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Chapter 20 Electrostatics
Matter contains electric charges. The study of forces between charges at
rest is called electrostatics.
Electric charges
Nucleus
Neutron
Proton
Where charges come from
Matter is made up of atoms (原子). An
atom consists of a nucleus (原子核) at
the center and electrons surrounding the
Electron
nucleus (Figure 20-1). The nucleus is
All atoms are built of protons,
made up of protons (質子) and neutrons electrons and neutrons. They
(中子). A proton has a positive charge normally have equal numbers of
(+). An electron has a negative charge (-),protons (+) and electrons(-) so they
are neutral.
which is equal in magnitude to the
charge on the proton. A neutron has no
charge. Usually atoms are electrically
neutral (中性), that is, the number of
protons is equal to the number of
1
electrons.
The electrons sometimes get loose
and can move around. When an
atom loses an electron, the atom
has a net (淨) positive charge
because it now has one proton
more than the number of electrons
remaining in the atom (Figure 202-a). It becomes a positive ion (正
離子). Similarly, when an atom
gains an electron, the atom has a
net negative charge and is called a
negative ion (負離子)(Figure 202-b). Therefore, ions are formed
when atoms lose or gain one or
more electrons.
Nucleus
Proton
Neutron
Electron
(a) A positive ion
Nucleus
Proton
Neutron
Electron
(b) A negative ion
2
Quantity of Electricity
The quantity of electric charge, symbol Q, is called coulomb (庫倫),
written as C.
The charge of an electron = - 1.6 x 10-19 C.
The charge of a proton = + 1.6 x 10-19 C.
1
18
1 coulomb = charge on
electrons
=
charge
on
6.25
x
10
1.6 × 10 −19
electrons.
Therefore, 1 coulomb is a lot of charge.
METHODS OF CHARGING
Charging By Friction
3
repel
repel
attract
4
repel
attract
attract to
positive
negative
repel
attract
induced
charges
5
Experiment:
1. Rub 2 acetate (醋酸酯) strips (條
片) with a dry woollen cloth (棉
布), and then held side by side
(Figure 20-3(a)). They repel each
other. The acetate strips are said to
be charged (充電).
Charged
acetate
strip
repulsion
Like charges repel
(a)
2. Rub 2 polythene (聚乙烯) strips
with a dry woollen cloth, and
then held side by side (Figure
20-3(b)). They repel each other.
Charged
polythene
strip
repulsion
Like charges repel
(b)
6
3. Rub an acetate strip and a
Charged
polythene strip with a dry
acetate
woollen cloth, and then held side strip
by side (Figure 20-3(c)). They
attract each other.
Conclusion:
Charged
polythene
strip
attraction
Unlike charges attract
1. Two charged strips made of the same material repel
each other. Two
(c)
charged strips made of different materials attract each other.
⇒ Two different kinds of charges are produced by friction on these
materials.
2. Because charges on the same material are the same, and charges on
different materials are different.
Like charges repel, unlike charges attract.
7
Note: 1. The two kinds of charges
are called positive (+) and
negative (-). When plastic
materials are rubbed with a
dry woollen cloth, the
charges on them depend on
the nature of the materials
as shown in Table 20.1.
Material
Charge
Acetate
+
Perspex 不碎透明膠
+
Polythene
-
PVC 聚氯乙烯
-
8
Explanation of charging by friction
1. Initially, a woollen cloth and an
acetate strip both have equal
number of electrons and protons.
Therefore, they are neutral or
uncharged (Figure 20-5(a)).
2. When they are rubbed together,
some of the electrons from the
atoms on the surface of the
acetate strip are pulled away and
transferred to the woollen cloth
(Figure 20-5(b)). Then the
acetate strip has a net positive
charge and has fewer electrons
than usual. The woollen cloth
has a net negative charge and
has more electrons than usual.
rubbing acetate
(a)
woollen cloth
acetate
Both charged
(b) after rubbing woollen cloth
9
rubbing acetate
(c)
Only net
charges are
shown
woollen cloth
acetate
Both charged
(d) after rubbing
Only net
charges are
shown
woollen cloth
3. Figure 20-5(c) and (d) show the same process but only the net
charges are shown in the figures.
rubbing polythene
(a)
woollen cloth
Only net
charges
are shown
polythene
Both charged
(b) after rubbing
woollen cloth
4. When a polythene strip is rubbed with a dry woollen cloth, electrons
are transferred from the cloth to the polythene strip (Figure 20-4).
