1
EL FORCE and EL FIELD HW-PRACTICE 2013
1.A difference between electrical forces and gravitational forces is that electrical forces include
a. separation distance.
b. repulsive interactions.
c. the inverse square law.
d. infinite range.
e. none of the above
2. In a good insulator, electrons are usually
a. free to move around.
b. free to move around after an impurity has been added.
c. semi-free to move around.
d. tightly bound in place.
e. not moving at all.
3. A conducting sphere has a net charge of 4.8  1017 C. What is the approximate number of excess electrons on the sphere?
a. 100
b. 200
c. 300
d. 400
e. 500
4. When a hard rubber rod is given a negative charge by rubbing it with wool:
a. positive charges are transferred from rod to wool
b. negative charges are transferred from rod to wool
c. positive charges are transferred from wool to rod
d. negative charges are transferred from wool to rod
e. negative charges are created and stored on the rod
5. An electrical insulator is a material:
a. containing no electrons
b. through which electrons do not flow easily
c. which has more electrons than protons on its surface
d. cannot be a pure chemical element
e. must be a crystal
6. A conductor is distinguished from an insulator with the same number of atoms by the number of:
a. nearly free atoms
b. electrons
c. nearly free electrons
d. protons
e. molecules
7. Two uncharged, conducting spheres, A and B, are held at rest on insulating stands and are
in contact. A positively charged rod is brought near sphere A as suggested in the figure.
While the rod is in place, the two spheres are separated.
How will the spheres be charged, if at all?
a.
b.
c.
d.
e.
Sphere A
positive
positive
zero
negative
negative
Sphere B
positive
negative
zero
positive
negative
2
8. The diagram shows two pairs of heavily charged plastic cubes. Cubes 1 and 2 attract each other and so do cubes 1 and 3.
Which of the following illustrates the forces of 2 on 3 and 3 on 2?
9. At what separation will two charges, each of magnitude 6 C, exert a force of 1.4 N on each other?
a. 0.48 m
b. 5.1  106 m
c. 0.23 m
d. 40 m
e. 2.0 m
10. Two positive point charges Q and 2Q are separated by a distance R. If the charge Q experiences a force of magnitude F when the
separation is R, what is the magnitude of the force on the charge 2Q when the separation is 2R ?
a. F
b. F/4
c. 4F
d. 2F
e. F/2
11. Two small charged objects repel each other with a force F when separated by a distance d. If the charge on each object is reduced to
one-fourth of its original value and the distance between them is reduced to d/2 the force becomes:
a. F/16
b. F/8
c. F/4
d. F/2
e. F
12. A 5.0-C charge is 10 m from a –2.0-C charge. The electrostatic force is on the positive charge is:
a. 9.0 × 108 N toward the negative charge
b. 9.0 × 108 N away from the negative charge
c. 9.0 × 109 N toward the negative charge
d. 9.0 × 109 N away from the negative charge
e. none of these
13. Two identical charges, 2.0 m apart, exert forces of magnitude 4.0 N on each other. The value of either charge is:
a. 1.8 × 10–9 C
b. 2.1 × 10–5 C
c. 4.2 × 10–5 C
d. 1.9 × 105 C
e. 3.8 × 105 C
3
14. Two protons (p1 and p2) and an eletron (e) lie on a straight line, as shown. The directions of the force of p2 on p1, the force of e on p1, and
the total force on p1, respectively, are:
a.
b.
c.
d.
e.
, , 
, , 
, , 
, , 
, , 
15. Two protons (p1 and p2) and an electron (e) lie on a straight line, as shown. The directions of the force of p1 on e, the force of p2 on e, and
the total force on e, respectively, are:
a.
b.
c.
d.
e.
, , 
, , 
, , 
, , 
, , 
16. Two particles have charges Q and –Q (equal magnitude and opposite sign). For a net force of zero to be exerted on a third charge it must
be placed:
a. midway between Q and –Q
b. on the perpendicular bisector of the line joining Q and –Q, but not on that line itself
c. on the line joining Q and –Q, to the side of Q opposite –Q
d. on the line joining Q and –Q, to the side of –Q opposite Q
e. at none of these places (there is no place)
17. Two point charges, initially 2 cm apart, are moved to a distance of 10 cm apart. By what factor do the resulting electric and gravitational
forces between them change?
a. 5
b. 25
c. 1/5
d. 1/25
18. If the charge and mass are tripled for two identical charges maintained at a constant separation, the electric and gravitational forces
between them will be changed by what factor?
a. 9
b. 2/3
c.
