PASCO SE-8657 MOTOR ACCESSORY - Manual (Page 35)

012-06247 A Motor Accessory
31 ¨
Part B: DC Generator
➀
(a) The voltage and current produced is always in the same direction (direct current) but is
pulsating, not steady. (The pulsating nature will be difficult to note with a digital meter.)
Assuming as before that the negative lead of the meter is connected to the upper brush and
the positive lead is connected to the lower brush, then if armature is rotated clockwise (as
viewed from above) the meter will read positive.
(b) It responds similarly to a but in the opposite direction.
(c) Just as the current is about to reverse direction (as, for example, between steps 10 a and
b), the commutator reverses the connections between the coil and the brushes in order to
main the consistent direction of the current.
(d) pulsating DC
➁
(a) The effect would be greater voltage and current.
(b) Answers will vary. Two possible explanations include: (1) The free electrons present
in the wire of the coil move through the wire as a current due to the force given by
F=qV x B, where the force is seen as being proportional to the velocity of the wire, and
thus the electrons contained in it. At greater rotational speeds, the velocity of the wire
would be greater, and thus the force causing electron movement. (2) Faraday’s law states
that the emf induced is proportional to the rate at which the flux in the loops of the coils is
changing. At higher rotational speeds, the rate of change to the flux is greater, and thus so
is the emf.
➂
(a) The voltage vs. time graph would be a “sine wave” whose amplitude and “wave-
length”, or period, is decreasing to zero.
(b) It would be the same as a except all portions that would be below the horizontal (time)
axis are instead reflected above it.
(c) It would be the same as b except everything is below the axis.
Experiment 3
Notes concerning the setup:
•
If the power source does not limit the current to 1 A, use an appropriate resistor in
series to limit the current.
•
If the AC power source does not quantify the current, use an appropriate ammeter, or
measure the voltage drop across a resistor of 0.51 ohm, 1 watt wired in series and
calculate the current.
•
Remind students not to prolong situations when the armature is not spinning and the
power is connected—the coils will overheat.
Questions
Part A
➄
a) Yes.
b) Yes. Since the coils are wound such that the north poles of each are in the same direc-
tion, they work together to produce a net magnetic flux for the armature that is the sum of
the magnetic flux of each of the coils.
c) True.
➅ Opposite
➆
Yes.
➇ a) The armature’s north pole alternates from end to end as the AC waveform alternates
from positive to negative.