Chapter 12 - Electricity
TRUE/FALSE
1. Opposite charges repel one another.
ANS: F
2. Object X has a negative charge. It attracts object Y. Object Y must have a positive charge.
ANS: F
3. Copper is a better conductor than glass.
ANS: T
4. A conductor does not transfer electrons very well.
ANS: F
5. The electric force between two charged objects decreases as they get closer to each other.
ANS: F
6. Electric field lines travel away from a positive charge and towards a negative charge.
ANS: T
7. The purpose of Millikan's experiment was to prove that oil droplets could be balanced in an electric
field.
ANS: F
8. The elementary charge has a value of 1.60 × 10–19 C.
ANS: T
9. An ammeter is used to measure the current in an electrical circuit.
ANS: T
10. The current in a circuit is the rate at which charge flows.
ANS: T
11. An ampere is equal to one coulomb per second.
ANS: T
12. A direct current always flows in the same direction in a circuit.
ANS: T
13. The direction of electron flow is from a negative terminal to a positive terminal in a circuit.
ANS: T
14. When a charged object loses energy, it gains electric potential.
ANS: F
15. The electric potential difference of a circuit is measured with a voltmeter.
ANS: T
16. One volt is equal to one joule per second.
ANS: F
17. A load in a circuit provides energy to the electrons.
ANS: F
18. In a parallel circuit, electrons have only one path to follow.
ANS: F
19. Georg Ohm discovered that the ratio of voltage to current is constant for a given conductor.
ANS: T
20. A 10-Ω resistor removes more energy from a circuit than a 20-Ω resistor.
ANS: F
21. Power is a measure of the rate of energy use.
ANS: T
22. The power used by a load can be calculated by the equation
.
ANS: F
MULTIPLE CHOICE
1. A positive charge attracts object X.
A negative charge repels object Y.
Object X is attracted to object Y.
Which is of the following is the most likely explanation?
a. Object X has a positive charge. Object Y has a negative charge.
b. Object X has a negative charge. Object Y has a negative charge.
c. Object X has a positive charge. Object Y has a positive charge.
d. Object X has a negative charge. Object Y has a positive charge.
e. Object X has a neutral charge. Object Y has a negative charge.
ANS: E
2. A sweater will end up with a negative charge when it
a. loses electrons
d. gains protons
b. loses protons
e. loses neutrons
c. gains electrons
ANS: C
3. The leaves of a negatively charged electroscope spread apart even more when a charged rod is
brought near. The charge on the rod must be
a. negative
d. either positive or negative
b. positive
e. very large
c. neutral
ANS: A
4. An uncharged pith ball is
a. attracted by another uncharged pith ball
b. attracted by a negatively charged pith ball
c. attracted by a positively charged pith ball
d. both A and B
e. both B and C
ANS: E
5. How many types of electrostatic charges have been discovered?
a. 1
d. 4
b. 2
e. 5
c. 3
ANS: B
6. A neutral object is touched with a positively charged rod. The object will become
a. uncharged
d. positively charged
b. grounded
e. magnetic
c. negatively charged
ANS: D
7. The electrons in a metal sphere can be made to move by placing a charged object near it. This
process is called
a. friction
d. contact
b. induction
e. induction and grounding
c. grounding
ANS: B
8. To give an electrically neutral object a positive charge, you must
a. add electrons to it
b. remove electrons from it
c. add protons to it
d. remove protons from it
e. remove neutrons from it
ANS: B
9. It is difficult to induce a charge separation on an insulator because
a. Insulators hold electrons tightly.
b. Insulators absorb electrons easily.
c. Insulators do not absorb electrons easily.
d. Insulators give up electrons easily.
e. Insulators are positively charged.
ANS: A
10. A negatively charged rod is used to give a neutral object a positive charge. This method of charging
is called charging by
a. friction
d. induction and grounding
b. induction
e. contact and grounding
c. contact
ANS: D
11. Which particles in an atom have an electrical charge?
a. proton and neutron
d. neutron and nucleus
b. electron and proton
e. all of the above
c. neutron and electron
ANS: B
12. An example of a good conductor is
a. glass
b. copper
c. plastic
d. ebonite
e. wood
ANS: B
13. When silk and glass are rubbed together, the resulting charges will be
a. silk negative, glass positive
d. silk positive, glass negative
b. silk positive, glass positive
e. silk positive, glass neutral
c. silk negative, glass negative
ANS: A
14. A balloon with a negative charge will attract
a. another balloon with a negative charge
b. a balloon with a positive charge
c. a balloon with no charge
d. both A and C
e. both B and C
ANS: E
15. What substance will give wool the most positive charge when they are rubbed together?
a. paraffin wax
d. sulphur
b. silk
e. acetate
c. glass
ANS: D
16. A negatively charged rod is held below a neutral pith ball. Which statement best describes the
induced charge separation on the pith ball?
a. Negative charges will move to the top.
b. Negative charges will move to the bottom.
c. Positive charges will move to the top.
d. Positive charges will move to the bottom.
e. both A and D
ANS: A
17. Two materials are rubbed together. One becomes positively charged, the other becomes negatively
charged. This method is of charging is called charging by
a. friction
d. induction and grounding
b. induction
e. contact and grounding
c. contact
ANS: A
18. Object X attracts object Z.
Object Z repels object Y.
Object X attracts object Y.
Which of the following is the most likely explanation?
a. X is positive. Y is negative. Z is positive.
b. X is negative. Y is positive. Z is negative.
c. X is positive. Y is negative. Z is negative.
d. X is positive. Y is neutral. Z is negative.
e. X is negative. Y is neutral. Z is positive.
