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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
          FOR ONLINE READING ONLY
                                                      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
              72
                                                                        Student’s Book Form Four
     Physics Form 4.indd   72                                                               14/07/2025   14:43
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