Thus, the polythene strip is negatively charge and the woollen cloth
is positively charged.
10
rubbing acetate
(c)
Only net
charges are
shown
woollen cloth
rubbing polythene
(a)
woollen cloth
Only net
charges
are shown
acetate
Both charged
(d) after rubbing
polythene
Both charged
(b) after rubbing
Only net
charges are
shown
woollen cloth
woollen cloth
Note:
1. Equal and opposite charges are produced by a redistribution (再分
配) of some electrons from one material to the other during rubbing.
⇒ Rubbing only separate charges, because the principle of
conservation of charge said that:
Charge cannot be created nor destroyed.
⇒ The net charge on the strip is equal to the net charge on the
woollen cloth but they are of opposite sign.
11
2. An uncharged object becomes positively charge if it loses electrons
and negatively charge if it gains electrons. Positive charge, i.e.
proton, cannot be transferred by rubbing.
Conductors (導電體) and insulators (絕緣體)
Definitions:
Conductors are materials which allow electrons to readily (容易地)
flow through them.
Insulators are materials, which do not allow electrons to readily flow
through them.
12
free electron
positive ion
electron
positive ion
In metals, some electrons, called free electrons (自由電子), are loosely
held by the atoms and can move freely between atoms (Figure 20-6). In
an insulator, the electrons are tightly bound to the atoms and are not free
to move from place to place (Figure 20-7). Table 20.2 lists some
common conductors and insulators.
Good conductors
Poor conductors
Insulators
metals, especially gold,
silver, copper, aluminium
water
human body
earth
rubber
plastic, e.g. acetate, perspex,
polythene, PVC
carbon
semiconductors, e.g.
germanium, silicon
glass
moist air
dry air
13
Note:
1. An insulator can be charged by friction. It is because the charge on the
insulator remains on it and does not escape easily.
2. Conductors can be charged by friction. However, the charge cannot be
detected, because it is immediately conducted from the conductor to
the hand and then to the earth (Figure 20-8(a)). The flow of electrons
to the earth can be prevented by adding an insulating handle on the
conductor so that the charge cannot not flow away (Figure 20-8(b)).
conductor
conductor
insulating
handle
(a)
electron
flows to
earth
(b)
free electron
14
Attraction of uncharged Objects
Conductors
acetate
conductor
When a positively charged acetate rod
attraction
is placed near a light uncharged
induced
conductor, the free electrons in the
charges
conductor are attracted and move near (a)
repulsion
the positively charged rod (Figure 20attraction > repulsion
9(a)). Therefore, the near side
Only net
becomes negatively charged and the
charges are
shown
far side becomes positively charged.
The positively charged rod attracts the near side and repels the far side.
Because the separation between positive charges on the rod and negative
charges on the conductor is smaller, the attraction is stronger than
repulsion. So there is a net attraction on the conductor and the conductor
is pulled towards the charged object.
15
If a negatively charged polythene rod
is brought near, the free electrons in
the conductor are repelled to the far
side (Figure 20-9(b)). Therefore, the
near side becomes positively charged
and the far side becomes negatively
charged. However, the net force on
the conductor is still attractive
because the positive charges on the
conductor is nearer to the rod than the
negative charges. The conductor is
thus pulled towards the charged
object.
polythene
attraction
(b)
induced
charges
conductor
repulsion
attraction > repulsion
Only net
charges are
shown
16
Film show time
Charge Propelled Cylinder
Start
17
Film show time
Metal Rod Attraction
Start
18
Insulators
When a positively charge rod is
placed near an uncharged insulator,
the electrons and the positively
charge nuclei of the atoms
experience attraction and repulsion
respectively (Figure 20-10). The
electrons tend to move nearer to the
rod and the whole atom is
deformed (使成畸形). The two
ends of each deformed atom bear (
帶) opposite charges. Thus, the
insulator behaves as if it has
negative charges at the near side
acetate
attraction
insulator
electron
positive ion
repulsion
attraction > repulsion
and positive charges at the far side. Since the negative charges on the
insulator is nearer to the rod, attraction is greater than repulsion. Thus,
the insulator is attracted by the charged rod.
19
Note: 1. The charges appear on an uncharged object because of the
presence of nearby charged object are called induced charges (
感生電荷).
20
21
B
22
C
Charging By An E.H.T. Power Supply
An E.H.T. supply (超高壓電源) pulls electrons from the positive
terminal and pushes the electrons to the negative terminal of the
supply.
