Chapter Questions
The Atom
Classwork
1. Which part on an atom carries a positive charge? Which carries the negative charge?
2. How many types of electric charge are there?
3. When a neutral atom captures a free electron what is the net charge on the atom? What do we call
this type of atom?
Homework
4. Eight electrons orbit an oxygen atom. How many protons are in the nucleus?
5. When we remove on electron from an atom what in the net charge on the atom? What do we call this
type of atom?
Charging by Rubbing
Classwork
6. What happens to a plastic rod when it is rubbed with a piece of animal fur? What happens to the
piece of fur?
7. How can you tell that whether two objects, after being rubbed together, have each acquired an
electric charge? How will the magnitude of the charge on each compare with one another? How
about the sign of the charge on each?
8. What happens with two glass rods rubbed with silk when we bring them close to each other?
Homework
9. What happens between a plastic rod rubbed with a piece of animal fur and a glass rod rubbed with a
piece of silk when we bring them close to each other?
10. What happens with two plastic rods rubbed with animal fur when we bring them close to each other?
11. Repeat question #8 for a charged glass rod.
Charging by Conduction & Induction, Electroscopes, Grounding
Classwork:
12. Describe two ways that you can give an electroscope a positive charge?
13. A student has a charged glass rod but he wants to charge an electroscope negatively. What should
he do to accomplish that? Why does that work?
14. A student brings a charged plastic rod near a tiny piece of paper that is resting on a tabletop. Will the
paper be attracted or repelled? Explain why and draw a diagram.
15. Why is it that when you take off a shirt or blouse in a dark room you can see tiny sparks and hear a
crackling sound?
Homework
16. A positively charged rod attracts a metal ball suspended at the end on an insulating string. Draw a
diagram and explain the mechanism of attraction.
Electric Charge & Force Problems - 1
v 1.3
©2009 Goodman & Zavorotniy
17. An electroscope is charged negatively and a charged rod is brought very close to it but not touching,
as a result the leaves move further. What kind of electric charge is on the rod?
18. Two light paper strips are suspended at the ends of two insulating strings. One paper strip is
charged the other is not charged. Design an experiment to determine which strip is charged.
19. A student touches an electroscope with his hand at the same time he brings a positively charged rod
close to the electroscope without touching. When he removes his hand first and then moves the rod
away from the electroscope the leaves move apart. Why? What kind of charge is on the leaves?
Electric Force
Classwork
20. Will two charged objects interact on the Moon, where there is no atmosphere?
21. A negatively charged oil drop falls down on a table surface. An uncharged metal sheet is placed on
the table. What kind of electric charge must we add to the sheet in order to stop the drop from
falling?
22. A metal sphere is charged positively. When we increase the positive charge on the sphere does it
change the number of protons?
Homework
23. Is it possible to add 0.5 of electron charge to an object? Explain.
24. When you discharge a positively charged object does it mean that the object no longer contains any
electric charges?
Chapter Problems
Coulomb’s Law
Classwork
1. Two positive charges of 1 mC and 10 mC are separated by a distance of 10 m. Find the direction
and the magnitude of electrostatic force between the charges.
2. A particle with a charge of +7.4 μC is separated from another charged particle with a charge of –3.6
μC by a distance of 1.4 m. Find the direction and the magnitude of electrostatic force between the
particles.
3. A +1.4 nC charge exerts a repulsive force of 20 mN on a second charge which is located a distance
of 2.2 m away from it. What is the charge (magnitude and sign) of the second charge?
4. Two spherical objects, whose centers are 8 cm apart, have equal negative charges and repel each
other with a force of 9 mN. What is the charge on each of them? How many extra electrons are on
each of them?
5. Two conducting spheres have net charges of +9 μC and -7 μC and attract each other with a force of
4 mN. The spheres are brought in a contact and then moved apart to the initial distance. What is the
new force between the spheres? Is this force attractive or repulsive?
Homework
6. Two negative charges of 2.5 μC and 9 μC are separated by a distance of 25 cm. Find the direction
and the magnitude of electrostatic force between the charges.
Electric Charge & Force Problems - 2
v 1.3
©2009 Goodman & Zavorotniy
7. Two charges of +2.6 μC and –5.4 μC experience an attractive force of 6.5 mN. What is separation
between the charges?
8. What is the distance between two charges +7.8 μC and +9.2 μC if they exert a force of 4.5 mN on
each other?
9. A –4.2 μC charge exerts an attractive force of 1.8 mN on a second charge which is a distance of 2.4
m away. What is the magnitude and sign of the second charge?
10. Two equal negative point charges repel each other with a force of 18 mN. What is the charge on
each object if the distance between them is 9 cm? How many extra electrons are on each object?
11. Two charged conducting spheres have net charges of +4 μC and -8 μC and attract each other with a
force of 16 mN. The spheres are brought into contact and then moved apart to the initial distance.
What is the new force between the spheres? Is this force attractive or repulsive?
12. What is the ratio of the electrostatic force to the gravitational force between two electrons?
General Problems
Classwork
-Q
13. A conducting sphere is carrying a negative charge of –6 μC and is placed on an insulated tabletop. A
0.2 g oil drop is suspended 1.5 m above the sphere.
a. Draw a free-body diagram showing all the forces acting on the drop.
b. What is the sign of the net charge of the drop?
c.
Determine the magnitude of the electric charge on the drop.
d. If we double the charge on the drop what will be its initial acceleration?
Electric Charge & Force Problems - 3
v 1.3
©2009 Goodman & Zavorotniy
Homework
14. A 0.14 kg metal ball is suspended at the end of a string and carries a positive charge of +10 nC. A
charged sphere with a negative charge of -25 μC is placed at 5 cm below the ball.
a. Draw a free-body diagram showing all the forces acting on the ball.
b. Find the tension force in the string.
c.
