Solutions to Period 17 Exercises
E.1 A current can be induced in a wire by
a) moving the wire in a magnetic field.
b) attaching one end of the wire to a
battery.
c)
dropping the wire into sulfuric acid.
d) heating the wire.
e) all of the above.
E.1 = a
17-Ex 1
E.2 Which of the following statements about
electric power generation is NOT true?
a) Electricity is generated by moving
magnets and coils of wire relative to
one another.
b) Generating plants convert kinetic
energy into electrical energy.
c)
At a generating plant, electricity is
stored as thermal energy by using it to
heat tanks of water.
d) Power plant turbines may be turned
using the gravitational potential energy
of falling water or the thermal energy
of steam.
e) ALL of the statements are true.
It is not possible to store large
amounts of electrical energy. At a
generating plant, tanks of water are
heated to produce steam, which turns
the turbines.
E.2 = c
17-Ex 2
E.3 A permanent magnet that forms the weight
at the end of a pendulum is suspended
directly over a loop of copper wire, the
ends of which are attached to a sensitive
galvanometer. If the pendulum is set into
motion so that is swings back and forth
over the wire, you should expect
a) a direct current to be induced in the
wire.
b) an alternating current to be induced in
the wire.
c)
a current to be induced in the wire
when the magnet comes to rest over
the wire.
d) no current to flow in the wire as there
is no source of electrical energy in the
circuit.
e) None of the answers is correct.
E.3 = b
17-Ex 3
E.4 The primary coil of a transformer has 100
turns, and the secondary has 200 turns. If
the primary is plugged into a 120 volt wall
outlet, what will be the output voltage of
the secondary?
a) 60 V
b) 100 V
c)
120 V
d) 200 V
e) 240 V
Solve Np/Ns = Vp/Vs for Vs:
Vs = Vp Ns
Np
= 120 V x 200 turns = 240 V
100 turns
Or using proportional reasoning, since the
number of turns doubles, the voltage will
double as well.
E.4 = e
17-Ex 4
E.5 You plan a trip to Europe, where the
household voltage is 240 volts. Your
electric razor, purchased in the U.S.,
operates on 120 volts. It will require a
transformer in order to operate properly in
Europe. If the transformer has 100 turns in
the primary coil, how many turns should it
have in the secondary coil?
a) 20
b) 50
c)
100
d) 200
e) 500
To solve equation 17.1 for Ns, first multiply
both sides by Ns and cancel:
Np Ns
Ns
=
Vp Ns
Vs
Then multiply both sides by Vs/Vp and cancel:
N p Vs
Vp
= Ns =
100 turns x 120 volts
240 volts
=
50 turns
If the voltage must be reduced by ½, the
number of turns will also be reduced by ½.
E.5 = b
17-Ex 5
E.6 Is the transformer described in E.5 a stepup or step-down transformer?
The transformer must reduce the 240 V to
120 V, so a step-down transformer is
needed.
E.7 One advantage of using alternating
current (AC) instead of direct current (DC)
is that
a) transformers, which change the
voltage, work only on AC.
b) AC does not produce heat in the power
lines, which the current flows through.
c)
electromagnets work only on AC.
d) AC is more practical to produce, since
DC can be obtained only from a battery.
e) None of the above is correct.
E.7 = a
17-Ex 6&7
Solutions to Period 17 Exercises
E.1 = a
E.2 = c
E.3 = b
E.4 = e
E.5 = b
E.6 = step down
E.7 = a