BW1 Ch 20: CQ1-12, 23, 27. P. 1-8
Q20.1. Reason: Two charges are alike because the react in the same way when brought in the vicinity of a third
charge. They also exert repulsive forces on each other.
Assess: Looks, smell, and taste are not characteristics we use to distinguish charges.
Q20.2. Reason: The statement of the question informs us that ball A is negative since charge is transferred to it from a
plastic rod that has been rubbed with wool. Recall that a plastic rod rubbed with wool has a negative charge.
The first statement informs us that balls B, C, and D are either neutral or positive. If they are neutral, their attraction to ball
A is due to charge polarization. If they are positive, their attraction to ball A is due to the fact that unlike charges attract.
The second statement informs us that balls B and D are neutral. If both are charged, there will be attraction or repulsion. If
either one is charged, there will be attraction due to charge polarization of the other. Since there is neither attraction nor
repulsion, neither is charged.
The last statement informs us that ball C must be charged. It is attracted to ball B due to polarization of charges on the
neutral ball B.
Knowing that C is charged and attracted to A and that unlike charges attract, we can conclude that C has a positive charge.
To recap, ball A is negative, balls B and D are neutral, and ball C is positive.
Assess: By careful application of our knowledge of electrostatics we have been able to account for all observations. In
some cases we had to combine two bits of information in order to make a conclusion regarding charge. All conclusions are
consistent with the knowledge that like charges repel, unlike charges attract, and charge polarization can occur for a
neutral object.
Q20.3. Reason: (a) Two like charges exert repulsive forces on each other. So the object must also have the same
charge as the plastic rod. Therefore it will attract the glass rod since the glass rod has the opposite charge from the plastic
rod.
(b) This time you cannot predict the outcome, because the object could either be charged like the glass rod (and therefore
repel the glass rod) or be neutral (and be attracted to the glass rod through polarization of the object). In either case it is
attracted to the plastic rod, so you can’t distinguish the two cases.
Assess: Don’t jump to conclusions that just because an object will attract one kind of rod that it will repel the other.
Neutral objects can be attracted to both kinds of rods through polarization.
Q20.4. Reason: (a) An insulator can be charged. We could rub it vigorously with either silk or wool. (b) A conductor
can be charged. We could touch it with an insulator that has been rubbed with either silk or wool and hence charge it by
transfer of charge.
Assess: The insulator obtained its charge by charge separation and the conductor obtained its charge by a transfer of
charge.
Q20.5. Reason: The clothes are charged by rubbing in the drier. Your body is neutral but the clothes can polarize you
a little and attract the opposite charge in your body.
Assess: This is the same way a balloon sticks to the wall after you rub it on your hair.
Q20.6. Reason: The ball is initially either neutral or has a charge opposite that of the rod. If the ball is neutral, the
attraction to the rod is due to polarization of the charge on the ball. If the ball is charged, it has a charge opposite that of
the rod since it is attracted to the rod.
When the metal ball touches the rod (which is charged), if the ball is neutral and the rod negative (excess electrons),
electrons will be transferred from the rod to the ball making it negative. The ball will now be repelled because it has the
same charge as the rod. If the ball is neutral and the rod positive (deficiency of electrons) electrons will be transferred
from the ball to the rod resulting in a deficiency of electrons on the ball hence a positive charge. Since the ball and rod
now have the same charge, the rod will repel the ball.
If the ball is charged, it must have a charge opposite that of the rod since they are attracted. So one is negative (excess of
electrons) and the other is positive (deficiency of electrons). When they touch there is a redistribution of electrons. If one
object is more negative than the other is positive, both objects will end up negative and hence repel. If one object is more
positive than the other is negative, both objects will end up positive and hence repel.
Assess: In order to answer this question, we have to be aware of charge polarization, that it is the electrons that are the
mobile charge carriers and that like charges repel and unlike charges attract.
Q20.7. Reason: Each sphere ends up with 1 unit of negative charge. Once they touch, the two spheres become
essentially one conductor. The overall net charge is – 4  2  – 2. Charge is spread uniformly over the surface of a
conductor, so each sphere ends up with –1.
Assess: The arithmetic is easy to compute the total charge. One must also realize that charge spreads out over
a conductor.
