Download Ch 14 Electrostatics

06-07 CE Physcis/Ch 14 Electrostatics
Chapter 14 Electrostatics 靜電學
14.1 Electrostatic phenomena 靜電現象
 Like charges and unlike charges
14.2 Where charges 電荷 come from?
 Charge in atom
 Conservation of charge
14.3 Different charging methods 起電的方法
 Charging insulator by rubbing 摩擦
 Charging conductor by sharing 授受
 Charging conductor by induction 感應
14.4 Electric field 電場
 Electric field line
14.5 Electrostatic hazard 靜電的危害
 Lightning 閃電
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06-07 CE Physcis/Ch 14 Electrostatics
14.1 Electrostatic phenomena (p. 3)
14.1 靜電現象
(p. 3)
Activity 1 Rubbing balloon (p. 3)
1. Electrostatic phenomena (p. 4) Fig. 14.1 (p. 4)
(a) In a dry winter day:
(i) While combing hair with a plastic comb, a
crackling sound 劈啪聲 was heard.
(ii) The hair attracted by a plastic comb.
(iii)The hand got an electric shock.
(b) Reason:
When the comb is rubbed with the hair, they
carry charges.
(c) Static charges:
The charges at rest.
(d) Electrostatic phenomena:
The phenomena due to the static charges.
(e) Other example:
(i) The screens of TV and monitors become
dusty easily.
(ii) Reason:
- The static charges accumulate on them.
- They attract dust particles in the air.
2. Like charges and unlike charges (p. 5)
Fig. 14.2, Fig. 14.3 (p. 5)
(a) (i) Rub two acetate strips 醋酸纖維素薄片
separately with dry cloths and bring them
together. Fig. 14.3(a) (p. 5)
(ii) Result:
They repel each other.
(iii)Reason:
They acquire the same kind of charge after
being rubbed.
(b) (i) Rub two polythene strips 聚乙烯薄片
separately with dry cloths and bring them
together. Fig. 14.3(b) (p. 5)
(ii) Result:
They repel each other.
(iii)Reason:
They acquire the same kind of charge after
being rubbed.
(c) (i) Rub an acetate strip and a polythene strip
separately with dry cloths and bring them
together. Fig. 14.3(c) (p. 5)
(ii) Result:
They attract each other.
(iii)Reason:
They acquire different kinds of charge.
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(d) Conclusion:
(i) There are two kinds of charges, positive and
negative.
(ii) Like charges repel and unlike charges
attract.
(iii)Rubbing causes objects to be electrically
charged.
(iv) Charged objects exert forces on other
objects.
14.2 Where charges come from? (p. 6)
3. Particles in atom (p. 6)
- All substances are made up of atoms.
- It consists of two parts:
(a) Nucleus 原子核:
Made up of
(i) Protons 質子: positively charged particles
(ii) Neutrons 中子: neutral particles (電中性)
的粒子
(b) Electrons:
(i) Negatively charged particles
(ii) Circulating around the nucleus
Class Practice 1 (p. 6)
4. Charge in atom (p. 6)
(a) Unit of charge: coulomb 庫倫(C)
(b) Charges of electrons and protons:
(i) The charge of an electron is 1.6  1019 C.
(ii) The charge of a proton is 1.6  1019 C.
(iii)ve sign means the charge is negative.
(iv) +ve sign means the charge is positive.
(c) One coulomb is the total charge quantity carried
by 6  1018 electrons.
(d) Charge of atoms:
Normally, an atom:
(i) Number of protons
= number of electrons
(ii) Amount of positive charges
= Amount of negative charges
(iii)Carries no net charge
- It is said to be electrically neutral
(uncharged).
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06-07 CE Physcis/Ch 14 Electrostatics
5. Transfer of electrons (p. 7) Fig. 14.4 (p. 7)
(a) When an atom is charged, either it gains or loses
electrons.
(i) When a neutral atom loses electrons, it is
positively charged.
Reason:
It carries more positive charges than negative
charges.
(ii) When a neutral atom gains electrons, it is
negatively charged.
Reason:
It carries more negative charges than positive
charges.
(b) (i) Charging 起電:
When a neutral object is given a net charge.
(ii) Discharging 放電:
When a net charge on the object is removed.
(c) The charging process involves a transfer of
electrons.
。
6. Conservation of charge (p. 7)
The principle of conservation of charge states
that:
(i) The total electric charges of bodies in an
isolated system is a constant.
(ii) When a body loses a certain amount of
charge, the other body gains the same amount of
charge at the same time.
Activity 2 Analogue of charges in atom (p. 8)
14.3 Different charging methods (p. 9)
Activity 3 Charging methods (p. 9)
7. Conductor and insulator (p. 11)
Materials can be classified into two groups:
Conductor 導電體 and insulator 絕緣體.
(a) Conductor:
(i) The charges can move easily from one place
to another.
Reason:
It has many loosely held electrons.
(ii) The electrons are responsible for the
conduction of electricity. So, conductor can
conduct electricity.
(iii)Example:
- Good conductors:
All metals (copper, aluminium, iron)
- Poor conductors:
Stone, human body, wood
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06-07 CE Physcis/Ch 14 Electrostatics
(b) Insulator:
(i) The charges cannot move easily.
Reason:
Its electrons are held tightly by nuclei.
(ii) In normal circumstance, it cannot conduct
electricity.