1/9
d. 18
19. The electric field around an isolated electron has a certain strength 1 cm from the electron. The electric field strength 2 cm from the
electron is
a. half as much.
b. the same.
c. twice as much.
d. four times as much.
e. none of the above
20. A beam of electrons accelerates from the back of your TV set to the screen. In order to make the beam hit the top of the screen,
oppositely charged parallel plates create a vertical electric field. How will the top plate be charged?
a. Positively
b. Negatively
21. Two parallel plates are oppositely charged. The left plate is negative and the right plate is positive. In which direction does the electric
field point?
a. To the right
b. To the left
4
22. Which of the following statements is not true?
A. Electric charge is quantized.
B. Electric charge is conserved.
C. The force between two point charges is proportional to the sum of the charges.
D. The force between two point charges is proportional to the inverse square of the separation of the charges.
23. A positive test charge q is released near a positive fixed charge Q.
As q moves away from Q, it will move with
A. constant velocity.
B. constant acceleration.
C. increasing acceleration.
D. decreasing acceleration.
24.
A small electrically charged sphere is suspended vertically from a thread. An oppositely charged
rod is brought close to the sphere such that the sphere is in equilibrium when displaced from
the vertical by an angle of 450 .
Which one of the following best represents the free body diagram for the sphere?
25. Three equal point charges X, Y and Z are fixed in the positions shown.
The distance between q1 and q2 and the distance between q2 and q3 is 1.0 m.
The electric force between the charges at X and Y is F.
The electric force between the charges at X and Z is
A. F/2.
B. F/v2.
C. F.
D. 2F.
26. Calculate the force acting between two point charges of +10.0 C and -5.0 C separated by a distance of 10.0 cm in a vacuum.
27. The force between two point charges is 20.0 N. If one charge is doubled, the other charge tripled and the distance between them is
halved, calculate the resultant force between them.
5
28. Charges of +1C are located at the corners of a 45 rightangled triangle as shown in the Figure below.
Determine the resultant force on the charge located at the right angle.
29. Charges of +1C are located at the corners of a 45 rightangled triangle as shown in the Figure below.
Determine the resultant force on the charge located at the right angle.
30. Which one of the field patterns below could be produced by two point charges?
31. The distribution of electric field lines in a certain region of space varies as shown in the figure.
The magnitude of the electric field in this region
A. increases to the right.
B. decreases to the right.
C. increases in the downward direction.
D. decreases in the downward direction.
32. Two charges of –e and +4e are fixed at the positions shown below. At which position along the line XY is the electric field due to these
charges equal to zero?
6
33. The diagram below shows two stationary point charges + 2Q and – Q.
At which point is the electric field strength greatest?
A. A
B. B
C. C
D. D
34. A point charge of 25 C experiences a force of 1.0 x 10-4 N. Calculate the electric field strength producing this force.
35. Calculate the electric field strength 1.5 cm from a point charge of 1.00 x 102 pC in a vacuum.
36. Calculate the electric field strength at X due to the charges shown in Figure
37. If an object is given a positive charge, does its mass increase, decrease, or stay the same? Explain.
38. If an object is given a negative charge, does its mass increase, decrease, or stay the same? Explain.
39. Explain why a comb that has been rubbed through your hair attracts small bits of paper, even though the paper is uncharged.
40. Small bits of paper are attracted to an electrically charged comb, but as soon as they touch the comb they are strongly repelled. Explain
this behavior.
7
41. Explain what happens when you vigorously rub your wool socks on a carpeted floor, touch a metal doorknob, and get a shock.
42. A charged rod is brought near a suspended object, which is repelled by the rod. Can we conclude that the suspended object is charged?
Explain.
43. A charged rod is brought near a suspended object, which is attracted to the rod. Can we conclude that the suspended object is
charged?
Explain.
44. An electron (charge –e) orbits a helium nucleus (charge +2e). Is the force exerted on the helium nucleus by the electron greater than,
less than, or the same as the force exerted on the electron by the helium nucleus?
45. An electron and a proton are released form rest in space far from any other objects. The particles move toward each other, due to their
mutual electrical attraction. When they meet, are they (a) midway between their initial positions, (b) closer to the initial position of the
electron, or (c) closer to the initial position of the proton? Explain.