Q20.8. Reason: The following figure shows a representation of the charges on the metal sphere before and after the
positively charged rod is brought near the neutral metal sphere.
(a) Notice that the positive charges on the metal sphere have not moved, just the electrons. Since the ball is metal, it is
easy for the electrons to move. Electrons associated with nuclei on the region of the ball nearest the positive rod will move
as close to the positive rod as possible but may not actually disassociate from the nucleus. In other regions of the metal
ball, the electrons may actually leave their “mother” nucleus in order to get closer to the very positive rod. In regions of the
ball far from the rod, some charge polarization may occur with the electrons “leaning” toward the positive rod but not
disassociated from the nucleus. Finally, in regions farthest from the rod, the influence of the “mother” nucleus may be so
much greater than that of the distant rod that the electrons essentially don’t feel the presence of the positively charged rod.
(b) Since the negative charges are concentrated on the region of the sphere closest to the rod, there will be a net attractive
force of the sphere to the rod.
Assess: This question uses our knowledge of charge polarization, the manner in which like and unlike charges interact,
and the fact that the electrons are the mobile charge carriers.
Q20.9. Reason: (a) No, we cannot conclude that the wall is charged. Attractive electric forces occur between (i) two
opposite charges, or (ii) a charge and a neutral object that is polarized by the charge. Rubbing the balloon does charge the
balloon. Since the balloon is rubber, its charge is negative. As the balloon is brought near the wall, the wall becomes
polarized. The positive side of the wall is closer to the balloon than the negative side, so there is a net attractive electric
force between the wall and the balloon. This causes the balloon to stick to the wall, with a normal force balancing the
attractive force and an upward friction force balancing the very small weight of the balloon.
(b)
Assess: You have probably wondered at birthday parties how the balloons stick. Now you know. Tell everyone at the
next party, and explain why they eventually fall off, too.
Q20.10. Reason: Put the two metal spheres in contact with each other. Then rub the glass rod on the silk to charge the
rod positively. Bring it near, but not touching, one of the spheres. This will make the near sphere negatively charged and
the far sphere equally positively charged. Separate the spheres and remove the rod.
Assess: We don’t want to touch the rod to the sphere and transfer charge, because it would be too easy to have both
spheres end up positive.
Q20.11. Reason: If the bees are charged and the grains of pollen are neutral, then the charge on the bees can polarize
the grains of pollen so that they are attracted to the charged bee with an attractive electric force. That is, the charge on the
pollen grain will be somewhat separated, with one side of the grain of pollen becoming slightly negative (and therefore
attracted to the positive bee) and the other side becoming slightly positive (repelled by the bee).
Assess: Bees also attract particles of dust and soil to their bodies the same way. In doing so they provide a chemical
survey of the area around their hive. This enables scientists to use bees to locate landmines and explosives.
Q20.12. Reason: Charge is free to move throughout the rod-sphere combination. Negative charges move into Rod A
toward the positively charged rod, leaving the sphere positively charged.
Assess: Rod A ends up negatively charged.
Q20.23. Reason: Because of Newton’s third law we shouldn’t choose choices D or E for any of the situations. We
ignore the gravitational attraction between the balls as it is many orders of magnitude smaller than usual electrical forces.
(a) Like charges repel, so the correct choice is B.
(b) Opposite charges attract so the correct choice is C.
(c) Like charges repel, so the correct choice is B.
(d) The changed rod will polarize the neutral rod and they will attract each other, so the correct choice is C.
Assess: It is important to remember that F12   F21.
Q20.27. Reason: Call the charge on the glass bead q1  35 nC and the charge on the plastic bead q2 . We need to
solve Coulomb’s law (Equation 20.1) for q2:
FK
| q1|| q2 |
r2
Where F  80  104 N and r  29 cm  0029 m, and K  90 109 N  m2/C2.
| q2 | 
Fr 2
(80  104 N)(0 029 m) 2

 21 nC
K | q1| (90  109 N  m2 /C2 )(35 109 C)
We know q2 must be a positive charge since it is repelled by another positive charge.
The correct choice is C.
Assess: The order of magnitude of q2 is in the same ballpark as q1 , so the number is probably reasonable. Checking
your number-crunching in your calculator twice never hurts either. As for units, the N and m 2 cancel out, leaving C.