ANS: C
19. A negatively charged rod is held near a neutral pith ball. Before the objects touch, they will _____
one another. After they touch, they will _____ one another.
a. attract, repel
b. attract, attract
c. repel, repel
d. repel, attract
e. attract, not affect
ANS: A
20. Two touching metal spheres, X and Y, are placed on insulated stands. A negatively charged rod is
brought close to sphere X. Sphere Y is then removed. As a result,
a. Both spheres will have a negative charge.
b. Both spheres will have a positive charge.
c. Both spheres will have neutral charges.
d. Sphere X will have a negative charge. Sphere Y will have a positive charge.
e. Sphere X will have a positive charge. Sphere Y will have a negative charge.
ANS: E
21. It is possible to give a neutral object a positive charge by using the method of induction and
grounding with a negative rod. Consider the following steps:
(i) remove the ground
(ii) hold the rod near the object
(iii) ground the object
(iv) remove the rod
The steps, in the correct order, are
a. (iii), (iv), (ii), (i)
b. (iii), (ii), (iv), (i)
c. (iv), (iii), (ii), (i)
d. (iii), (ii), (i), (iv)
e. (ii), (iii), (iv), (i)
ANS: E
22. The Earth is a good ground because
a. It has a large mass.
b. It has a large charge.
c. It can absorb or give up extra electrons easily.
d. It attracts negative charges.
e. It attracts positive charges.
ANS: C
23. The electric force field around a positive charge may be represented by field lines that are
a. in circles around the charge
b. pointing away from the charge
c. pointing towards the charge
d. very close together
e. very far apart
ANS: B
24. Objects X, Y, and Z are at the corners of an equilateral triangle.
Objects X and Y are both fixed and both have equal, positive charges.
Object Z has a negative charge and is free to move.
X
Y
Z
If the top of the page is "north", in which direction will object Z move when it is released?
a. north
d. southwest
b. south
e. northeast
c. northwest
ANS: A
25. The diagram below shows two oppositely charged plates with an electric field between them. The
top plate is positively charged.
Which statement best describes this situation?
a. The direction of the electric field is upward.
b. The direction of the electric field is downward.
c. A positive charge would move upward in the field.
d. both A and C
e. both B and C
ANS: B
26. The electric field between two oppositely charged parallel plates is
a. strongest near the positive plate
b. strongest near the negative plate
c. strongest at the edges of the plates
d. equal everywhere between the plates
e. equal to the gravitational field between the plates
ANS: D
27. Two charged objects have a force of 0.040 N between them. If the distance between the objects is
reduced by half, the force between them will be
a. 0.010 N
d. 0.080 N
b. 0.020 N
e. 0.16 N
c. 0.040 N
ANS: E
28. Two identical charges are 36 mm apart. The electrostatic force between them is 1.3 N. What is the
size of each charge?
a. 1.9 × 10–13 C
d. 2.2 × 10 –6 C
–7
b. 4.3 × 10 C
e. 4.3 × 10–6 C
–12
c. 5.2 × 10 C
ANS: B
29. Which part of an atom has no electric charge?
a. neutron
d. nucleus
b. proton
e. both A and D
c. electron
ANS: A
30. Which part of an atom has the greatest mass?
a. electron
b. proton
c. neutron
ANS: D
d. nucleus
e. ion
31. How many electrons are in one coulomb of charge?
a. one
d. 6.25 × 1018
–19
b. 1.6 × 10
e. 6.25 × 10–18
c. 1.6 × 10 19
ANS: D
32. A charge of +1.3 µC is positioned 25 mm from a charge of +3.4 µC. The electrostatic force
between the charges is
a. 20 N
d. 1.6 N
b. 0.51 N
e. 64 N
c. 2.8 N
ANS: E
33. An atom with 9 protons, 10 electrons, and 10 neutrons has a net charge of
d. –1.6 × 10–18 C
a. –1.6 × 10–19 C
–19
e. –1.4 × 10–18 C
b. +1.6 × 10 C
c. +3.0 × 10–18 C
ANS: A
34. An electrically neutral pith ball gains 4.0 × 1023 electrons. Its charge is now
a. –2.5 × 104 C
d. –4.0 × 1023 C
–19
e. –1.4 × 10–18 C
b. +1.6 × 10 C
c. –6.4 × 10 4 C
ANS: C
35. How many electrons must be transferred to a neutral pith ball to give it a charge of –8.0 × 10–16 C?
d. 8.0 × 103
a. 8.0 × 1016
2
b. 5.0 × 10
e. 2.0 × 10 4
3
c. 5.0 × 10
ANS: C
36. The conclusion to Millikan's experiment was
a. The mass of an electron is very small.
b. The electron has a negative charge.
c. The mass to charge ratio of an electron is very small.
d. There is a basic electrical charge.
e. Charged objects may be balanced in an electrical field.
