1
The combined charge of all electrons in a nickel coin is
hundreds of thousands of coulombs, a unit of electrical charge.
Does that imply anything about the net charge on the coin?
Explain.
6
The fact that the electron has a negative charge and the proton
has a positive charge is due to a convention established by
Benjamin Franklin. Would there have been any significant
consequences if Franklin had chosen the opposite convention?
Is there any advantage to naming charges plus and minus as
opposed to, say, A and B?
Answer
Answer
No. Net charge is the difference between positive and
negative charges. It can still be zero.
2
No, the basic physics of electric charges would not have
been affected at all by an opposite assignment of positive
and negative labels. The use of + and – signs, as opposed
to labels such as A and B, has the distinct advantage that it
gives zero net charge to an object that contains equal
amounts of positive and negative charge.
The figure shows how a charged rod placed near an uncharged
metal object can attract (or repel) electrons. There are a great
many electrons in the metal, yet only some of them move as
shown. Why not all of them?
7
Answer the following question Based on the materials listed,
(a) A charged balloon is placed near a stream of water flowing
from a faucet is the charge of the balloon more likely to be
positive or negative? Explain.
(b) If the charge on the balloon is reversed, will the stream of
water deflect toward or away from the balloon? Explain.
Table 19-1
Triboelectric Charging
Material
Rabbit fur
Glass
Human hair
Answer
Nylon
Silk
For each atom in a conductor, only a small number of its
electrons are free to move. For example, every atom of
copper has 29 electrons, but only 1 or 2 from each atom
are free to move easily. Also, not even all of the free
electrons move. As electrons move toward a region,
causing an excess of negative charge, that region then
exerts a large repulsive force on other electrons, preventing
them from all gathering in one place.
3
Consider an electrically neutral object. If this object is given a
positive charge, does its mass increase, decrease, or stay the
same? Explain.
Paper
Relative charging with rubbing
++++++
+++++
++++
+++
++
+
Cotton
-
Wood
--
Amber
---
Rubber
----
PVC
Teflon
----------
Answer
Answer
(a) The balloon is made from rubber and the cloth that is
rubbed against it is probably made of cotton or a similar
material. It followed from the table that the rubber balloon
will have a negative charge. In fact, the only two materials
more negative then rubber in the table are materials one
would not associate with”cloth”
Answer
Its mass decreases because a positive charge is generally
due to a loss of electrons, each of which has a small mass
4
Consider an electrically neutral object. If this object is given a
negative charge, does its mass increase, decrease, or stay the
same? Explain.
Answer
Giving an object a negative charge means transferring
electrons to the object. This, in turn, increases its mass.
5
When an object that was neutral becomes charged, does the
total charge of the universe change? Explain.
Answer
No when an object becomes charged it is becaause of a
transfer of charge beween it and another object
(b) the t\stream of water will be deflected toward the
balloon regardless of whether its charge is positive or
negative, just as bits of paper are attracted equally well to a
charged amber (-) rod or a charged glass (+) rod.
8
Answer the following questins using the triboelec tric chart
(a) If rabbit fur is rubbed against glass, what is the sign of the
charge each acquires? Explain.
(b) Repeat part (a) for the case of glass and rubber.
(c) Comparing the situations described in parts (a) and (b), in
which case is the magnitude of the triboelectric charge the
greatest? Explain.
10
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.
Table 19-1
Triboelectric Charging
Material
Rabbit fur
Glass
Human hair
Nylon
Silk
Paper
Relative charging with rubbing
++++++
+++++
++++
+++
++
+
Cotton
-
Wood
--
Amber
---
Rubber
----
PVC
Teflon
Answer
-----
Initially, the bits of paper are uncharged and are attracted
to the comb by polarization effects. accompanying
discussion.) When one of the bits of paper comes into
contact with the comb, it acquires charge from the comb.
Now the piece of paper and the comb have charge of the
same sign, and hence we expect a repulsive force between
them.
------
Answer
(a) Referring to the table, we see that rubbing rabbit fur
against glass will result in a positive charge for the rabbit
fur, and a negative charge for the glass.
(b) For glass and rubber, we see that the rubber acquires a
negative charge. Note that in this case, the charge on the
glass is positive; hence, the charge acquired by a material
depends not only on the material itself, but also on the
material that it is rubbed against.
11
(c) Noting that rabbit fur and glass are adjacent in the
table whereas glass and rubber are widely separated, we
conclude that the magnitude of triboelectric charge is
greater in the glass-rubber case.
9
Explain why a comb that has been rubbed through your hair
attracts small bits of paper, even though the paper is
uncharged.
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.
Answer
Yes, If the suspended object were neutral, it would be
attracted to the charged rod by polarization effects. The
fact the suspended object is repelled indicates tit has a
charge of the same sign as that of the rod.
12
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.
Answer
Answer
The charged comb causes the paper to become polarized
with the side nearest the comb aquiring a charge opposite
to the charge of the comb. The result is an attractive
interaction between the comb and the paper.
No. Even uncharged objects will be attracted to a charged
rod, due to polarization effects. See Figure 19-5 and the
accompanying discussion.
13
An electron and a proton are released from rest in space, far
from any other objects. The particles move toward each other,
due to their mutual electrical attraction. When they meet, is the
kinetic energy of the electron greater than, less than, or the
same as the kinetic energy of the proton? Explain.
Answer
The electron has the greater kinetic energy (about a factor
of 2000). This is due to the fact that the momentum of the
system is conserved hence meve = mpvp when the particles
meet. It follows that 1/2 meve2 = mp/me)(1/2 mpvp2) when
mp/me = 2000
14
Describe some of the similarities and differences between
Coulomb’s law and Newton’s law of gravity.
21
Answer
15
Sketch the direction and relative magnitude of the electric
force experienced by each of the three charges shown in the
figure.
When adhesive tape is pulled from a dispenser, the detached
tape acquires a positive charge and the remaining tape in the
dispenser acquires a negative charge. If the tape pulled from
the dispenser has 0.14 µC of charge per centimeter, what
length of tape must be pulled to transfer 1.8 x 1013 electrons to
the remaining tape?
Answer
21 cm
22
A system of 1525 particles, each of which is either an
electron or a proton, has a net charge of -5.456 x 10-17.
(a) How many electrons are in this system?
(b) What is the mass of this system?
Answer
(a) 933
(b) 9.91 x 10-25 kg
Answer
16
23
-6.2 x 10
17
24
-12
C
Find the net charge of a system consisting of 212
electrons and 165 protons.
Answer
-7.5 x 10-18 C
18
How much negative electric charge is contained in 2
moles of carbon?
Answer
-1 x 106 C
19
Find the total electric charge of 1 kg of electrons
Answer
-2 x 1011 C
20
Answer
Find the net charge of a system consisting of 3.9 x 19-19
electrons.
Answer
A container holds a gas consisting of 1.50 moles of oxygen
molecules. One in a million of these molecules has lost a single
electron. What is the net charge of the gas?
Answer
0.145 C
Find the net charge of a system consisting of 165
protons.
Calculate the net charge on a substance consisting of a
combination of 7.0 x 1013 protons and 4.0 x 1013 electrons.
Answer
4.8 x 10-6 C