P20.1. Prepare: We will use the charge model. An electron has a negative charge of magnitude 1.6  1019 C.
Solve: (a) In the process of charging by rubbing, electrons are removed from one material and transferred to the other
because they are relatively free to move. Protons, on the other hand, are tightly bound in nuclei. So, electrons have been
removed from the glass rod to make it positively charged.
(b) The number of electrons removed is
5  109 C
10
 3.1  10
1.6  1019 C
Assess: A large number of electrons are needed to create a modest charge.
P20.2. Prepare: Use the charge model. An electron has a negative charge of magnitude 1.6  1019 C.
Solve: (a) In the process of charging by rubbing, electrons are removed from one material and transferred into the other
because they are relatively free to move. Protons, on the other hand, are tightly bound in the nuclei. So, electrons have
been added to the plastic rod to make it negatively charged.
(b) The number of electrons added is
20  109 C
 1.25  1011
1.6  1019 C
which to two significant figures is 1.3  1011.
Assess: To create a modest charge of 20 nC, a large number of electrons need to be transferred from one material to the
other.
P20.3. Prepare: Use the charge model. Each oxygen molecule has 16 protons (8 per atom), and there are
6.02  1023 oxygen molecules in 1.0 mole of oxygen. The proton has a positive charge of magnitude 1.6  1019 C.
Solve: The amount of positive charge in 1.0 mole of oxygen is
6.02  1023  (16  1.6  1019 C)  1.5  106 C
Assess: Coulomb is a “big” unit of charge, so 1 mole of oxygen has a lot of positive charge.
P20.4. Prepare: We will use the charge model and the model of a conductor as material through which electrons
move. An electron has a negative charge of magnitude 1.60  1019 C.
Solve: (a) The charge of a plastic rod decreases from 15.0 nC to 10.0 nC. That is, 5.0 nC charge has been removed
from the plastic. Because it is the negatively charged electrons that are transferred, 5.0 nC has been added to the metal
sphere.
(b) Because each electron has a charge of 1.60  1019 C and a charge of 5.0 nC was transferred, the number of electrons
transferred from the plastic rod to the metal sphere is
5.0  109 C
 3.1  1010
1.60  1019 C
Assess: A modest charge of 5 nC contains over 30 billion electrons!
P20.5. Prepare: Use the charge model and the model of a conductor as a material through which electrons move. An
electron has a negative charge of magnitude 1.6  1019 C.
Solve: (a) The charge of the glass rod decreases from 12 nC to 8 nC. Because it is the electrons that are transferred,
4 nC of electrons has been added to the glass rod. Thus, electrons are removed from the metal sphere and added to the
glass rod.
(b) Because each electron has a charge of 1.6  1019 C and a charge of 4 nC was transferred, the number of electrons
transferred from the metal sphere to the glass rod is
4  109 C
 2.5  1010
1.6  1019 C
Assess: 25 billion electrons constitute a charge of 4 nC.
P20.6. Prepare: We will use the charge model and the model of a conductor as a material through which electrons
move. An electron has a negative charge of magnitude 1.6  1019 C.
Solve: Because the metal spheres are identical, the total charge is split equally between the two spheres. That is, qA  qB
11
 5  1011 electrons. Thus, the charge on metal sphere A and B is (5  10 )(1.6  1019 C)  80 nC.
Assess: Flow of charge from one charged conductor to another occurs when they come into contact.
P20.7. Prepare: Use the charge model and the model of a conductor as a material through which electrons move. An
electron has a negative charge of magnitude 1.6  1019 C.
Solve: Plastic is an insulator and does not transfer charge from one sphere to the other. The charge of metal sphere A is
(1.0 1012 )(1.6 1019 C)  160 nC and the charge of metal sphere B is 0 C.
Assess: Flow of charge does not occur between a charged conductor and an insulator when they are brought into contact.
P20.8. Prepare: When two identical conducting spheres are in contact the charge is evenly distributed between them.
Solve: (a) The following grid shows the initial charge on each of the spheres and the charge after each event.
(b) The following grid shows the initial charge on each of the spheres and the charge after each event.
Assess: The end result depends on the order in which the various events occur.