ANS: D
37. The SI unit for measuring charge is the
a. coulomb
b. ampere
c. volt
d. joule
e. watt
ANS: A
38. The SI unit for measuring current is the
a. coulomb
b. ampere
d. joule
e. watt
c. volt
ANS: B
39. One ampere is equal to one
a. N/s
b. C/s
c. s/C
d. V/C
e. V/s
ANS: B
40. If 15 C passes a point in a circuit in 5.0 s, the current in the circuit is
a. 75 A
d. 15 A
b. 3.0 A
e. 5.0 A
c. 0.33 A
ANS: B
41. If 2.75 × 1019 electrons pass a point in a circuit in 30 s, the current is
a. 52 A
d. 0.15 A
b. 9.2 A
e. 1.3 × 102 A
c. 4.4 A
ANS: D
42. A circuit has a current of 345 mA. How much charge passes a point in the circuit in 2.00 min?
a. 0.690 A
d. 20.7 A
b. 2.88 A
e. 41.4 A
c. 5.75 A
ANS: E
43. How many seconds will it take for 10.0 C of charge to pass through a 12.0 A circuit?
a. 120 s
d. 100 s
b. 0.120 s
e. 1.20 s
c. 0.833 s
ANS: C
44. A current of 20.0 mA runs in a circuit for 60.0 s. The charge transferred through the circuit in this
time is
d. 1.20 C
a. 1.20 × 103 C
b. 12.0 C
e. 0.333 C
c. 3.00 C
ANS: D
45. A 0.90-A circuit operated for 2.0 min. The total charge that passed through the source in this time
was
a. 12.8 C
d. 54 C
b. 0.45 C
e. 1.3 C
c. 1.1 × 102 C
ANS: C
46. A current of 12.0 A travelled through a circuit for 120 s. How many electrons passed through a
point in the circuit in this time?
d. 6.25 ×1012
a. 6.25 × 1017
b. 7.50 × 1020
e. 9.00 × 1021
19
c. 6.25 ×10
ANS: E
47. How long will it take 6.02 × 1023 electrons to pass through a point in a circuit if the current is 15.0
A?
a. 107 min
d. 150 min
b. 401 min
e. 345 min
c. 160 min
ANS: A
48. An electric current is generally comprised of
a. moving electrons
b. moving protons
c. moving negative ions
d. moving positive ions
e. a build-up of electric charge
ANS: A
49. The direction of electron flow in a circuit is
a. the same as the direction of the electric current
b. opposite to the direction of the electric current
c. from the positive terminal to the negative terminal of the source
d. both A and C
e. both B and C
ANS: B
50. In circuits, the term electron flow refers to
a. the direction of electron movement
b. the direction of the electric current
c. the rate of electron movement
d. the number of electrons
e. the total charge of the electrons
ANS: A
51. The SI unit for measuring energy is the
a. coulomb
b. ampere
c. volt
d. joule
e. watt
ANS: D
52. Which of these units is equal to one volt ?
a. 1 J/s
b. 1 J/C
c. 1 C/s
d. 1 C/J
e. 1 J/C·s
ANS: B
53. One volt is equal to one
a. J/C
b. C/s
d. A/s
e. C/A
c. V/A
ANS: A
54. The SI unit for measuring potential difference is the
a. coulomb
d. joule
b. ampere
e. watt
c. volt
ANS: C
55. Which of the following equations could be used to calculate the potential difference in a circuit, if
all variables are given?
a.
d. all of the above
b.
e. none of the above
c.
ANS: D
56. How much work is done on 10.0 C of charge to move it through a potential difference of 9.0 V in
10.0 s?
a. 9.0 J
d. 0.90 J
b. 90 J
e. 9.0 × 102 J
c. 1.1 J
ANS: B
57. When a current of 4.0 A travels in a conductor for 15 s at a voltage of 12 V, the energy transferred
is
a. 3.2 J
d. 0.31 J
b. 5.0 J
e. 7.2 × 102 J
c. 45 J
ANS: E
58. What is the potential difference of a battery if it provides 250 J of energy to a load in a 12.0 A
circuit in 2.3 s?
a. 9.1 V
d. 1.35 × 103 V
b. 6.9 × 103 V
e. 28 V
c. 48 V
ANS: A
59. How long will it take a 12.0-V battery to provide 2.00 × 103 J of energy to a load in a circuit with a
current of 0.30 A?
a. 50 s
d. 8.0 × 105 s
2
b. 5.6 × 10 s
e. 7.2 × 103 s
c. 1.8 × 103 s
ANS: B
60. A hair dryer operates from a 120-V source. If it has consumed 840 J of energy, what total charge
has passed through it?
d. 7.0 C
a. 1.0 × 105 C
2
b. 1.0 × 10 C
e. 0.14 C
c. 71 C
ANS: D
61. What current travels through a 120-V load if it uses 6.7 × 104 J in 3.0 min?
a. 3.7 A
d. 1.9 A
b. 5.6 A
e. 9.3 A
c. 3.1 A
ANS: C
62. An electron is accelerated through a potential difference of 675 V. How much energy has the
electron gained?
a. 1.08 ×10 –16 J
d. 6.75 × 102 J
21
b. 4.22 × 10 J
e. 2.21 × 106 J
c. 1.52 × 103 J
ANS: A
63. An electron has a mass of 9.11 × 10–31 kg. Originally, it is at rest. What speed will the electron have
after it is accelerated though a potential difference of 5.67 × 103 V?
a. 5.67 × 103 m/s
d. 3.16 × 107 m/s
b. 1.99 × 10 15 m/s
e. 3.00 × 108 m/s
7
c. 4.46 ×10 m/s
ANS: C
64. A positively charged particle had a mass of 6.70 ×10–27 kg. When it was accelerated through a
potential difference of 4.50 ×103 V, it achieved a speed of 6.56 × 105 m/s. What charge did the
particle have?
a. 1.44 × 10–15 C
d. 2.63 × 10–12 C
–19
b. 3.20 × 10 C
e. 6.40 × 10–19 C
–10
c. 5.66 × 10 C
ANS: B
65. Which device would most likely be classified as a load in an electrical circuit?
a. battery
d. cell
b. light bulb
e. none of the above
c. ammeter
ANS: B
66. Electrons possess potential energy. In a circuit, this potential energy may be transformed into
a. heat energy
d. any of the above
b. light energy
e. none of the above
c. motion
ANS: D
67. A circuit that has a break in it is called a(n)
a. short circuit
b. open circuit
c. closed circuit
d. parallel circuit
e. series circuit
ANS: B
68. In what type of circuit do electrons follow one path from the source, to a load, and back to the
source?
a. short circuit
d. series circuit
b. open circuit
e. connected circuit
c. parallel circuit
ANS: D
69. Kirchoff's law of voltages states that in a complete circuit,
a. The sum of voltage decreases equals the sum of voltage increases.
b. All paths in a circuit have equal voltage increases.
c. All electrons must travel the same path.
d. Electrons provide the necessary voltage for the circuit to operate.
e. The voltage is equal to the energy divided by the current.