23
Film show time
20.1 Charging by an EHT supply
Start
24
the same
25
the same as
do
do not
26
Experiment:
wooden rods on
retort stand
1. Connect the circuit as
shown in Figure 20-11(a).
aluminium
Both aluminium strips are
strip
connected to the positive
terminal and the negative
terminal is earthed (接地 E.H.T. power supply
). When the E.H.T. supply
is switched on, electrons
repulsion
are drawn from the strips
Like charges repel
connected to the positive (a)
terminal to the negative
terminal and then to the
earth. The strips lose
electrons and become
positively charged. Hence,
they repel each other.
27
2. Connect the circuit as
shown in Figure 2011(b). Both aluminium
strips are connected to
the negative terminal
and the positive terminal
is earthed. When the
E.H.T. supply is
switched on, electrons
are drawn from the earth
through the positive
terminal to the strips
connected to the
negative terminal. The
strips become negatively
charged and repel each
other.
wooden rods on
retort stand
aluminium
strip
E.H.T. power supply
repulsion
(b)
Like charges repel
28
3. Connect the circuit as
wooden rods on
retort stand
shown in Figure 2012(a). When the supply
aluminium
is switched on,
strip
electrons are drawn
from the strip
E.H.T. power supply
connected to the
positive terminal to the
strip connected to the
attraction
negative terminal. The
Unlike charges
strip connected to the (a)
attract
positive terminal
becomes positively
charged and the strip
connected to the negative terminal becomes negatively charged.
Therefore, they attract each other.
29
4. When an aluminium strip
wooden rods on
is connected to the
retort stand
negative terminal and the
rubbed
positive terminal is
polythene
earthed, the aluminium
strip
strip becomes negatively
E.H.T. power supply
charged. A negatively
charged polythene strip,
aluminium
strip
which is charged by
friction, is then brought
Like charges repel
(b)
near the aluminium strip
(Figure 20-12(b)). They
repel each other.
Therefore, electric charges are the same, whether produced by
friction or a power supply. It is because electrons are identical (完全
相同), only the method of producing the charges changes.
30
Van de Graaff Generator (范德格拉夫起電機)
A Van de Graaff generator (Figure
20-13) produces a large and
continuous supply of electric
charge. A rubber belt is charged by
friction. The charge produced is
carried by the belt to the metal
dome (圓頂) where it is collected.
metal
dome
receives and stores
positive charge
insulating column
rotating
belt
polythene roller
rubber
motor
metal base
31
Film show time
Experiments with Van de Graaff Generator
Start
32
Film show time
20.2 Van de Graaff Generator
- electric forces
Start
33
Being uncharged, the
polystyrene ball is first
attracted to the dome. On
touching the dome, the ball
acquires the same charge as
that on the dome and so is
immediately repelled.
34
Film show time
Van der Graaff Generator
Start
35
Film show time
20.3 Van de Graaff Generator
- hair rising experiment
Start
36
Film show time
20.4 Van de Graaff Generator
- electric sparks
Start
37
The hairs stand on ends. Carrying the same kind of charge, the hairs
repelled one another and so stand on ends.
Sparks are produced as electric charges pass from the dome through the
38
air to the metal sphere and then to the earth.
An electric current passes through the
microammeter. This is due to the flow of
charges from the dome to the earth via
the microammeter.
electric charges
current
39
Film show time
Jumping Pie Plates
Start
40
Experiment:
1. Suspend a polystyrene ball by a
nylon thread from an insulating
rod (Figure 20-14 (a)). Since the
polystyrene ball is initially
uncharged, it is attracted to the
dome. On touching the dome, it
acquires the same charge as that
on the dome and so is repelled.
Like charges repel
(a)
41
2. Connect a small metal
sphere S on a conducting
stand to the base B of the
Van de Graaff generator
(Figure 20-14 (b)). With a
continuous supply of charge
to the dome D, sparks (火花
(b)
) are produced between D
and S if their separation is
small. The charges on the
dome D jump across to S,
flow down the conducting
base and through the
connecting wire to the base
of the generator.
D
spark
S
The flow of
positive
charge
Connecting wire Conducting
base B
42
3. Put a head of hair on the dome
and start the Van de Graaff
generator (Figure -(a)). The hairs
on the dome share some charges
from the dome. Since the hairs
carry the same type of charge,
they repel each other and so stand
on end (豎起).
4. Connect the Van de Graaff
generator to a light beam
galvanometer (光束電流計),
which can measure a small
current, with two wires (Figure (b)). When the generator is
switched on, the light beam
deflects showing that the flow of
charge is the same as the flow of
electric current.
