Induction and Alternating Current with teacher's notes

25. The rms potential difference across high-voltage
transmission lines in Great Britain is 220 000 V.
What is the maximum potential difference?
(See Sample Problem 22C.)
26. The maximum potential difference across certain
heavy-duty appliances is 340 V. If the total resistance
of an appliance is 120 Ω, calculate the following:
a. the rms potential difference
b. the rms current
(See Sample Problem 22C.)
27. The maximum current that can pass through a light
bulb filament is 0.909 A when its resistance is 182 Ω.
a. What is the rms current conducted by the filament
of the bulb?
b. What is the rms potential difference across the
bulb’s filament?
c. How much power does the light bulb use?
(See Sample Problem 22C.)
28. A 996 W hair dryer is designed to carry a maximum
current of 11.8 A.
a. How large is the rms current in the hair dryer?
b. What is the rms potential difference across the
hair dryer?
(See Sample Problem 22C.)
INDUCTANCE
Review questions
29. Describe how mutual induction occurs.
30. What is the difference between a step-up transformer
and a step-down transformer?
31. Does a step-up transformer increase power? Explain
your answer.
32. Describe how self-induction occurs.
Conceptual questions
33. In many transformers, the wire around one winding
is thicker, and therefore has lower resistance, than the
wire around the other winding. If the thicker wire is
wrapped around the secondary winding, is the device
a step-up or a step-down transformer? Explain.
34. Would a transformer work with pulsating direct
current? Explain your answer.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Practice problems
35. A transformer is used to convert 120 V to 9.0 V for
use in a portable CD player. If the primary, which is
connected to the outlet, has 640 turns, how many
turns does the secondary have?
(See Sample Problem 22D.)
36. A transformer is used to convert 120 V to 6.3 V in
order to power a toy electric train. If there are 210
turns in the primary, how many turns should there
be in the secondary?
(See Sample Problem 22D.)
MIXED REVIEW PROBLEMS
37. A student attempts to make a simple generator by
passing a single loop of wire between the poles of a
horseshoe magnet with a 2.5 × 10 −2 T field. The
area of the loop is 7.54 × 10 −3 m 2 and is moved perpendicular
to the magnetic field lines. In what
time interval will the student have to move the
loop out of the magnetic field in order to induce
an emf of 1.5 V? Is this a practical generator?
38. The same student in item 37 modifies the simple
generator by wrapping a much longer piece of
wire around a cylinder with about one-fourth the
area of the original loop (1.886 × 10 −3 m 2 ). Again
using a uniform magnetic field with a strength of
2.5 × 10 −2 T, the student finds that by removing the
coil perpendicular to the magnetic field lines during
0.25 s, an emf of 149 mV can be induced. How
many turns of wire are wrapped around the coil?
39. A coil of 325 turns and an area of 19.5 × 10 −4 m 2 is
removed from a uniform magnetic field at an angle
of 45° in 1.25 s. If the induced emf is 15 mV, what is
the magnetic field’s strength?
40. A transformer has 22 turns of wire in its primary
and 88 turns in its secondary.
a. Is this a step-up or step-down transformer?
b. If 110 V ac is applied to the primary, what is
the output potential difference?
41. The potential difference in the lines that carry electric
power to homes is typically 20.0 kV. What is the
ratio of the turns in the primary to the turns in the
secondary of the transformer if the output potential
difference is 117 V?
Induction and Alternating Current
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22 REVIEW & ASSESS
25. 3.1 × 10 5 V
26. a. 2.4 × 10 2 V
b. 2.0 A
27. a. 0.643 A
b. 117 V
c. 75.2 W
28. a. 8.34 A
b. 119 V
29. The changing B field produced
by a changing current in one
circuit induces an emf and
current in a nearby circuit.
30. A step-up transformer uses
the B field of an alternating
current to induce an increased
emf in the secondary. A stepdown
transformer uses the
same principle to induce a
smaller emf in the secondary.
31. no; The change in potential
difference in a transformer is
accompanied by an inverse
change in the current. In an
ideal transformer, power is
unchanged, as expected from
energy conservation.
32. Changing current in a circuit
produces a changing magnetic
field, which induces an opposing
emf and current in the
same circuit.
33. a step-down transformer; I is
larger in the secondary, so wire
with a lower R is needed to
reduce energy dissipaton.
34. yes; Current changes continually,
so its changing B field can
induce an emf in a transformer’s
secondary.
35. 48 turns
36. 11 turns
37. 1.3 × 10 −4 s; no
38. 790 turns
39. 4.2 × 10 −2 T
40. a. a step-up transformer
b. 440 V
41. 171:1
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