ANS: A
70. If ten 9.0-V sources are connected in series, the total potential difference in the circuit will be
a. 0.90 V
d. 90 V
b. 1.1 V
e. 900 V
c. 9.0 V
ANS: D
71. If ten 9.0-V sources are connected in parallel to each other, the total potential difference in the
circuit will be
a. 0.90 V
d. 90 V
b. 1.1 V
e. 900 V
c. 9.0 V
ANS: C
72. In a circuit diagram, the symbol
a. resistor
b. light bulb
c. transformer
represents a(n)
d. switch
e. cell
ANS: A
73. In a circuit diagram, the symbol
a. cell
b. resistor
c. light bulb
represents a(n)
d. open switch
e. motor
ANS: C
74. In a circuit diagram, the symbol
a. a resistor
b. connecting wires
represents
d. an open switch
e. a closed switch
c. crossing wires
ANS: D
75.
In a circuit diagram , the symbol
a. ground
b. cell
c. resistor
represents a
d. lamp
e. fuse
ANS: B
76. This diagram that follows shows
a.
b.
c.
d.
e.
two lamps in series and two resistors in parallel
two lamps in series and two resistors in series
two lamps in parallel and two resistors in series
two lamps in parallel and two resistors in parallel
two lamps in series, one resistor in series, and one resistor in parallel
ANS: A
77. The value of V2 in the following circuit is
a. 40 V
b. 35 V
c. 25 V
ANS: C
78. The value of V2 in the following circuit is
d. 15 V
e. 10 V
a. 40 V
b. 30 V
c. 20 V
d. 15 V
e. 10 V
ANS: B
79. The value of I3 in the following circuit is
a. 10 A
b. 1.0 A
c. 2.5 A
d. 2.0 A
e. 20 A
ANS: A
80. Assuming the two resistors are identical, the value of I3 in the following circuit is
a. 10 A
b. 5.0 A
c. 7.5 A
d. 3.3 A
e. 1.0 A
ANS: B
81. The opposition to the flow of electrons in a circuit is called the
a. current
d. charge
b. potential difference
e. work
c. resistance
ANS: C
82. A circuit has a 120-V source. If the current is 10.0 A, the resistance in the circuit must be
a. 10.0 Ω
d. 1.20 × 102 Ω
–2
e. 12.0 Ω
b. 8.33 × 10 Ω
c. 1.20 × 103 Ω
ANS: E
83. The units used to measure charge, current, potential difference, and resistance are, respectively,
a. ampere, joule, ohm, volt
d. ampere, joule, volt, ohm
b. coulomb, ampere, volt, ohm
e. ampere, coulomb, ohm, volt
c. coulomb, ampere, ohm, watt
ANS: B
84. A 30.0-Ω resistor is connected in series to a 10.0-Ω resistor, and a 120-V supply. The current
through the 30.0-Ω resistor will be
a. 3.0 A
d. 15 A
b. 4.0 A
e. 0.33 A
c. 12 A
ANS: A
85. Which combination of resistors will produce the lowest total resistance in a circuit?
a. two 10-Ω resistors in series
d. three 10-Ω resistors in parallel
b. two 10-Ω resistors in parallel
e. four 10-Ω resistor in series
c. three 10-Ω resistors in series
ANS: D
86. Electrons travelling through a circuit may be compared to red blood cells moving through the
circulatory system. Which of the following best corresponds to the energy of the electrons?
a. red blood cells
d. lungs
b. veins and arteries
e. muscles and other tissues of the body
c. oxygen
ANS: C
87. The electrons travelling through a circuit may be compared to red blood cells moving through the
circulatory system. Which of the following best corresponds to the load in a circuit?
a. red blood cells
d. lungs
b. veins and arteries
e. muscles and other tissues of the body
c. oxygen
ANS: E
88. The electrons in a circuit may be compared to red blood cells moving through the circulatory
system. Which of the following best corresponds to the source in the circuit?
a. red blood cells
d. lungs
b. veins and arteries
e. muscles and other tissues of the body
c. oxygen
ANS: D
89. If the voltage in a circuit is doubled and the resistance is halved, the current will be
a. increased by a factor of 2
d. decreased by a factor of 4
b. decreased by a factor of 2
e. unaffected
c. increased by a factor of 4
ANS: C
90. A toy train set runs on a current of 250 mA. It has a resistance of 20 Ω. If the train ran for 60 min,
how much energy was did the train use?
a. 5.0 J
d. 4.5 × 103 J
b. 12 J
e. 5.0 × 102 J
c. 75 J
ANS: D
91. A 9.0-V battery produces a current of 8.0 A. What is the resistance in the circuit?
a. 1.1 Ω
d. 5.3 Ω
b. 0.89 Ω
e. 72 Ω
c. 0.75 Ω
ANS: A
92. A toy train set has a resistance of 20.0 Ω and uses a current of 250 mA. If it ran for
one hour, what is the power of the train?
