Page 80 - Physics_Form_4

Electromagnetism
electromagnet. Most cars,
for example, are fitted with
alternators which produce
electricity for running the
car after it is ignited. The
produced current (a.c) is
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changed into direct current
(d.c) using diodes. Part of this
d.c is used for charging the car
battery. For a car alternator,
the armature is rotated by a
belt which is moved by the
crankshaft of the engine.
In power stations, much
larger alternators are used to
generate a.c electricity. The
energy needed to rotate the
such large armatures in power
stations is obtained from
sources like falling water from
a dam, compressed steam
from a nuclear reactor, or
steam from underground. The
basic principle of generating
electricity in all alternators is
the same.
The d.c generator
A d.c generator is made
by replacing the slip rings
of the a.c generator with a
commutator. Each half of a
commutator ring is called
a commutator segment and
is insulated from the other
half. Each end of the rotating
loop of wire is connected to
a commutator segment. Two
carbon brushes connected to
Physics Form 4.indd 73
the outside circuit rest against the rotating commutator.
The commutator rotates with the loop of wire, just as
the slip rings do in the a.c generator. Figure 2.36 shows
a simple d.c generator.
Magnetic flux
N S
Commutator
Carbon
brush Wire coil
(the conductor)
V
Axis of
rotation
Figure 2.36: Simple d.c. generator
When the loop is rotated in the magnetic field, the
induced e.m.f is still an alternating e.m.f. However,
after a rotation of 180°, instead of the current
reversing, the connections to the external circuit are
reversed so that the direction of current direction in
the external circuit remains the same. The output of a
d.c generator is as shown in Figure 2.37.
Inducted e.m.f (V)
Time (s)
Figure 2.37: Output of a d.c generator
Note that, the lower half of the cycle is not cut off but
reversed.
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