Electrostatics Practice and Review
Multiple Choice
Identify the choice that best completes the statement or answers the question.
____
1. An attracting force occurs between two charged objects when the charges are of
a. unlike signs.
c. equal magnitude.
b. like signs.
d. unequal magnitude.
____
2. When a glass rod is rubbed with silk and becomes positively charged,
a. electrons are removed from the rod.
c. protons are added to the silk.
b. protons are removed from the silk.
d. the silk remains neutral.
____
3. Which statement is the most correct regarding electric insulators?
a. Charges within electric insulators do not readily move.
b. Electric insulators have high tensile strength.
c. Electric charges move freely in electric insulators.
d. Electric insulators are good heat conductors.
____
4. The process of charging a conductor by bringing it near another charged object and then grounding the
conductor is called
a. contact charging.
c. polarization
b. induction.
d. neutralization.
____
5. The figure shown above demonstrates charging by
a. grounding.
c. polarization.
b. induction.
d. contact.
____
6. Both insulators and conductors can be charged by
a. grounding.
c. polarization.
b. induction.
d. contact.
____
7. If the charge is tripled for two identical charges maintained at a constant separation, the electric force between
them will be changed by what factor?
a.
c. 9
b.
____
8. Two point charges, initially 1 cm apart, are moved to a distance of 3 cm apart. By what factor do the resulting
electric and gravitational forces between them change?
a. 9
c.
b. 3
____
d. 18
d.
9. Which is the most correct statement regarding the drawing of electric field lines?
a. Electric field lines always connect from one charge to another.
b. Electric field lines always form closed loops.
c. Electric field lines can start on a charge of either polarity.
d. Electric field lines never cross each other.
____ 10. Where is the electric field of an isolated, uniformly charged, hollow metallic sphere greatest?
a. at the center of the sphere
c. at infinity
b. at the sphere’s inner surface
d. at the sphere’s outer surface
____ 11. For a conductor that is in electrostatic equilibrium, any excess charge
a. flows to the ground.
b. resides entirely on the conductor’s outer surface.
c. resides entirely on the conductor’s interior.
d. resides entirely in the center of the conductor.
____ 12. Electrical forces between charges are strongest when the charges are
a. far apart.
b. close together.
c. The electrical force is constant everywhere.
____ 13. A difference between electrical forces and gravitational forces is that electrical forces include
a. infinite range.
b. repulsive interactions.
c. the inverse square law.
d. separation distance.
e. none of the above
____ 14. To be safe in the unlikely case of a lightning strike, it is best to be inside a building framed with
a. steel.
b. wood.
c. either A or B.
____ 15. A negatively charged rod is brought near a metal can that rests on a wood table. You touch the opposite side
of the can momentarily with your finger. The can is then
a. positively charged.
b. negatively charged.
c. charged the same as it was.
d. uncharged.
____ 16. The reason a charged balloon will stick to a wall is that
a. induced opposite charges in the wall are closer than other wall charges.
b. the rubber of the balloon simply sticks to walls.
c. electrons transfer back and forth between the wall and the balloon.
d. the charge is slightly sticky and acts like glue.
e. none of the above
____ 17. A positive charge and a negative charge held near each other are released. As they move, the force on each
particle
a. increases.
b. stays the same.
c. decreases.
Short Answer
18. When an object loses electrons, it acquires a(n) ____________________ electric charge.
19. When a conductor is given a negative charge, the charge will move on the conductor until the repulsive forces
between the free electrons are in ____________________.
20. A large conducting sphere with a large net negative charge is brought close to but does not touch a small,
uncharged conducting small sphere. A wire connects the sphere to a metal plate. If the wire to the metal plate
is disconnected and the large sphere is removed, what are the charges on the smaller sphere and on the metal
plate?
21. A negatively charged rubber rod is brought near a neutral, conductive sphere that has no charge. As a result,
the part of the sphere closest to the rod becomes positively charged. Explain how this positive charge occurs.
22. Any force between two objects that are not touching is called a(n) ____________________ force.
23. Draw the lines of force representing the electric field surrounding two objects that have equal magnitude
charges of opposite polarity.
24. Draw the lines of force representing the electric field around an electron.
25. The space around a charged object contains an electric ____________________.
Problem
26. State Coulomb's law. Explain what each symbol in it stands for, and use it to find the force between a 0.002-C
charge and a 0.006-C charge that are 10 meters apart.
27. What is the electric force between an electron and a proton that are separated by a distance of 3.4  10
Is the force attractive or repulsive? (e 1.60 10 C, k
8.99  10 N•m /C )
m?
28. A thundercloud has an electric charge of 48.8 C near the top of the cloud and –41.7 C near the bottom of the
cloud. The magnitude of the electric force between these two charges is 7.98  10 N. What is the average
separation between these charges? (k
8.99  10 N•m /C )
29. Charge A and charge B are 1.3 m apart. Charge A is 1.40 C, and charge B is 2.40 C. Charge C, which is 2.40
C, is located between them and is in electrostatic equilibrium. How far from charge A is charge C?