a. 1.2 W
d. 2.2 × 102 W
b. 5.0 W
e. 4.5 × 103 W
c. 75 W
ANS: A
93. A fuse is rated at 15.0 A. If it is part of a 120-V circuit, which of the following could be connected
without blowing the fuse?
a. 200-W mixer
d. any of the above, individually
b. 800-W toaster
e. none of the above
c. 1000-W hair dryer
ANS: D
94. Three 10-Ω resistors are connected in series to one another in a 12.0-V circuit. What is the total
resistance in the circuit?
a. 3.3 Ω
d. 20 Ω
b. 10 Ω
e. 30 Ω
c. 12 Ω
ANS: E
95. Three 10.0-Ω resistors are connected in parallel to one another in a 12.0-A circuit. The total
resistance in the circuit is
a. 0.0333 Ω
d. 20.0 Ω
b. 0.300 Ω
e. 30.0 Ω
c. 3.33 Ω
ANS: C
96. In the circuit diagram below, the meters marked A, B, and C are connected correctly if
a.
b.
c.
d.
e.
A is an ammeter and B and C are voltmeters.
A is a voltmeter and B and C are ammeters.
A and C are voltmeters and B is an ammeter.
A and C are ammeters and B is a voltmeter.
A, B, and C are ammeters.
ANS: D
97. An electric toaster draws 5.00 A from a 120-V outlet for 3.0 h. If electricity costs 9.90¢ per kilowatt
hour, the total cost would be
a. $0.18
d. $18.00
b. $1.80
e. $20.20
c. $6.00
ANS: A
98. A 100-W light bulb burns for 25 h. How much energy has it used in this time?
a. 2.5 × 103 J
d. 9.0 × 10 6 J
3
b. 2.5 × 10 kW·h
e. 42 J
5
c. 1.5 × 10 J
ANS: D
99. 3.0 kW·h is equivalent to
a. 1.1 × 107 J
b. 1.8 × 106 J
c. 1.1 × 103 J
d. 2.2 × 102 J
e. 0.83 J
ANS: A
100. A load has a resistance of 20.0 Ω. If 40.0 C of charge pass through it in 10.0 s, what is the potential
difference of the load?
a. 5.00 V
d. 8.00 × 102 V
b. 20.0 V
e. 8.00 × 103 V
c. 80.0 V
ANS: C
COMPLETION
1. An atom will be electrically neutral when it has an equal number of protons and
____________________.
ANS: electrons
2. A device used to detect the presence of an electric charge is a(n) ____________________.
ANS: electroscope
3. A charged atom is called a(n) ____________________.
ANS: ion
4. When a neutral object gains electrons, it becomes ____________________ charged.
ANS: negatively
5. A chart that shows a substance's tendency to gain or lose electrons is called a(n)
_________________________.
ANS: electrostatic series
6. A substance that does not allow electrons to travel easily through it is called a(n)
____________________.
ANS: insulator
7. The method used to remove excess charge from an electrically charged object is called
____________________.
ANS: grounding
8. The charge of a single electron is called the _________________________.
ANS: elementary charge
9. The coulomb is a unit used to measure ____________________.
ANS: charge
10. The ____________________ in a circuit is measured with an ammeter.
ANS: current
11. In a(n) ____________________ current, the electrons periodically reverse direction.
ANS: alternating
12. The unit used to measure electric potential difference in a circuit is the ____________________.
ANS: volt
13. The current in a ____________________ circuit is the same at every load.
ANS: series
14. A ____________________ circuit is created if electrons can return to the source without passing
through a load.
ANS: short
15. The unit for measuring electric power is the ____________________.
ANS: watt
MATCHING
Match each definition with the letter of the correct term. A letter may be used more than once, or
not at all.
a. energy
d. power
b. resistance
e. potential difference
c. current
f. work
1.
2.
3.
4.
5.
6.
7.
rate at which charge travels in a circuit
opposition to the flow of electrons
rate at which energy is used
amount of work needed to move a unit charge between two points
measured in volts (V)
measured in amperes (A)
measured in watts (W)
1.
2.
3.
4.
5.
6.
7.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
C
B
D
E
E
C
D
Match the name of each scientist with the contribution that he made. A letter may be used more
than once, or not at all.
a. Andre Ampere
e. Charles Coulomb
b. Robert Millikan
f. Gustav Kirchoff
c. Georg Ohm
g. James Watt
d. Benjamin Franklin
h. Alessandro Volta
8. determined the elementary charge of an electron
9. identified and named positive and negative charges
10. determined that in an electric circuit, the total electric current going into a junction is equal to the
total current coming out of that junction
11. determined that the potential difference between any two points in a conductor varies directly as
the current between the two points
12. determined that the size of the force between two charged objects is directly proportional to the
product of the charges
13. determined that the size of the force between two charged objects is inversely proportional to the
square of the distance between the objects
14. determined that in a complete electrical circuit, the total decrease in electric potential is equal to the
total increase in electric potential
8.
9.
10.
11.
12.
13.
14.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
B
D
F
C
E
E
F
Match each term to the letter of the circuit type it best describes. A letter may be used more than
once, or not at all.
a. short circuit
d. parallel circuit
b. open circuit
e. closed circuit
c. series circuit
15.
16.
17.
18.
19.
20.
21.
electrons all travel the same path in the circuit
circuit with a break in it
electrons can return to a source without passing through a load
potential decrease is constant across all loads
electrons may travel different paths
current is constant in all parts of the circuit
circuit in which current cannot flow
15.
16.
17.
18.
19.
20.
21.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
C
B
A
D
D
C
B
Match each term with the letter of the circuit component that it best describes. A letter may be used
more than once, or not at all.
a. battery
d. ground
b. light bulb
e. wire
c. ammeter
f. voltmeter
22.