Like
charges
repel
(c)
Flow of
electric
charge is
electric
current
(d)
43
Light beam galvanometer
44
Gold leaf electroscope (金泊檢電器) (Out of Syllabus)
A gold leaf electroscope consists of a
metal cap
piece of gold leaf suspended (懸掛)
from a metal plate with a metal cap
on top. The gold leaf is enclosed
inside a metal case with glass
windows (Figure 20-15). A gold leaf
electroscope is used to
1. detect charges, and
insulation
metal plate
gold leaf
glass window
metal case
2. determine which type of charges
an object has.
45
Detection of charges
When a charged
object (either
positively or
negatively charged) is
brought near an
uncharged
electroscope, the leaf
(a)
(b)
rises showing
When the gold leaf of an uncharged electroscope
charges on the object
rises, a charged object is brought near.
(Figure 20-16). When
the charged object is
removed, the leaf
drops.
46
Determining the sign of charge on an object
To determine the type of charge on an object, an electroscope is charged
so that charges are distributed on the cap, metal plate and the gold leaf.
Since like charges repel, the charges on the metal plate repel the
charges on the gold leaf and therefore the gold leaf rises. If the
electroscope gains more charges, the leaf rises more due to larger
repulsion.
47
Induced charges
on an initially
uncharged
object
(a)
(b)
(c)
Induced charges
on an initially
uncharged
object
(d)
(e)
(f)
When the gold leaf of a charged electroscope rises, a charged
object with the same kind of charge as the electroscope is
brought near.
48
From Figure 20-17, the only sure test of charge on a body is to obtain a
rise of the gold-leaf of a charged electroscope. The charge on the
electroscope and the test charge are of the same kind. Table 20.3
summarizes the result in Figure 20-17.
Charge on
electroscope
Charge brought near
electroscope
Effect on gold-leaf
+
+
rises
-
-
rises
+
-
drops
-
+
drops
+ or -
Uncharged body
drops
49
Film show time
Charging By Sharing (授受起電)
Start
50
Experiment:
1. Charge two electroscopes with the same
amount of charge with an E.H.T. supply.
metal plate
insulating
handle
(a)
2. Use two metal plates of different sizes to touch
the electroscopes and observe the drops of the
gold leaves when the plates are removed.
Result and conclusion:
1. After the metal plate is removed, the leaf of the
electroscope drops showing that charges flow
from the electroscope to the metal plate (Figure
20-18).
⇒ Charges flow between charged conductors in
contact giving them both a share of the same
charge. The electroscope is said to be
discharged (放電) and the metal plate is charged
(充電).
⇒ Charges are shared between the two charged
conductors.
51
2. If a larger metal plate touches
the electroscope, the leaf falls
more (Figure 20-18(b)).
⇒ A larger conductor gets a
larger share of charge. If
the conductor is very large,
there is almost no charge left
on the electroscope.
Note:
1. If the size of an uncharged metal ball
B is equal to the size of a charged
metal ball A (Figure 20-19), both
balls will share equal amount of
charge.
(b)
B
A
A
B
Size of A = size of C
⇒ Charge on A = Charge on C
52
2. When two equal size but oppositely
charged conductors are brought together
(Figure 20-20), the negative charges are
neutralized by the positive charges. The
excess positive charges are then shared
equally between the two conductors.
Y
X
X
Y
Size of X = size of Y
⇒ Charge on X = Charge on Y
Earthing (接地)
Definition:
Earthing is the process of
sharing charge with the
earth.
Earth a Negatively Charged Electroscope
Stop
53
Earth a Positively Charged Electroscope
Play
To remove the net charge on a
conductor, an extremely large
conductor is needed to share the
charges with the conductor. The
largest conductor is the earth. When a
charged conductor is connected to the
earth by a metal wire (Figure 20-21),
the net charge flows from the
conductor to the earth, and so
practically no net charge remains on
the conductor. Therefore, the
conductor becomes uncharged.
Charged metal
sphere
electron
flow
connected to
earth
Earth
54
Discharging Through Air
Air contains some positive
ions and electrons. When
charged objects are left in air,
charges of opposite kind will
be attracted towards the
charged objects (Figure 20acetate
22). The charged objects are
then gradually neutralized by
the charges in air and are
electron
discharged. Charges with the
same kind to that of the
charged objects are also
repelled by the objects and so
moved away from them.