46. An electron with the velocity v and located infinitely far away moves directly toward another electron that is free and at rest. What will
happen to each electron?
47. Given the (comparatively) vast spaces between atoms, why don’t we simply fall through the floor?
48. Consider the three electric charges shown. List the charges in order of the magnitude of the force they experience, starting with the
smallest.
8
49. In the operating room, technicians and doctors must take care not to create an electric spark, since the presence of oxygen gas used
during an operation increases the risk of a deadly fire.
nonconducting? Explain.
Should the operating room personnel wear shoes that are (a) conducting or (b)
50. Do most or all of the conductors have something in common? If so, what?
51. Do most or all of the insulators have something in common? If so, what?
52. What is the difference between an insulator and a conductor? Which would you guess copper is? Wood? Distilled water?
53. A 2-C charge experiences a force of 20 N when put at a certain location in space. The electric field at that location is
54. The electric field in a certain region of space is 40 N/C. What is the force on a 10-C charge placed in that region?
55. The electrical force on a 2-C charge is 60 N. What is the value of the electric field at the location of the charge?
56. 10,000 electrons are removed from a neutral plastic ball. What is its charge?
57. What is the magnitude of the force of an electron in an electric field of 300 N/C?
58. The diagram shows the electric field lines in a region of space containing two small charged spheres (Y and Z). Then:
a.
b.
c.
d.
e.
Y is negative and Z is positive
the magnitude of the electric field is the same everywhere
the electric field is strongest midway between Y and Z
a small negatively charged body placed at X would be pushed to the right
Y and Z must have the same sign
9
59. The diagram shows a positive charge Q and a negative charge –Q with the same magnitude.
The electric field at point P on the perpendicular bisector of the line joining them is:
a.

b.

c.

d.

e.
zero
60. An electron traveling north enters a region where the electric field is uniform and points north. The electron:
a. speeds up
b. slows down
c. veers east
d. veers west
e. continues with the same speed in the same direction
61. Five particles are shot from the left into a region that
contains a uniform electric field. The numbered lines
show the paths taken by the five particles.
A negatively charged particle with a charge - 3Q follows
path 2 while it moves through this field.
Which path would be followed by a helium atom (an electrically neutral particle)?
A) path 5
B) path 2
C) path 3
D) path 1
E) path 4
In which direction does the electric field point?
A) toward the left of the page
B) toward the right of the page
C) out of the page, toward the reader
D) toward the bottom of the page
E) toward the top of the page
a.
Which path would be followed by a charge +6Q?
path 5
b. path 2
c. path 1
d.
path 4
e. path 3
62. An isolated point charge produces an electric field with magnitude E at a point 2 m away from the charge. A point at which the field
magnitude is E/4 is:
a. 1 m away from the charge
b. 0.5 m away from the charge
c. 2 m away from the charge
d. 4 m away from the charge
e. 8 m away from the charge
63. An isolated point charge produces an electric field with magnitude E at a point 2 m away. At a point 1 m from the charge the magnitude of
the field is:
a. E
b. 2E
c. 4E
d. E/2
e. E/4
64. In the figure, point A is a distance L away from a point charge Q. Point B is a distance 4L away from Q. What is the ratio of the electric
field at B to that at A, EB/EA?
a.
b.
c.
d.
e.
1/9
1/3
This cannot be determined since neither the value of Q nor the length L is specified.
1/4
1/16
10
65. What is the magnitude and direction of the electric force on a 1.2 µC charge at a point where the electric field is 2500 N/C and is
directed along the +y axis.
a. 0.0030 N, +y direction
b. 0.15 N, +y direction
c. 0.15 N, y direction
d. 4.3 N, +x direction
e. 0.0030 N, y direction
66. A electric dipole is released from rest in a uniform electric field with the orientation shown. Which entry in the table below correctly
describes the rotation and the net force on the dipole?
a.
b.
c.
d.
e.
rotation
clockwise
zero
counterclockwise
counterclockwise
clockwise
net force
non-zero
zero
non-zero
zero
zero
67. At which point (or points) is the electric field zero N/C for the two point charges shown on the x axis?
a.
b.
c.
d.
e.
The electric field is zero somewhere on the x axis to the right of the 2q charge.
