Electricity PracTest ANSWERS
1) Sphere A carries a net charge and sphere B is neutral. They are placed near each other on an insulated
table. Which statement best describes the electrostatic force between them?
A) There is no force between them since one is neutral.
B) There is a force of repulsion between them.
C) There is a force of attraction between them.
D) The force is attractive if A is charged positively and repulsive if A is charged negatively.
Answer: C
Diff: 2 Page Ref: Sec. 16.5-16.6
2) Two charges are separated by a distance d and exert mutual attractive forces of F on each other. If the
charges are separated by a distance of d/3, what are the new mutual forces?
A) F/9
B) F/3
C) 3F
D) 9F
Answer: D
Diff: 2 Page Ref: Sec. 16.5-16.6
3) Is it possible to have a zero electric field value between a negative and positive charge along the line
joining the two charges?
A) Yes, if the two charges are equal in magnitude.
B) Yes, regardless of the magnitude of the two charges.
C) No, a zero electric field cannot exist between the two charges.
D) cannot be determined without knowing the separation between the two charges
Answer: C
Diff: 1 Page Ref: Sec. 16.7-16.8
4) The electric field shown
A) increases to the right.
B) increases down.
C) decreases to the right.
D) decreases down.
E) is uniform.
Answer: A
Diff: 2 Page Ref: Sec. 16.7-16.8
5) If a solid metal sphere and a hollow metal sphere of equal diameters are each given the same charge, the
electric field (E) midway between the center and the surface is
A) greater for the solid sphere than for the hollow sphere.
B) greater for the hollow sphere than for the solid sphere.
C) zero for both.
D) equal in magnitude for both, but one is opposite in direction from the other.
Answer: C
Diff: 2 Page Ref: Sec. 16.9
6) A piece of plastic has a net charge of +2.00 mC. How many more protons than electrons does this piece
of plastic have?
A) 1.25 ˛ 1013
B) 1.25 ˛ 1019
C) 2.50 ˛ 1013
D) 2.50 ˛ 1019
Answer: A
Diff: 1 Page Ref: Sec. 16.5-16.6
7) A 1.0-C charge is 15 m from a second charge, and the force between them is 1.0 N. What is the
magnitude of the second charge?
A) 25 C
B) 1.0 C
C) 0.025 C
D) 25 nC
Answer: D
Diff: 1 Page Ref: Sec. 16.5-16.5
8) Two point charges, initially 2.0 cm apart, experience a 1.0-N force. If they are moved to a new
separation of 8.0 cm, what is the electric force between them?
A) 4.0 N
B) 16 N
C) 1/4 N
D) 1/16 N
Answer: D
Diff: 2 Page Ref: Sec. 16.5-16.6
9) Three point charges are placed on the x-axis. A charge of +2.0 mC is placed at the origin, -2.0 m C to the
right at x = 50 cm, and +4.0 mC at the 100 cm mark. What are the magnitude and direction of the
electrostatic force which acts on the charge at the origin?
A) 0.072 N right
B) 0.072 N left
C) 0.14 N right
D) 0.14 N left
Answer: A
Diff: 2 Page Ref: Sec. 16.5-16.6
10) A point charge of +Q is placed at the center of a square, and a second point charge of -Q is placed at the
upper-left corner. It is observed that an electrostatic force of 2.0 N acts on the positive charge at the center.
What is the magnitude of the force that acts on the center charge if a third charge of -Q is placed at the
lower-left corner?
A) zero
B) 2.8 N
C) 4.0 N
D) 5.3 N
Answer: B
Diff: 2 Page Ref: Sec. 16.5-16.6
11) A 5.0-C charge is 10 m from a small test charge. What is the direction of the electric field?
A) toward the 5.0 C
B) away from the 5.0 C
C) perpendicular to a line joining the charges
D) none of the given answers
Answer: B
Diff: 2 Page Ref: Sec. 16.7-16.8
12) Two point charges each have a value of 3.0 C and are separated by a distance of 4.0 m. What is the
electric field at a point midway between the two charges?
