Chapter 12 Electromagnetic Induction
213
as magnetic coupling. The changing magnetic
fi eld in one turn of the coil produces an emf in
the other turns of the coil through which the
fi eld passes.
Inductance
The induced voltage produced in a coil
by a changing magnetic fi eld affects the fl ow
of circuit current through the coil. Because
the induced voltage opposes the voltage of
the power source in the circuit, the induced
voltage opposes change in the current fl ow
of the circuit. This opposition to a change in
current fl ow is called inductance.
Let us look again at what happens in
a circuit consisting of a dc power source,
a switch, and a coil. This time, we will look
at how induced voltage and the current it
produces affects the current from the circuit’s
power source. To distinguish this current
from the induced current, we will refer to it as
circuit current.
Figure 12-2. A dc circuit with a switch and a coil.
A—Voltage is induced in the coil when the switch
is fi rst closed. This induced voltage opposes the
polarity of the source voltage. B—No voltage is
induced in the coil once the current reaches its
maximum and becomes constant.
Expanding
magnetic
field
induces
voltage
A
B
–
+
Constant
magnetic
field
does not
induce
voltage
Figure 12-3. An ac circuit with a switch and a
coil. When the fi eld collapses during the positive
and negative cycles of alternating current, the
polarity of the coil reverses so that it opposes the
polarity of the ac power source. A—Collapsing
fi eld during positive cycle. B—Collapsing fi eld
during negative cycle.
A
–
+
–
+
B
+
–
+
–
Figure 12-4. The changing magnetic fi eld
created by alternating current in each turn of
the coil cuts through the other coils, creating a
magnetic coupling.
Field from one portion of
wire coil cuts through other section
AC power
supply