A charged object can be discharged easily when:
polythene
Only Net charges
on the rods are
shown
positive ion
1. the air contains more ions which can be done by heating the air with
flame or put a radioactive source in air,
2. the air contains a lot of water, that is, the humidity is high.
55
Charge Distribution On A Conductor
The net charge on a conductor
1. stays on the surface of the
conductor, and
2. is mostly concentrated at the
places where the surfaces are
sharply curved (Figure 2023).
spherical
conductor
pear shaped
conductor
insulating stands
Note: 1. The above results only apply to the surfaces of conductors and
not to insulators because the charge cannot flow to establish
any particular distribution in an insulator.
56
Charging By Induction (感應起電)
Definition:
Electric induction (電感應) is the change in the charge
distribution of an object under the influence of a nearby
charged object.
Charging By Induction: Separating
Conductors
Play
57
Charging by induction: separating conductors
1. Place two insulated metal
spheres A and B in contact and
thus they form a single
conductor (Figure 20-24(a)).
2. Bring a negatively charged rod
near to A (Figure 20-24(b)). The
rod pushes free electrons from A
to B. Therefore a positive charge
is induced on A and a negative
charge is induced on B.
spherical
conductor
B
A
insulating stands
Sphere A and B are put in contact.
(a)
free electrons flow
B
A
Free electrons in sphere B are pulled
towards sphere A.
(b)
58
3. Move B a short distance away
from A while keeping the
charged rod in position (Figure
20-24(c)). Therefore, the excess
electrons on B cannot return to
A.
4. Remove the charged rod (Figure
20-24(d)). A ends up with
positive charge and B with
negative charge. The charges on
A and B spread over their
conducting surfaces so that they
are far enough apart not to affect
each other.
B
A
Sphere A and B are separated while
the charged rod is held in position.
(c)
B
A
Remove the charged rod.
(d)
59
e.g. Two metal spheres can be charged by using a positively charged
rod. Use Figure 20-25 to show how this can be done.
spherical
conductor
B
A
free electrons flow
B
A
insulating stands
(a)
(b)
B
(c)
B
A
A
(d)
Simulation Programs
20.1 Charging by induction and separation
60
Note:
1. The method of charging conductors without contact is called
charging by induction. The charges on the spheres are called induced
charges.
2. Sphere A has received charge of opposite sign to that on the charged
rod. Sphere B has received the same kind of charge as that on the
charged rod.
3. Induction only separates
charges. It does not create
charges. The charges on sphere
A are equal in magnitude to the
charges on sphere B but of the
opposite sign.
B
B
A
A
4. No charge is loss from the
charged rod.
61
Charging By Induction: Earthing a Conductor
Play
62
Charging by induction: earthing a conductor
free electrons flow
spherical
conductor
insulating stands
Electron
B
A
Bring a charged rod near an insulated sphere.
(a) Electrons move from B to A.
flow
B
A
Earth the sphere with a finge r. Electrons are
(b) attracted from earth to replace the missing electrons
on B.
1. Bring a positively charged rod near an insulated metal sphere (Figure
20-26(a)). The free electrons in the sphere are pulled to the side A.
Therefore negative and positive charges are induced on side A and B
respectively.
2. Earth the sphere by touching it with a finger (Figure 20-26(b)). Some
electrons are attracted from the earth to the sphere to replace the
missing electrons on side B. Therefore side B becomes uncharged.
63
B
(c)
A
Remove the finger. Electrons cannot flow
back to earth.
B
A
(d) Remove the charged rod.
3. Remove the finger from the sphere so that electrons cannot flow back
to the earth (Figure 20-26(c)). Therefore, the sphere has a net negative
charge.
4. Take away the charged rod and the sphere is left with a net negative
charge (Figure 20-26(d)). The negative charges spread over the
surface of the sphere to reduce the force of repulsion.
64
e.g. A metal sphere can be charged positively by using a negatively
charged rod. Use Figure 20-27 to show how this can be done.
spherical
conductor
B
insulating
(a) stands
(c)
B
A
B
A
(b)
A
(d)
B
A
Simulation Programs
20.2 Charging by induction and earthing
65
Film show time
Charging an electroscope by induction
Start
66
Film show time
Charging an electroscope by induction
Start
67
Charging an Electroscope Positively By Induction
Play
68
Charge a Metal Plate Positively By Induction
Play
69
Film show time
Pitch Balls
Start
70
Film show time
Beer Can Pitch Balls
Start
71
Note:
1. The induced charge on the sphere is always
opposite to that of the charged rod.