A) zero
B) 9.0 ˛ 107 N/C
C) 18 ˛ 107 N/C
D) 4.5 ˛ 107 N/C
Answer: A
Diff: 2 Page Ref: Sec. 16.7-16.8
13) Two point charges of +3.0 mC and -7.0 mC are placed at x = 0 and x = 0.20 m. What is the magnitude
of the electric field at the point midway between them?
A) 1.8 ˛ 106 N/C
B) 3.6 ˛ 106 N/C
C) 4.5 ˛ 106 N/C
D) 9.0 ˛ 106 N/C
Answer: D
Diff: 2 Page Ref: Sec. 16.7-16.8
14) A Styrofoam ball of mass 0.120 g is placed in an electric field of 6000 N/C pointing downward. What
charge must be placed on the ball for it to be suspended?
A) -16.0 nC
B) -57.2 nC
C) -125 nC
D) -196 nC
Answer: D
Diff: 2 Page Ref: Sec. 16.7-16.8
15) It takes 50 J of energy to move 10 C of charge from point A to point B. What is the potential difference
between points A and B?
A) 500 V
B) 50 V
C) 5.0 V
D) 0.50 V
Answer: C
Diff: 1 Page Ref: Sec. 17.1-17.4
16) Starting from rest, a proton falls through a potential difference of 1200 V. What speed does it acquire?
A) 1.2 x 105 m/s
B) 2.4 x 105 m/s
C) 3.6 x 105 m/s
D) 4.8 x 105 m/s
Answer: D
Diff: 2 Page Ref: Sec. 17.1-17.4
17) A uniform electric field, with a magnitude of 500 V/m, is directed parallel to the +x axis. If the
potential at x = 5.0 m is 2500 V, what is the potential at x = 2.0 m?
A) 500 V
B) 1000 V
C) 2000 V
D) 4000 V
Answer: D
Diff: 2 Page Ref: Sec. 17.1-17.4
18) Consider a uniform electric field of 50 N/C directed toward the east. If the voltage measured relative to
ground at a given point in the field is 80 V, what is the voltage at a point 1.0 m directly south of that point?
A) zero
B) 30 V
C) 50 V
D) 80 V
Answer: D
Diff: 2 Page Ref: Sec. 17.1-17.4
19) What charge appears on the plates of a 2.0-mF capacitor when it is charged to 100 V?
A) 50 mC
B) 100 mC
C) 150 mC
D) 200 mC
Answer: D
Diff: 1 Page Ref: Sec. 17.7-17.9
20) A parallel-plate capacitor is filled with air, and the plates are separated by 0.050 mm. If the capacitance
is 17.3 pF, what is the plate area?
A) 4.9 x 19-5 m2
B) 9.8 x 10-5 m2
C) 2.4 x 10-4 m2
D) 4.8 x 10-4 m2
Answer: B
Diff: 2 Page Ref: Sec. 17.7-17.9
21) A charge of 60 mC is placed on a 15 mF capacitor. How much energy is stored in the capacitor?
A) 120 J
B) 4.0 J
C) 240 mJ
D) 120 mJ
Answer: D
Diff: 1 Page Ref: Sec. 17.7-17.9
22) If a 10-mF capacitor is charged so that it stores 2.0 x 10-3 J of energy, what is the voltage across it?
A) 5.0 V
B) 10 V
C) 15 V
D) 20 V
Answer: D
Diff: 2 Page Ref: Sec. 17.7-17.9
23) A parallel-plate capacitor consists of plates of area 1.5 x 10-4 m2 and separated by 1.0 mm. The
capacitor is connected to a 12-V battery. What is the charge on the plates?
A) 1.6 x 10-11 C
B) 3.2 x 10-11 C
C) 1.6 x 10-14 C
D) 3.2 x 10-14 C
Answer: A
Diff: 2 Page Ref: Sec. 17.7-17.9
24) A parallel-plate capacitor is constructed with plate area of 0.40 m2 and a plate separation of 0.10 mm.
How much charge is stored on it when it is charged to a potential difference of 12 V?
A) 0.21 mC
B) 0.42 mC
C) 0.63 mC
D) 0.84 mC
Answer: B
Diff: 2 Page Ref: Sec. 17.7-17.9