23.
24.
25.
26.
27.
28.
29.
30.
measures the electric current
provides energy to the circuit
provides a path for electrons to travel
removes energy from the circuit
removes charge from the circuit
measures electric potential difference across a load
converts electrical energy to light energy
classified as a source
classified as a load
22.
23.
24.
25.
ANS:
ANS:
ANS:
ANS:
C
A
E
B
26.
27.
28.
29.
30.
ANS:
ANS:
ANS:
ANS:
ANS:
D
F
B
A
B
Match each quantity with the letter of the device that is most suitable for measuring it. A letter may
be used more than once, or not at all.
a. voltmeter
e. compass
b. ammeter
f. stopwatch
c. electroscope
g. meter stick
d. balance
31.
32.
33.
34.
35.
presence of an electrical charge
amount of time a circuit is in operation
current flowing in a circuit
length of the wire used in a circuit
relative size of an electric charge
31.
32.
33.
34.
35.
ANS:
ANS:
ANS:
ANS:
ANS:
C
F
B
G
C
SHORT ANSWER
1. Why does a negative rod attract a neutral pith ball?
ANS:
The rod induces a charge separation in the pith ball, with the "positive" side next to the rod. These
opposite charges now attract one another.
2. In the winter, a spark may jump from your finger when you touch a light switch. Why does this
happen?
ANS:
As you move around, electrons may collect on you. When you touch a light switch, the electrons
jump to the switch, and are visible as a spark.
3. After you pull a sweater off over your head, your hair may stand up away from you head. Why does
this happen?
ANS:
As the sweater is removed, electrons are transferred to your hair. Since each strand is negatively
charged, the strands repel one another.
4. A negative object will attract a neutral object. What happens when these objects touch, and why?
ANS:
The objects will repel one another. Electrons will transfer from the negative object to the neutral
one, giving it a negative charge. Since both objects now have the same charge, they will repel one
another.
5. Two balloons attract one another. Does this mean that they have opposite charges?
ANS:
Not necessarily. There are two possibilities: either the balloons have opposite charges, or one
balloon is charged, and the other is neutral.
6. When silk is rubbed with wool, the silk becomes negatively charged. Explain how and why this
happens.
ANS:
Wool has a weaker hold on electrons than silk does. When the wool and silk are rubbed together,
electrons are transferred to the silk, giving it a negative charge.
7. If you are given two identical balloons, describe how you could give each of them an identical
quantity of charge.
ANS:
Charge one balloon, then touch it to the other balloon. The charge will distribute itself equally
between the two balloons.
8. A pith ball gains 2.8 × 107 electrons. What charge does it have now?
ANS:
4.5 × 10–12 C
9. What is the current in an electrical circuit if 20.0 C of charge pass a point in 45 s?
ANS:
0.44 A
10. Two light bulbs are connected to a source in an electrical circuit. Explain what would happen if one
of the bulbs burned out, if the bulbs were connected
(a) in parallel with one another
(b) in series with one another
ANS:
(a) The electrons in the circuit would still have a complete path to follow. The second bulb would
stay lit and would burn brighter, since the other load has been removed.
(b) The path of electrons in the circuit would be interrupted. The second bulb would go out as well.
11. What is the difference between the direction of electric current and the direction of electron flow in
a circuit?
ANS:
Electric current travels from the positive to the negative terminal of a source in a circuit. Electron
flow travels in the opposite direction.
12. How much energy can a 9.0-V battery provide to a load in a 3.0-A circuit in 1.0 min?
ANS:
1.6 × 103 J
13. State Kirchoff's laws for electric circuits.
ANS:
In any complete circuit, the total increase in electric potential is equal to the total decrease in
electric potential.
The total current entering a junction in an electric circuit must be equal to the total current leaving
the junction.
14. Draw a complete circuit containing three lamps that allows the other two lamps to stay lit if any one
of them is turned off.
ANS:
15. A particular circuit contains three lamps. Lamp A turns off when lamp B is turned off, but stays on
when lamp C is turned off. Lamp C stays on when either lamp A or lamp B is turned off. Draw a
circuit diagram to show the arrangement of the lamps.
ANS:
16. Explain why ammeters and voltmeters are connected differently to electrical circuits.
ANS:
An ammeter is connected in series to a load in a circuit since it must measure the flow of charge
through the load and, therefore, all the charge must flow through it. A voltmeter is connected in
parallel to a load since it must compare the electric potential of the charge before and after it passes
through the load.
17. Why are loads in a household circuit connected in parallel to the circuit, rather than in series?
ANS:
Connecting in parallel allows each appliance to be turned on or off without interrupting the flow of
current to the others. In a series circuit, if one appliance is turned off, the current would have no
path to the others.
18. Draw a circuit containing a source, two bulbs in series, and one resistor in parallel.
ANS:
Other variations are possible.
19. Explain how a 15-A fuse works to protect a circuit.
ANS:
If too many loads are connected to a circuit, the current increases and the wires may overheat,
producing a potentially dangerous situation. The fuse contains a wire or element which melts or
"blows" when the current exceeds 15 A, thus, breaking the circuit.
20. This graph shows the relationship between measurements of V and I for two different loads. Which
load has the greater resistance? Explain.
ANS:
Load 1 has the greater resistance. The slope of the line for load 1 is greater, indicating a larger
potential difference for the same current.
21. An increase in temperature causes an increase in the resistance of most materials. Describe or
sketch a graph showing the relationship between potential difference and time for a resistor that
heats up as the circuit operates.
ANS:
The graph of V versus t (V dependent) will be a line with a positive slope. Resistance increases as
time goes by, causing the current to decrease and the potential difference to increase so long as I is
kept constant.