If the maximum tension force that the sting can withstand is 3 N, how much charge must be
added to the ball in order to break the string?
d. What will be the tension force in the string if we changed the charge on the sphere from -25
μC to +25 μC and leave the charge of the hanging ball at +10 nC?
15. Two identical balls (B and C) with a mass of 0.5 g are suspended from two strings as show above.
The balls carry equal charges +10 nC each and are separated by a distance of 4 cm.
a. Draw free-body diagram and show all forces applied to ball C.
b. Find the tension force in the string BC.
c.
Draw free-body diagram and show all forces applied on ball B.
d. Find the tension force in string AB.
e. Answer questions a, b, c, d for the situation when the balls have equal but opposite charges
(charge on B is positive and charge on C is negative).
Electric Charge & Force Problems - 4
v 1.3
©2009 Goodman & Zavorotniy
Classwork
16. A positive charge Q1 = 2.6 μC is located at a point X1 = -3 m and a positive charge Q2 = 1.4 μC is
located at a point X2 = +4 m.
a.
b.
c.
d.
Draw free body diagrams for the electric force acting on Q1 and Q2.
Find the magnitude the electric force between Q1 and Q2.
Find the magnitude and direction of the electric force acting on Q1.
Find the magnitude and direction of the electric force acting on Q2.
17. **A positive charge Q1 = 7.4 μC is located at a point X 1 = -2 m, a negative charge Q2 = -9.7 μC is
located at a point X2 = 3 m and a positive charge Q3 = 2.1 μC is located at a point X3 = 9 m.
a.
b.
c.
d.
e.
f.
g.
Draw free body diagrams for the electric force acting on Q1, Q2 and Q3.
Find the magnitude of the force between Q1 and Q2.
Find the magnitude of the force between Q1 and Q3.
Find the magnitude of the force between Q2 and Q3.
Find the magnitude and direction of the net electric force on charge Q1.
Find the magnitude and direction of the net electric force on charge Q 2.
Find the magnitude and direction of the net electric force on charge Q 3.
Homework
18. **A negative charge Q1 = -25 μC is located at a point X1 = -2 m, a positive charge Q2 = 15 μC is
located at a point X2 = 3 m and a positive charge Q3 = 18 μC is located at a point X3 = 9 m.
a.
b.
c.
d.
e.
f.
g.
Draw free body diagrams for the electric force acting on Q1, Q2 and Q3.
Find the magnitude of the force between Q1 and Q2.
Find the magnitude of the force between Q1 and Q3.
Find the magnitude of the force between Q2 and Q3.
Find the magnitude and direction of the net electric force on charge Q 1.
Find the magnitude and direction of the net electric force on charge Q 2.
Find the magnitude and direction of the net electric force on charge Q 3.
Electric Charge & Force Problems - 5
v 1.3
©2009 Goodman & Zavorotniy
Answers
16. The electrons from the conductor move
towards the closest side of the rod and attract
to the positive rod.
Chapter Questions
1.
Proton, electron
2.
2
17. The charge on the rod is negative since the
leaves separate.
3.
Negative, negative ion
18. Bring another charged object and see how it
interacts with the charged paper strips.
4.
8 protons
5.
Positive, positive ion
19. The charge in the leaves was negative since
due to induction, electrons travel to the leaves
to attract the positively rod.
20. Yes
6.
7.
Plastic rod becomes negative, fur becomes
positive
22. No then it would become a different
substance
Use a metallic piece to see of it repels or
attracts the object; the magnitude of the
charge is the same for the two objects
23. No you cannot break up electrons
24. No, it just means it will become neutral
8.
Repulsion
9.
Attraction
10.
Repulsion
11.
Nothing – neutral
12.
Conduction & Induction
13.
By conduction, the electroscope will be
charged negatively
14.
The positive charged rod will attract the
paper since the electrons inside the paper
will move towards the rod.
15.
This is an example of static electricity where
electrons are being moved from one
clothing to another.
Electric Charge & Force Problems - 6
21. Negative
v 1.3
©2009 Goodman & Zavorotniy
Chapter Problems
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
15.
900 N repelling
0.122 N attracting
0.0077 C or 7.7mC
-8
11
-8 x 10 C, 5 x 10
electrons
-5
6.35 x 10 N repelling
3.24 N repelling
4.4 m
11.98 m
-7
2.7 x 10 C
-7
-1.27 x 10 C, 7.95 x
11
10 electrons
0.002 N repelling
kqeqe : Gmeme
-28
-71
2.3 x 10 : 5.5 x 10
42
4.2 x 10 : 1
17.
a.
a.
b.
0.0055 N
b.
c.
d.
e.
f.
g.
c.
d. 0.0098 N
e.
0.02584 N
0.001156 N
0.005093 N
+0.02468 N
-0.02075 N
-0.00393 N
18.
General Problems
13.
i.
ii. 0.0043 N
a.
b. Negative
-8
c. 8.17 x 10 C
2
d. 9.8 m/s upward
iii.
iv. 0.0098 N
14.
16.
F21
a.
a.
b.
c.
d.
e.
f.
g.
0.135 N
0.03347 N
0.0675 N
+0.1685 N
-0.2025 N
+0.03403 N
b. 0.000669 N or 669 μC
c. -0.000669 N or -669 μC
d. +0.000669 N or +669 μC
a.
b. 2.272 N
-9
c. 8.1 x 10 C
d. 0.472 N
Electric Charge & Force Problems - 7
v 1.3
©2009 Goodman & Zavorotniy