Induction and Alternating Current with teacher's notes

22 REVIEW & ASSESS
12. −0.63 V
13. B field (induces emf in
turning coil), wire coil (conducts
induced current), slip
rings (maintain contact with
rest of circuit by means of
conducting brushes)
14. turn the handle faster
15. Frequency indicates how often
each second the current goes
from a maximum value in one
direction to a maximum value
in the other direction and back.
16. Replace the slip rings with a
commutator, which prevents
the reversal of the current
every half-cycle.
17. Battery current is constant,
while dc generator current
fluctuates.
18. an emf with polarity opposite
that of the emf powering the
motor; The coil’s rotation in
the B field induces a back emf
that reduces the net potential
difference across the motor.
19. The magnetic forces are greatest
on charges in the sides of a
loop that move perpendicular
to the B field (that is, when the
plane of the loop is parallel to
the field lines).
20. The B field of an induced current
opposes the change (due to
coil rotation) in the external B
field. Faster rotation of the coil
increases this induced current
and thus the opposing field.
21. Maximum values are maintained
only for an instant, whereas
rms values remain steady
and thus are easier to measure.
22. a, b; The B field lines in these
cases are in a plane perpendicular
to the plane of the loop, so
the loop crosses the field lines.
23. 3.88 × 10 −2 V
24. a. 2.4 × 10 2 V
b. 6.9 A
822
12. A 52-turn coil with an area of 5.5 × 10 −3 m 2 is
dropped from a position where B = 0.00 T to a new
position where B = 0.55 T. If the displacement
occurs in 0.25 s and the area of the coil is perpendicular
to the magnetic field lines, what is the
resulting average emf induced in the coil?
(See Sample Problem 22A.)
ALTERNATING CURRENT,
GENERATORS, AND MOTORS
Review questions
13. List the essential components of an electric generator,
and explain the role of each component in generating
an alternating emf.
14. A student turns the handle of a small generator
attached to a lamp socket containing a 15 W bulb.
The bulb barely glows. What should the student do
to make the bulb glow more brightly?
15. What is meant by the term frequency in reference to
an alternating current?
16. How can an ac generator be converted to a dc generator?
Explain your answer.
17. In what way is the output of a dc generator different
from the output of a battery?
18. What is meant by back emf? How is it induced in an
electric motor?
Conceptual questions
19. When the plane of a rotating loop of wire is parallel
to the magnetic field lines, the number of lines passing
through the loop is zero. Why is the current at a
maximum at this point in the loop’s rotation?
20. The faster the coil of loops, or armature, of an ac
generator rotates, the harder it is to turn the armature.
Use Lenz’s law to explain why this happens.
21. Voltmeters and ammeters that measure ac quantities
measure the rms values of emf and current,
respectively. Why would this be preferred to measuring
the maximum emf or current? (Hint: Think
about what a meter reading would look like if ac
quantities other than rms values were measured.)
822
Chapter 22
22. A bar magnet is attached perpendicular to a rotating
shaft. It is then placed in the center of a coil of
wire. In which of the arrangements shown in
Figure 22-26 could this device be used as an electric
generator? Explain your choice.
Practice problems
(a)
(b) (c)
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Figure 22-26
23. A generator can be made using the component of
Earth’s magnetic field that is parallel to Earth’s surface.
A 112-turn square wire coil with an area of
4.41 × 10 −2 m 2 is mounted on a shaft so that the
cross-sectional area of the coil is perpendicular
to the ground. The shaft then rotates with a frequency
of 25.0 Hz. The horizontal component of
Earth’s magnetic field at the location of the loop is
5.00 × 10 −5 T. Calculate the maximum emf induced
in the coil by Earth’s magnetic field.
(See Sample Problem 22B.)
24. An ac generator consists of 45 turns of wire with
an area of 0.12 m 2 . The loop rotates in a magnetic
field of 0.118 T at a constant frequency of
60.0 Hz. The generator is connected across a circuit
load with a total resistance of 35 Ω. Find the
following:
a. the maximum induced emf
b. the maximum induced current
(See Sample Problem 22B.)
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