B
2. Insulators cannot be charged by induction,
because the electrons are not free to move in
B
an insulator.
Compare different methods of charging
Table 20.4 compares different methods of charging.
A
A
Methods of charging
Conductor
Insulator
By friction
By E.H.T.
supply
By sharing
By induction
9(with
insulating handle)
9
9
9
9
x
x
x
72
1. For an insulator, the charges cannot move. Once the insulator is
charged, the charges stay in the place where they are produced.
Therefore, insulators can be charged by friction.
2. A conductor readily shares the charges with other conductors in
contact because the charges in a conductor are free to move. However,
an insulator does not share charges with other conductors or
insulators. Therefore an earthed conductor discharges immediately,
but an earthed insulator does not.
73
3. The advantages of charging by induction over charging by sharing are
shown in Table 20.5.
Charging by induction
Charging by sharing
Charge on the original charged rod
remains constant.
Charge on the original charged conductor
is shared.
Can produce many charged conductors by
repeating the process of charging.
Charge on the original charged conductor
is getting less and less as more conductors
are charged.
For charging by induction using single
conductor and earthing, the charges on the
charged conductor always have the
opposite polarity to that of the original
charged rod.
The charged conductor produced has the
same type of charge as the original
charged conductor.
74
C
B
75
ELECTRIC FIELDS
When a charged object B is
brought near another charged
object A (Figure 20-28), B
may move away from or
towards A depending on the
charges on B. Such effects are
the result of an electric force,
which acts on any charged
object B by A. This electric
force can act at a distance (遙
距作用). We call this
influence around a charged
object A its electric field.
B
repulsive force
A
attractive force
B
76
Definitions:
An electric field is a space or a
region where an electric charge
experiences an electric force.
F
F
The direction of an electric field at
a particular place is the direction
of the force it produces on a
positively charged object (Figure
(a)
20-29(a)).
Therefore, the force acting on a
negatively charged object will be in
the opposite direction to that of the
electric field causing the force
(Figure 20-29(a)).
P
The thin arrow shows the direction of
the electric field and the line itself is an
electric field line.
The arrows labelled F show the
direction of the force on a charge in the
electric field.
77
Electric Field Lines
Definition:
An electric field line is a line drawn in an electric field such that its
direction at any point gives the direction of the force on a small
positive charge placed at that point (Figure 20-29).
F
F
F
P
(a)
N
F
(b)
The thin arrow shows the direction of the electric field and the line itself is an
electric field line.
The arrows labelled F show the direction of the force on a charge in the electric
field.
78
If a small positive charge were free to move, it would move along an
electric field line. However, the force acting on a negative charge is
opposite to the direction of the electric field lines (Figure 20-29).
F
F
F
P
(a)
N
F
(b)
Java Applet*
Electric field Pattern
Properties of Electric Field Lines
1. Electric field lines start at a positive charge (Figure 20-29(a)) and
end at a negative charge (Figure 20-29(b)). They do not start or end
in empty space.
79
2. The direction of the electric field lines gives the direction of the
electric field.
Strong field
region
Weak
region
P
(a)
field
Triple the magnitude of the
charge is represented by
triple the electric field lines.
Density of the field lines is
proportional to the
magnitude of electric field at
a place.
P
(b)
3. The number of electric field lines attached to a charge is
proportional to the magnitude of the charge (Figure 20-30).
4. The density of the electric field lines is proportional to the
magnitude of the electric field (Figure 20-30(b)).
80
5. Electric field lines do not have branches and
do not cross each other. It is because electric
field has only one direction at a point
(Figure 20-31).
P
Java Applet*
Electric field Pattern
81
Field Patterns
Metal electrodes of
different shapes
E.H.T. power supply
Experiment:
Shallow glass dish containing caster oil to a depth
about 5 mm, covering the electrodes
1. Place a pair of metal electrodes in a shallow glass dish so that they just
covered by a layer of castor oil (菎油) (Figure 20-32).
2. Connect the electrodes to an E.H.T. power supply, which is set at
about 4000 V.
3. Sprinkle (灑) tiny grains or needles of an insulating material onto the
surface of the oil. (Grass seed, semolina powder (粗粒小麥粉) or
hairs work quite well.)
4. Repeat the experiment with electrodes of different shapes
82
Film show time
20.5 Electric field patterns
Start
83
84
electric field
positive
negative
85
Results:
The needles receive induced
opposite charges at their ends. As
the electric field between the
electrodes causes forces to act on
these charges (Figure 20-33), the
needles become aligned (排列) in
the direction of the electric field.