22. An electrical circuit contains two loads connected in series to one another. Which would cause the
greater increase in the total resistance of the circuit: adding another load in series or adding the
same load in parallel?
ANS:
Adding the new load in series. Total resistance in a series circuit is the sum of the separate
resistances. Adding resistance in parallel actually decreases the total resistance of the circuit.
23. Examine the following circuit diagram and state the value of
(a) V2 (b) I2 (c) R1 (d) R2 (e) RT
ANS:
(a)
(b)
(c)
(d)
(e)
30.0 V
5.0 A
1.0 Ω
6.0 Ω
4.0 Ω
24. A stove has a power rating of 8000 W. Yet, measurements of the energy transferred to water being
heated on the stove showed that the power output was only 4000 W. Explain this observation.
ANS:
The power rating of the stove indicates the power used by the stove. However, not all of the energy
transferred during its operation goes into the water—some is lost to the air and other elements of
the stove.
25. What is the power of an iron that uses 1.4 × 106 J in 30 min?
ANS:
7.8 × 102 W
26. What is the value of one kilowatt hour, in joules?
ANS:
3.6 × 106 J
27. How long could you leave 60-W light bulb on, for the same cost as leaving a 100-W light bulb on
for 24 h?
ANS:
40 h
PROBLEM
1. A neutral pith ball was given a charge of –3.0 × 10–7 C. Calculate the number of electrons
transferred to the pith ball.
ANS:
Q = –3.0 × 10–7 C
e = –1.6 × 10–19 C
N=?
The pith ball gained 1.9 × 1012 electrons.
2. In a circuit, 150 C of charge passed the source in 45 s. What was the current in the circuit?
ANS:
Q = 150 C
∆t = 45 s
I=?
The current was 3.3 A.
3. A circuit has a current of 10.0 A. Calculate the number of electrons that pass a point in the circuit in
1 s.
ANS:
I = 10.0 A
∆t = 1.00 s
e = 1.60 × 10–19 C
N=?
6.25 × 1019 electrons pass the point in 1 s.
4. An oven operates on a 15.0-A current from a 120-V source. How much energy will it consume in
3.0 h of operation?
ANS:
I = 15.0 A
V = 120 V
∆t = 3.0 h = 1.08 × 104 s
E=?
The oven will use 1.9 × 107 J of energy.
5. A radio operated from a 120 V source. If it consumed 840 J of energy, what total charge passed
through it?
ANS:
V = 120 V
E = 840 J
Q=?
A total charge of 7.0 C has passed through the radio.
6. What is the potential difference when 3.45 ×103 J of work is done on a 12.0-C charge?
ANS:
Q = 12.0 A
E = 3.45 × 103 J
V=?
The potential difference is 2.88 × 102 V.
7. Immersion heaters are placed into liquids to warm them up. If 250 g of water in a cup is heated from
15ºC to 97 ºC in 5.0 m, what current must the heater draw from a 12.0-V battery?
(cw = 4.184 J/g·ºC)
ANS:
m = 250 g
T = 97ºC – 15ºC = 82ºC
cw = 4.184 J/g·ºC
V = 12.0 V
∆t = 5.0 min = 3.00 × 102 s
I=?
(Note: Qw refers to the heat transferred to the water.)
The current is 24 A.
8. A flashlight operates with a 6.0-V battery, using a current of 1.75 A. How much charge passes
through the bulb in 15.0 min?
ANS:
V = 120 V
I = 1.75 A
t = 15.0 min = 9.00 × 102 s
Q=?
A total charge of 1.58 × 103 C passed through the bulb.
9. A 10.0-A circuit has a potential difference of 120 V. Calculate the resistance in the circuit.
ANS:
I = 10.0 A
V = 120 V
R=?
The resistance is 12.0 Ω.
10. A 120-V circuit contains a 10.0-Ω resistor, a 20.0-Ω resistor, and a 30.0-Ω resistor in series. What
is the current in the circuit?
ANS:
V = 120 V
R1 = 10.0 Ω
R2 = 20.0 Ω
R3 = 30.0 Ω
IT = ?
The current is 2.0 A
11. A 125-V circuit contains a 10.0-Ω resistor. What resistance must be added in series for the circuit
to have a current of 5.00 A?
ANS:
VT = 125 V
IT = 5.00 A
R1 = 10.0 Ω
R2 = ?
A resistance of 14.0 Ω must be added.
12. What is the drop in potential difference produced by a 15.0-Ω heating element in a 12.0 A circuit?
ANS:
R = 15.0 Ω
I = 12.0 A
V=?
The voltage drop is 180 V.
13. A 120-V circuit contains a load with a resistance of 15.0 Ω. What resistance must be added in
parallel with the load to result in a current of 12.0 A in the circuit?
ANS:
VT = 120 V
R1 = 10.0 Ω
IT = 15.0 A
R2 = ?
An additional 30.0 Ω of resistance is needed.
14. A 120-V circuit contains a 20.0-Ω resistor and a 40.0-Ω resistor connected in parallel.
(a) What is the current through the 40.0-Ω resistor?
(b) What is the total current in the circuit?
ANS:
(a) VT = 120 V
R1 = 20.0 Ω
R2 = 40.0 Ω
I2 = ?
The current through the 40.0-Ω
Ω resistor is 3.0 A.
(b) VT = 120 V
R1 = 20.0 Ω
R2 = 40.0 Ω
I2 = 3.0 A
IT = ?
The current in the circuit is 9.0 A.
15. A kettle uses 5.66 × 102 W from a 120-V circuit. How much current does it use?
ANS:
P = 5.66 × 102 W
V = 120 V
I=?