The electric field is zero somewhere on the x axis between the two charges, but this point is nearer to the
2q charge.
The electric field is never zero in the vicinity of these charges.
The electric field is zero at two points along the x axis; one such point is to the right of the 2q charge and
the other is to the left of the +4q charge.
The electric field is zero somewhere on the x axis to the left of the +4q charge.
68. Two point charges, q1 and q2, are placed a distance r apart. The electric field is zero at a point P between the charges on the line segment
connecting them. We conclude that:
a. q1 and q2 must have the same magnitude and sign
b. P must be midway between q1 and q2
c. q1 and q2 must have the same sign but may have different magnitudes
d. q1 and q2 must have equal magnitudes and opposite signs
e. q1 and q2 must have opposite signs and may have different magnitudes
11
EL FORCE and EL FIELD HW-PRACTICE 2013
Answer Section
1.
11.
21.
27.
28.
B
C
B
2.D
12.A
22.C
3.C 4.D
13.C 14.D
23.D 24.D
5.B 6.C
15.B 16.E
25.A 26.F = -45 N
7.D 8.B 9.A
17.D 18.A 19.E
10.B
20.A
(attractive)
F = 480 N
F1 = 9x109 N
F2 = 9x109 N
F = 12.7 x 109 N
 = 450
The resultant force is 1.3 x 1010 N in a direction of 45 from the horizontal and downwards.
29. F1 = 9x109 N
F2 = 9x109 N
FR = 12.7 x 109 N
 = 450
The resultant force is 1.3 x 1010 N vertical downward
30. C
31.A
32.A
33.B
34. E = 4.0 N/C (in the direction of the force).
35. E= 4.0 x 103 N/C (radially outwards).
36. E = 10.4 x 102 N/C
 = 440
E = 10.4 x 102
N/C
at 10 below a horizontal line drawn through X.
37. Its mass decreases. This is because a positive charge is generally due to a loss of electrons, each of which has a small mass.
38. Its mass increases. This is because a negative charge is generally due to a gain of electrons, each of which has a small mass.
39. The charged comb causes the paper to become polarized, with the side nearest to the comb acquiring a charge opposite to the charge of
the comb.
The result is an attractive interaction between the comb and the paper.
40. the paper gets the same charge so there is repulsive force now between balloon and paper bits.
41. Loosely held electrons are transferred from the carpet to the socks when the socks are rubbed against the carpet. At that point, the body
and socks have an excess of electrons and are negatively charged. Touching the doorknob allows the electrons to escape. The shock felt is
the sudden movement of charges as the body and socks return to a neutral state.
42. Yes. If the suspended object were neutral it would be attracted to the charged rod by polarization effects. The fact that the suspended
object is repelled indicates that it has a charge of the same sign of that of the rod.
43.
44.
45.
46.
47.
48.
No. It can be neutral and then polarized and consequently attracted to the rod.
By Newton’s third law, the forces exerted on the electron and the nucleus are equal in magnitude and opposite in direction.
the force is the same; electron is faster – it has greater acceleration (smaller mass) so (c)
They’ll repel each other, and eventually start moving in opposite direction.
Because of
very strong repulsion between outer electrons in our body’s molecules and floors’ molecules.
The charge in the middle experiences the greatest force. Next in magnitude is the net force exerted on the charge at the left end, and
the smallest net force is exerted on the charge to the right.
49. (a) Their shoes should be conduction so any charge they transfer during walking can travel into the ground.
50. Any material through which charge can move easily is called conductor.
12
In conductors many of electrons are bound very loosely to the nuclei and can move about freely within the material (although they cannot
leave the material easily). They are often called free electrons or conduction electrons.
Metals are generally good conductors.
51. Any material that impedes the flow of charge is called insulator.
Electrons in insulators are tightly bound to atomic nuclei and so cannot easily be made to drift from one atom to the next.
Most materials other then metals are insulators.
52. A good electrical conductor is any material composed of atoms with "loose" outer electrons that are free to move about in the material,
such as copper. A good electrical insulator is a material composed of atoms that tightly bind electrons. Wood and distilled water are
examples of good electrical insulators.
53. 10 N/C
54. 400 N
55. 30 N/C
56. 1.6 x 10-15 C
57. 4.8 x 10-17 N
58. D
59.A
65. E
66.E
60.B
67.A
68.C
61.C,D,A
62.D
63.C
64.E