The electric field line and the shape
of the electric field are made visible
as the needles link together forming
lines between two electrodes.
metal electrodes
semolina powder
86
Figure 20-34 shows
the electric field
patterns produced
by different
electrodes.
Java Applet*
Electric field Pattern
(a)
(c)
Parallel electrodes with
unlike charge
Two like charges on
point electrodes
(b)
(d)
Two unlike charges on
point electrodes
Two like charges on
point electrodes
A point charge and a
straight electrode
(e)
A point charge surrounded by
circular electrode
(f)
87
1. The electric field between two parallel
electrodes are called parallel field (Figure 2034(a)) because the electric field lines are
parallel to each other. The density of the
electric field lines is constant and so the electric
field is a uniform electric field.
(a)
Parallel electrodes
with unlike charge
(e)
A point charge surrounded
by circular electrode
2. In the space between a point charge surrounded
by circular electrode, the electric field lines
extend from the point charge to the circular
electrode along the radii (Figure 20-34(e)). This
electric field is called radial (徑向) field.
3. Electric field lines are perpendicular to metal
surfaces (Figure 20-34(a), (e) & (f))
A point charge and a
straight electrode
(f)
88
B
A
89
HAZARDS AND APPLICATIONS.
Electrostatic Hazards
Charges accumulate (積聚) easily on insulators due to friction. These
charges can be very dangerous and may cause explosion.
Electric shock when getting on or off a car
When a car moves, air is rubbing it
continually. The car is then charged up. A
passenger may get an electric shock when
he touches the car (Figure 20-35). The
chance of getting an electric shock (電震)
can be minimized if the car is earthed by
connecting a metal strip to the car to pass
the charge to the earth (Figure 20-36). For
the same reason, an oil truck carries a
metal chain at the back as it moves.
90
Wheels of aircraft are made of conducting rubber. Any charge built up
on the body during flight is conducted to the ground on landing. A metal
chain is connected to the wheel of an aircraft when it is being refuelled.
If any charge is developed in the tyres, it will pass to the earth through
the chain.
91
Nuisances in everyday life
Static charges cause paper sheets to stick together. They also make dirt to
stick on woollen clothes, fibres, carpets and television screens because of
electrostatic attraction. Dirt attached to magnetic tapes by static charges
can degrade (降級) the quality of sound and picture reproduced by tape
recorders. Therefore, head of tape recorder should be cleaned regularly
to remove this dirt. High quality tapes are anti-static, that is, they are
slightly conducting and are hence not easily charged by friction.
92
Application of Electrostatics
Electrostatic Precipitation (靜電沉澱法)
Film show time
Start
93
Film show time
20.7 Electrostatic Precipitator
Start
94
positively charged
An electrostatic precipitator (靜電
particles are
沉澱器) (Figure 20-37) removes
deposited on the
smoke and dust from the waste
metal plates
gases going up the chimneys of
positively charged
factories and power stations. A wire
wire grid
grid is kept highly charged so that a
metal plates
continuous stream of ions occurs
between the grid and the earthed
chimney
metal plates. The ions attach
themselves to the dust particles in
the gas going up the chimney. The
connection to earth
charged dust particles are now
repelled from the wire grid and
smoke and dust particles
attracted to the earthed plates.
rising with the waste gases
These plates are tapped (輕敲) from
time to time so that the dust and smoke particles fall down the chimney
and are removed. The removed particles are buried in nearby swamp (沼
澤) area and used to make bricks and cement.
95
PhotocopyingPhotocopying
Photocopiers print by an
electrostatic process. The
main part of the machine
is a rotating lightsensitive drum (鼓) onto
which the image of the
document is projected
(Figure 20-38). The
photocopying process is
shown by steps in Figure
20-39.
toner
Light sensitive
drum
negatively
charged toner
Charged grid
paper
96
(a) A charged grid
moves over a light
sensitive drum.
(b) The drum is then
positively charged.
(d) Negative charged
carbon particles
(toner) is poured over
the drum and stick to
the positively charged
image.
(e) A sheet of paper is
passed over the drum
and receives a positive
charge from the grid.
(c) The document is projected on
the drum. Positive charges
disappear in area exposed to
light.
(f) The positively
charged paper attracts
toner from the drum
and forms and image
on it.
(g) The paper is
heated under infrared
light for a few
seconds to fixed the
print.
97
Electrostatic paint spraying
Makers of products like automobile
bodies, file cabinets, and refrigerators
have relied, almost only, on electrostatic
paint spraying for years.
