indicated, and with the S pole of the magnet directly under the top portion of the
loop and the N pole directly over the bottom portion, the induced voltage, as
determined by the Right-Hand Rule, will cause conventional current to flow in the
circuit in the direction shown. Since conventional current flows from positive to
negative through the external or load circuit, the end of the loop of wire marked
"A" will be positive (+) polarity and the end marked "B" will be negative (-).
Figure 18.
Rotating field.
b. After the bar magnet has moved through one-half revolution, the N pole
will have moved directly under the top conductor and the S pole directly over the
bottom conductor. The induced voltage as determined by the Right-Hand Rule will
now cause current to flow in the opposite direction. The end of the loop of wire
marked "A" will become negative (-) polarity and the end marked "B" will become
positive (+). Therefore, the polarity of the ends of the wire has changed. After
a second one-half revolution, the bar magnet will be back at the starting point
where "A" is positive (+) and "B" negative (-). Consequently, current will flow
through the load or external circuit first in one direction and then in the other.
This is an alternating current which is developed by a generator.
c. A generator made with a bar magnet rotating inside a single loop of wire
is not practical, since very little voltage and current are produced. The
performance is improved when both the loop of wire and the magnet are placed inside
an iron frame (fig 19). The iron frame not only provides a place onto which the
loop of wire can be assembled, but also acts as a con-ducting path for the magnetic
lines of force. Without the iron frame, magnetism, after leaving the N pole of the
rotating bar magnet, must travel through air to get to the S pole. Because air has
a high reluctance to magnetism, only a few lines of force will come out of the N
pole and enter the S pole. Since iron conducts magnetism very easily, adding the
iron frame greatly increases the number of lines of force between the N pole and S
pole. This means that more lines of force will be cutting across the conductor
which lies between the bar magnet and frame.
OS 010, 1-P9
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