UNIT
E
Self-Quiz
SUGGESTED ANSWERS
1.
(b); Choice (b) is correct. In order for an atom to have an electric charge, it must have an unequal number
of protons and electrons. An atom with an electric charge is also called an ion. Choice (a) is incorrect
because the sodium atom has an equal number of protons and electrons; it would be neutral. Choice (c)
is incorrect because the helium atom has an equal number of protons and electrons; it would be neutral.
Choice (d) is incorrect because the iron atom has an equal number of protons and electrons; it would be
neutral.
2.
(d); Choice (d) is correct. Charging by conduction occurs when an object contacts another object with
a different net charge, and electrons move from one object to the other. Choice (a) is incorrect because
it describes the process of grounding. Choice (b) is incorrect because the charges neutralize each other,
so neither object is being charged. Choice (c) is incorrect because it describes temporary static electric
charging.
3.
(a); Choice (a) is correct. A load converts electrical energy into another form of energy. Light bulbs and
computer hard drives are examples of loads. Choice (b) is incorrect because a switch controls the flow of
electrons; it does not convert electrical energy. Choice (c) is incorrect because a power source provides the
electricity; it does not convert electricity. choice (d) is incorrect because the connecting wire provides a
conductor through which the electrical current flows; it does not convert electrical energy.
4.
(a); Choice (a) is correct. Ohm’s law can be described with the equation R 5 V/I, where R is resistance,
V is voltage, and I is current. Choice (b) is incorrect because current equals resistance divided by current.
Choice (c) is incorrect because resistance is not independent of voltage and current. Choice (d) is incorrect
because voltage is resistance divided by current.
5.
True. Electrons in the neutral object shift away from a negatively charged object or toward a positively
charged object. The neutral object will not gain or lose any electrons.
6.
False. Tidal generation does not cause water or air pollution. A true version of the statement would
be: One disadvantage of tidal generation plants is that they can only be built near coastlines that have
significant tidal changes.
7.
True. A proton has a positive charge and an electron has a negative charge. Each of these charges can
either repel or attract other charges, depending on the two charges involved. So protons and electrons are
examples of electric charges that exert an electric force on other charges.
8.
attract; According to the Law of Electric Charges, objects that have opposite charges attract each other.
(11.1)
9.
insulators; Materials that prevent the movement of electrons are insulators. (11.4)
10.
secondary cells; primary cells; Secondary cells are are electric cells that can be recharged, while primary
cells are electric cells that can be used only once. (12.3)
11. (a)(iv); (b)(ii); (c)(i); (d)(v); (e)(iii)
12.
Sample answer: Aluminum, nickel, and copper are good conductors. Silk, plastic, and glass are good insulators.
13.
The electrostatic series can help engineers predict whether the materials in their designs have a tendency to
gain or lose electrons.
NEL
55219_04_ch13_p851-918_pp4.indd 917
Unit E Self-Quiz
917
12/8/09 1:52:57 PM
14.
Four factors that affect the internal resistance of a material are type of material, cross-sectional area, length,
and temperature.
15.
Static electricity involves electrons that collect in one area of an object and may be transferred to another
object in an uncontrolled manner. Current electricity refers to the controlled flow of electrons through
conductors. The movement of charges in current electricity is more predictable than the movement of
charges in static electricity.
16. (a)
(b)
As clothes come in contact with human skin, electrons are transferred from the skin to the clothes.
As a result, the negatively charged clothes are attracted to the positively charged skin, and the clothes
cling to the body.
Clothes made of polyester would most likely cause static cling. While human skin is at the far positive
end of the electrostatic series, polyester is near the far negative end. So contact between skin and
polyester is likely to cause a large buildup of static electricity.
17.
Use the equation % efficiency 5 E(out)/E(in) 3 100 %;
Percent efficiency of model A 5 (350 J 4 850 J) 3 100 % 5 41 %
Percent efficiency of model B 5 (175 J 4 600 J) 3 100 % 5 29 %
Percent efficiency of model C 5 (250 J 4 750 J) 3 100 % 5 33 %
Model A is most efficient.
18.
The water in a typical home contains many minerals and ions. This means that this water conducts
electricity. Throwing water on an electrical fire would allow electrons to move more freely.
19.
Use Ohm’s Law to calculate the average resistance for each resistor (divide voltage by current to find
resistance; R 5 V/I ). Using this method, the resistance of resistor A is about 81 Ω and that of resistor B is
about 109 Ω. So resistor B has the greater resistance.
20.
ⴚ
ⴚⴙ ⴚ
ⴚ ⴚⴚ
ⴙ ⴙ
21. (a)
(b)
(c)
22.
(b)
918
ⴚⴙ ⴚ
ⴚ ⴚⴚ
ⴙ ⴙ
ⴚ ⴚ
ⴙⴙ ⴚ
ⴙⴚ ⴚ
ⴚ
ⴙⴙ
ⴚ
ⴙ
ⴙ
ⴚ ⴚ
ⴚⴙ ⴙ ⴚ ⴚ
ⴚ ⴙⴚ ⴚ
ⴚ
ⴙⴙ
ⴚⴙ
ⴙ
ⴚ
ⴚ
ⴚ
ⴚ ⴚ ⴙ ⴚⴙ
ⴙ
Sample answer: When I brush my cat, his fur lifts and sticks to the brush. During the act of brushing,
electrons are transferred from the fur to the brush, and each object becomes oppositely charged. As a
result, the fur and brush are attracted to each other.
Sample answer: When I turn on my computer, the screen begins to attract dust. This happens because
the charge on the screen induces an opposite charge on the near side of each dust particle. As a result,
the dust particles are drawn to the screen.
Sample answer: My house has a metal lightning rod connected to the ground with a wire. If lightning
hits the rod, the electrons are transferred down the wire into the ground.
Sample answer: I would use copper for the wire because it is a good conductor of electricity. I would
also add a rubber casing as an insulator around the wire, to prevent unsafe electrical discharges from the
wire.
23. (a)
24.
ⴚ ⴚ
ⴙⴙ ⴚ
ⴙⴚ ⴚ
ⴚ
ⴙⴙ
ⴚ ⴚⴙ
ⴙ
Sample answer: What challenges do you expect the wind farm to bring to the community? Do you
think the community will benefit in any way from the wind farm? Do you plan to use wind power
from the wind farm?
Sample answer: Wind power is much cleaner than fossil fuels, which contribute to pollution and
climate change. In addition, wind is a renewable resource, so we do not have to worry about running
out of it. The new wind farm could also give your community easier access to electricity.
Sample answer: A floor lamp uses a circuit. The energy source in this circuit is an electricity generating
plant that supplies electricity to my home. The electricity flows through the connecting wires and reaches
the light bulb, which is the load. In the light bulb, electrical energy is converted into light energy and some
thermal energy. The switch on the lamp controls the flow of electricity through the circuit.
Unit E: The Characteristics of Electricity
55219_04_ch13_p851-918_pp4.indd 918
NEL
12/8/09 1:52:58 PM