30. Two equal positive charges, both q 2.0  10 C, interact with a third charge, Q 4.0 10 C, as shown in
the figure above. What is the magnitude of the electric force on Q? (k
8.99  10 Nm /C )
31. An electric field of 2715 N/C is produced by a charge of 4.68  10
distance to the charge? (
C. For this field strength, what is the
Electrostatics Practice and Review
Answer Section
MULTIPLE CHOICE
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
KEY:
ANS:
KEY:
ANS:
KEY:
ANS:
KEY:
ANS:
KEY:
ANS:
KEY:
A
PTS: 1
A
PTS: 1
A
PTS: 1
B
PTS: 1
C
PTS: 1
D
PTS: 1
C
PTS: 1
D
PTS: 1
D
PTS: 1
D
PTS: 1
B
PTS: 1
B
PTS: 1
force | electric
B
PTS: 1
electric | gravitation
A
PTS: 1
lightning | conductor
A
PTS: 1
induction | positive | negative
A
PTS: 1
charge | balloon
A
PTS: 1
force | charge
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
DIF:
BLM:
DIF:
BLM:
DIF:
BLM:
DIF:
BLM:
DIF:
BLM:
DIF:
BLM:
I
OBJ:
I
OBJ:
I
OBJ:
I
OBJ:
I
OBJ:
I
OBJ:
II
OBJ:
II
OBJ:
II
OBJ:
I
OBJ:
I
OBJ:
L2
OBJ:
comprehension
L2
OBJ:
comprehension
L2
OBJ:
comprehension
L2
OBJ:
analysis
L2
OBJ:
comprehension
L2
OBJ:
analysis
16-1.1
16-1.1
16-1.2
16-1.3
16-1.3
16-1.3
16-2.2
16-2.2
16-3.2
16-3.3
16-3.3
32.3 Coulomb's Law
32.3 Coulomb's Law
32.4 Conductors and Insulators
32.6 Charging by Induction
32.7 Charge Polarization
32.3 Coulomb's Law
SHORT ANSWER
18. ANS:
positive
PTS: 1
19. ANS:
equilibrium
DIF: I
OBJ: 16-1.1
PTS: 1
DIF: I
OBJ: 16-1.2
20. ANS:
positive on the sphere and negative on the plate
PTS: 1
DIF: I
OBJ: 16-1.3
21. ANS:
The negatively charged rod repels electrons from the part of the sphere nearest the rod. As a result, this part
becomes deficient in electrons, thus acquiring a positive charge.
PTS: 1
22. ANS:
field
DIF: II
OBJ: 16-1.3
PTS: 1
23. ANS:
DIF: I
OBJ: 16-2.2
PTS: 1
24. ANS:
DIF: I
OBJ: 16-3.2
PTS: 1
25. ANS:
field
DIF: I
OBJ: 16-3.2
PTS: 1
DIF: I
OBJ: 16-3.1
PROBLEM
26. ANS:
The force between two charges depends directly on the magnitude of each charge and inversely on the square
of the distance between the charges.
, where k (
) is the proportionality constant, q1 and q2
are the magnitudes of the charges, d is the distance between the charges, and F is the force between the
charges.
PTS: 1
DIF: L2
KEY: Coulomb | distance | force
27. ANS:
2.0  10 N; attractive
Given
q –e
q
OBJ: 32.3 Coulomb's Law
BLM: application
C
+e
C
r
m
N•m /C
Solution
The force is negative, and therefore attractive.
PTS: 1
28. ANS:
1.51
m
DIF: IIIA
OBJ: 16-2.1
1.51 km
Given
q
48.8 C
q
C
F
7.98
N
N•m /C
Solution
Rearrange to solve for r. Use the magnitude of each charge to determine the separation distance.
r
r
1.51
m
1.51 km
PTS: 1
29. ANS:
0.56 m
DIF: IIIB
OBJ: 16-2.1
DIF: IIIC
OBJ: 16-2.3
Given
r
1.3 m
r
d
r
1.3 m – d
q
C
q
C
q
C
N
Solution
PTS: 1
30. ANS:
0.46 N
Given
q
C
q
C
Q
C
0.50 m
 tan (0.30 m/0.40 m) 37°
k
8.99
Solution
For
:
For
:
(
)(cos 37°)
–(
)(sin 37°)
(
)(cos 37°)
(
)(sin 37°)
(0.29 N)(cos 37°)
–(0.29 N)(sin 37°)
(0.29 N)(cos 37°)
(0.29 N)(sin 37°)
+
0.23 N + 0.23 N
0.46 N
+
–0.17 N + 0.17 N
0N
0.23 N
–0.17 N
0.23 N
0.17 N
0.46 N
PTS: 1
31. ANS:
1.24  10
DIF: IIIC
OBJ: 16-2.3
m
Given
E 2715 N/C
q
C
Solution
E=
Rearrange to solve for r.
m
PTS: 1
DIF: IIIB
OBJ: 16-3.1