1. The paint droplets (小滴)
emitted from the spray gun are
charged with positive charges.
The similarly charged droplets
repel one another to form a large
cloud (Figure 20-40 (a)).
2. As the charged droplets
approach a target, they induce
negative charges on the target’s
surfaces. These negatively
charged target surfaces pull the
positively charged droplets
toward them (Figure 20-40 (a)).
Positively charged paint droplets
follow the electric field lines
Spray gun
Electric field line
Grounded target
(a)
98
(b)
3. The charged droplets deposit
uniformly on target surfaces
because as each one is deposited
on a target surface, the electrical
charges balance at that place,
making it no longer attractive to
other incoming charged droplets.
Therefore, the charged droplets
following are pulled to
remaining surface places that are
still exerting attractive
electrostatic forces (Figure 20-40
(b))
99
4. A very desirable result of
electrostatic spray systems is
"electrostatic wraparound". By
electrostatic charging of the paint,
the individual paint droplets
follow the electric field lines and
encircle (包圍) the target (Figure
20-41). This means that,
depending on the target, it is often
necessary to spray only from one
direction and the target is painted
"all around".
(a)
(b)
100
Lightning conductor
Action of Points
Electrons
attracted
Positive ions
repelled
causes the
electric wind
Electron flow
flame
Dome of Van de Graaff
generator
Charges
concentrate at the
sharp point
candle
positive ion
electron
101
Film show time
20.6 Point Action
Start
102
Electrons
attracted
Positive ions
repelled
causes the
electric wind
Electron flow
flame
Dome of Van de Graaff
generator
Charges
concentrate at the
sharp point
candle
positive ion
electron
Ordinary air contains a certain number of positive and negative ions.
Therefore, when a pin is connected to a Van de Graaff generator, ions of
the same sign will be strongly repelled and ions of opposite sign will be
strongly attracted (Figure 20-42). When the fast moving ions collide with
air molecules, they often knock electrons out of them, creating more
ions. Thus, a strong stream of ions may be created near a sharp point in a
very short time. If a candle flame is put near the pin (Figure -), the flame
is blown away from the point of the pin by the stream positive ions. Free
electrons or negative ions in the air will also be attracted to the point of
the pin and flow in the opposite direction along the wire back to the
dome of the generator. This sometimes causes part of the flame to be 103
attracted to the pin.
Lightning Conductor
A lightning conductor (避雷針) is a very thick
copper strip which connects to some sharp metal
points fitted above the highest part of a building to a
large metal plate buried deeply in the earth below
the building. The conductor provides a path for
electrons to flow easily in large numbers from the
top of the building to the earth.
104
When a negatively charged
thunder-cloud passes overhead
(Figure 20-43), positive charges
and negative charges are induced Positive ions
spray off the
on the sharp metal points and the points
on the
end of the
earthed plate respectively. The
conductor
negative charges move to the earth.
At the same time, negative ions in Induced positive
charge at the points
the air are attracted to the metal
points and give up their excess
electrons. These electrons then pass
down the conductor and escape to
the earth. The upward stream of
positive ions spreads out and
cancels some of the negative
charges on the cloud. This lowers
the chance of lightning striking the Large metal plate
buried deeply in
house.
damp earth
Negatively charged cloud
induces positive charges on the
ground and the buildings below
Electrons attracted towards
the points on the end of the
conductor
Electrons flow (theses come from
the electrons above the points of
the lightning conductor
Thick copper strip fixed to the
side of the building = the
lightning conductor
Electrons spread negative
charge around in the earth
105
If lightning does occur, the charges pass harmlessly to the earth through
the copper conductor. So damage to the building is prevented.
1
2
106
107
108
mirror
109
C
A
End of Chapter 20
110
Play
Total internal reflection in water
Simulation Programs
3.2 Total Internal Reflection
Film show time
diverging
diverging
behind
Start
111
Initial Position
-100
Initial velocity
400.00
Strobe time
2
0
Present Position
Present velocity
100.00
No
Acceleration
10
Present Time
10.00
Take Strobe
Photo (Yes/No)
Time interval
0.5
400.0
100.0
Start
112
lens (focus an
image on the film)
shutter (control
the length of
time the film is
exposed)
film
(record the
image)
Go to shutter
speed
focusing ring
(move the lens)
aperture ring
(control the size
of the aperture)
aperture
diaphragm
(adjust the
amount of light
entering the
camera)
Go to
aperture
113
Stop
114