start.
Some people say these examples are the
result of friction or some other function. In
electricity, there is a counter-electromotive
force (also called cemf or counter emf) which
opposes the applied voltage. This results in
a reduction or delay in any increase (at start-
up) or decrease (at shutdown) in the fl ow of
current.
Look at the dc circuit in Figure 12-2. This
circuit contains a dc power source, switch,
and coil. When the switch is closed, circuit
current begins to increase. The current creates
a magnetic fi eld around the coil, Figure 12-2A.
As the current increases, the magnetic fi eld
expands. Because it is changing, the magnetic
fi eld induces voltage in the coil that opposes
the polarity of the source voltage.
Once the circuit current reaches its
maximum and remains constant, the magnetic
fi eld stops expanding and also remains
constant, Figure 12-2B. There is no longer any
induced emf in the coil because a constant
magnetic fi eld does not induce voltage in a
coil.
Suppose the dc power source in this
circuit is replaced by an ac power source,
Figure 12-3. The current in an ac circuit is
always changing. This means there is always
a changing magnetic fi eld around the coil and,
therefore, always induced emf in the coil. As
alternating current changes its direction, the
polarity of the coil reverses so that it opposes
the polarity of the ac power source.
Note in Figure 12-4 that the changing
fi eld set up by each turn of the coil passes
through other turns of the coil. This is known
Figure 12-1. Recall that the changing magnetic
fi eld created by the sudden movement of a
magnet inside a coil of wire will create current in
the wire.
Magnet
Wire
coil
Galvanometer