The kettle uses 4.7 A of current.
16. How much time, in hours, will it take a 855-W toaster to use 3.39 × 107 J of energy?
ANS:
P = 855 W
∆E = 3.39 × 109 J
∆t = ?
It will take 11.0 h.
17. An 100-W light bulb burned for 25 h. How much energy did it use during this time?
ANS:
P = 100 W
∆t = 25 h = 9.0 × 104 s
∆E = ?
The light bulb used 9.0 × 106 J of energy.
18. An iron uses 975 W from a 120-V circuit. How much current does it draw?
ANS:
P = 975 W
V = 120 V
I=?
It draws 8.1 A of current.
19. What is the resistance of a 1500-W heater if it uses a current of 10.0 A?
ANS:
P = 1500 W
I = 10.0 A
R=?
The resistance is 15 Ω.
20. A stove has a power rating of 8.00 ×103 W. The energy transferred to a pan of water during a
two-minute test period was 3.25 × 105 J. What was the efficiency of the stove in this trial?
ANS:
Ps = 8.00 × 103 W
∆t = 120 s
∆Es = 3.25 ×103 J
efficiency = ?
The efficiency of the stove was 33.9%.
21. A string of lights has six lamps connected in parallel. If each lamp has a resistance of
120 Ω and the string is connected to a 120-V supply, calculate
(a) the total resistance in the circuit
(b) the current in the circuit
ANS:
(a) Rn = 120 Ω
VT = 120 V
The total resistance is 20 Ω.
(b) R = 20 Ω
V = 120 V
I=?
The current is 6.0 A.
22. A string of lights has ten lamps connected in series. If each lamp has a resistance of 25 Ω and the
string is connected to a 120-V source, calculate
(a) the total resistance in the string
(b) the current drawn by the string
ANS:
(a) Rn = 25 Ω
V = 120 V
RT = ?
The total resistance is 2.5 × 102 Ω.
(b) R = 250 Ω
V = 120 V
I=?
The current is 0.48 A.
23. How many 100-W light bulbs connected to a 120-V supply can be turned on at the same time
without blowing a 15.0-A fuse?
ANS:
Pn = 100 W
V = 120 V
I = 15.0 A
number of bulbs = ?
Eighteen bulbs can be turned on at once.
24. What is the resistance of a 650-W microwave oven that has a potential difference of 110 V?
ANS:
P = 650 W
V = 110 V
R=?
The resistance is 19 Ω.
25. A 725-W hair dryer operates on a current of 6.5 A. Calculate
(a) the potential difference
(b) the resistance
ANS:
(a) P = 725 W
I = 6.5 A
V=?
The potential difference is 1.1 × 102 V.
(b) P = 725 W
I = 6.5 A
R=?
The resistance is 17 Ω.
ESSAY
1. Lightning rods are made of metal and they are connected to a conducting wire that leads to the
ground. Explain how a lightning rod can protect a building in case of a lightning strike.
ANS:
If lightning strikes an unprotected building, the energy it contains can cause damage to electrical
components and start fires. A lightning rod is attached to the roof of a building. If lightning strikes,
it will most likely strike the rod since this will be the highest point on the building. The conducting
wire carries the charge to the ground, where it is absorbed safely by the Earth. This way, the charge
does not pass through the building, but around it.
2. Millikan performed an experiment to determine the elementary charge. Describe his method and
his analysis of the results.
ANS:
-Millikan used two parallel charged plates with a uniform electric field between them.
-He sprayed charged oil droplets between the plates.
-He adjusted the voltage between the plates until a droplet was balanced in the field.
-Since the gravitational force on the droplet was balanced by the electrostatic force, he was able to
calculate the actual charge on the droplet.
-By comparing the charges of thousands of droplets, Millikan was able to arrive at the lowest
common factor—the elementary charge.
3. Coulomb devised an experimental method to measure the size of the electric force between two
charged objects. Describe his method and his findings.
ANS:
Coulomb suspended a light bar with a charged sphere on one end, so that the bar was free to pivot
horizontally around its center. When a charged object was brought near the sphere, attractive
and/or repulsive forces caused the bar to rotate. Coulomb used the amount of rotation to determine
the size of the force. By changing the distance between the sphere and the object and changing the
magnitude of the charges, he was able to determine that the force between two charged objects is
directly proportional to the product of the size of their charges and inversely proportional to the
square of the distance between them.
4. Discuss the similarities and differences between gravitational forces and electrical forces.
ANS:
Similarities
-act at a distance
-occur between a minimum of two objects
-are inversely proportional to the square of the distance between the objects
-electrical forces are directly proportional to sizes of the charges; gravitational forces are directly
proportional to the masses of the objects
Differences
-gravitational forces are forces of attraction, while electric forces may be attractions or repulsions
-gravitational forces are much weaker than electric forces
5. Explain how and why the convention of electron flow was introduced as a counter to the original
and more common direction of electric current.
ANS:
-Franklin proposed "positive" charges (excess of charges) and "negative" charges (deficit of
charges)
-assumed that electric current flowed from excess to deficit of charge (positive to negative)
-when electron was discovered, an excess of charge was now negative
-electron flow moves from excess of electrons to deficit of electrons (negative to positive), which is
opposite to direction of electric current
6. In an analogy, compare the movement of electrons in a circuit with the movement of skiers on a ski
hill. Include comparisons to current, load, and source.
ANS:
-circuit path: skiers move up the hill, then down, and back to lift
-source: ski lift (gravitational potential energy provided to skiers)
-load: downhill part of ski run (skiers lose energy)
-current: rate of skiers moving around path on ski hill