Page 79 - Physics_Form_4
P. 79
Physics for Secondary Schools
hence the motion of the coil results to an If there is an external circuit, the current
induced e.m.f in the coil. through it would also have a maximum
value at 90° and 270°.
N Therefore, the induced current starts
S from zero at the vertical position and
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continuously increase to the maximum
value at 90° position. It then starts
to decrease to zero at 180° and then
Figure 2.34: Coil at the horizontal position
increases in opposite direction towards
After a 180° turn, starting from the the maximum at 270° position. It then
vertical position, the sides of the loop reduces to zero at 360° (vertical) position.
interchange and the current in the loop The cycle then repeats as the coil
is reversed as illustrated in Figure 2.35. continues to rotate.
This means that the e.m.f is positive for
one half of the cycle and negative for the This kind of current is called an alternating
current (a.c) and the corresponding
other half. The maximum induced e.m.f voltage (e.m.f) is an alternating voltage.
is obtained when the coil is at 90° and The number of cycles produced per
270° positions from the vertical position. second is called the frequency of the a.c
However, the direction of the induced current. The a.c obtained is tapped by an
current at 90° position is opposite to the external circuit through the slip rings and
direction of the current at 270° position. the carbon brushes.
Stage 1 Stage 2
The plane of When the armature
the armature is rotates 90 the plane
0
perpendicular of the armature is Stage 3 Stage 4 Stage 5
to the magnetic parallel to the lines Armature after a Armature after a Armature after a
lines of force of force ratation of 180 0 ratation of 270 0 ratation of 360 0
R R R R Q Q Q Q R R R R
S S S S Q Q Q Q R R R R P R R R R Q Q Q Q S S S S
N N N N S S S S N N N N S S S S N N N N S S S S N N N N S S S S N N N N Q Q Q Q S S S S
B B B B Q Q Q Q A A A A R R R R B B B B
P P P P S S S S
A A A A A A A B B B B S S S S B B B B B B B A A A A P P P P A PPPP
B SSSS
A A A
A PPPP
90º
180º
180º
270º
90º
270º
90º 180º 270º 360º
90º
360º
180º
270º
360º
360º timetimetimetime
3T
3T
3T
0 0 0 0 T T T T T T T T 3T T T T T
4 4 4 4
4 4 4 4
2 2 2 2
Figure 2.35: Graphical representation of the induced current in the a.c generator
Most electric generators are designed to produce a.c. These generators are often called
alternators. Practical alternators usually have a fixed set of coils arranged around a rotating
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Student’s Book Form Four
Physics Form 4.indd 72 14/07/2025 14:43