(iii)Example:
Glass, plastic, paper, air
8. Charging insulator by rubbing (p. 12) Fig. 14.5 (p. 12)
(a) Insulators can be charged by rubbing.
(b) Example:
A polythene rod or an acetate rod is rubbed with
a neutral dry cloth. Fig. 14.5(a) (p. 12)
(i) In the polythene rod, Fig. 14.5(b) (p. 12)
Result:
- Electrons are transferred from the cloth to the
polythene rod.
- The polythene rod is charge negatively while
the cloth is charged positively.
(ii) In the acetate rod, Fig. 14.5(c) (p. 12)
Result:
- Electrons are transferred from the acetate rod
to the cloth.
- The acetate rod is charge positively while the
cloth is charged negatively.
(c) Reason:
(i) The friction between their surfaces provides
energy to the electrons in the outermost shells of
their atoms.
(ii) Electrons are transferred.
(d) Direction of flow of electrons depends on:
the materials of the objects.
(e) (i) Rubbing only reallocates charges in the
objects.
(ii) No charges are created, nor destroyed.
Class Practice 2 (p. 13)
9. Charging conductor by sharing (p. 14)
Fig. 14.6 (p. 14)
(a) The method to charge an insulated metal sphere
negatively:
- Use an insulated conducting rod that carries
negative charges.
(b) Procedures:
(i) Touch the metal sphere with the negatively
charged rod.
(ii) Electrons flow from the rod to the metal
sphere and the metal sphere is charged
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06-07 CE Physcis/Ch 14 Electrostatics
negatively.
(iii)When the rod is removed, the electrons are
distributed over the surface of the metal sphere.
(iv) The sphere is charged by sharing.
10. Uneven sharing 不平均的授受 of charges (p. 14)
Fig. 14.7 (p. 14)
When a small charged sphere is connected to a
large uncharged sphere, most of the charges on
the small sphere move to the large one.
11. Van de Graaff generator 范德格拉夫起電機(p. 15)
Fig. 14.8, Fig. 14.9 (p. 15)
(a) A Van de Graaff generator is used to generate
charges.
(b) How the hairs stand on their ends:
In Van de Graaff generator: Fig. 14.9(a) (p. 15)
(i) When the generator is turned on, the plastic
conveyor belt is charged by rubbing.
(ii) Electric charges are continuously conducted
by the conveyor belt to the metal dome.
(iii)A large amount of charge is accumulated on
the surface of the dome.
In hairs: Fig. 14.9(b) (p. 15)
(i) When the hand touches the dome, the
charges flow from the dome to the body.
(ii) They carry like charges and repel each other.
(iii)The hairs share the charges with the dome
and stand on their ends.
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06-07 CE Physcis/Ch 14 Electrostatics
12. Earthing 接地(p. 16)
(a) Definition:
A charge sharing phenomenon between a body
and the earth.
(b) Symbol of earthing Fig. 14.10 (p. 16)
(c) Process of earthing: Fig. 14.11 (p. 16)
(i)When body touches a charged metal sphere
with a figure, or connects a conducting wire
from the sphere to the ground, the electrons will
move.
- When a positively charged metal sphere is
earthed:
The electrons flow from the earth to the metal
sphere to neutralize it.
- When a negatively charged metal sphere is
earthed:
The electrons on the sphere flow to the earth and
the sphere becomes natural.
(ii) The charges are shared between the sphere
and the earth.
(iii)Since the size of the earth is much greater
than that of the sphere, most of the net electrons
go to the earth.
(iv) The sphere becomes electrically neutral.
13. Charging conductor by induction (p. 16)
Fig. 14.12 (p. 17)
(a) The method to charge a metal sphere negatively:
- Use a positively charged metal rod.
(b) Procedures:
(i) The insulated charged rod close to the metal
sphere induces negative and positive charges on
both sides of the sphere.
Reason:
Positive charges on the rod:
- Attract the negative charges.
- Repel the positive charges on the sphere.
(ii) Touching the sphere with a finger
momentarily makes electrons flow from the
earth to the sphere to neutralize the positive
charges.
(iii)Since the negative charges are attracted by
the positive charges on the rod, they are not free
to move.
(iv) The rod is removed and the sphere carries
negative charges.
(v) The sphere is charged by induction.
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06-07 CE Physcis/Ch 14 Electrostatics
Example 1 (p. 17), Class Practice 3 (p. 18)
14.4 Electric field (p. 19)
14. Electric field (p. 19)
A charged object:
(a) Exerts an electrostatic force (attractive or
repulsive) on nearby objects.
(b) Sets up an electric field in the space around it.
15. Electric field patterns (p. 19)
(a) The method to demonstrate electric field
patterns:
- Use an electric field apparatus
Experiment 14A Electric field (p. 19)
(b) Experimental procedures: Fig. 14.13 (p. 19)
(i) Fill a dish with castor oil and semolina is
sprinkled evenly over its surface.
(ii) Immerse the electrodes in the oil and
connect it to the positive and negative terminals
of an extra high tension (E.H.T.) power supply.
(iii)Earth the negative terminal. Turn on the
power supply.
(c) Result and conclusion: Fig. 14.14 (p. 20)
(i) The semolina experiences an electric force
due to the electric field produced by the
electrodes.
(ii) The semolina lines up to show the electric
field pattern.
(iii)Different shapes of electrodes form different
electric field patterns.
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06-07 CE Physcis/Ch 14 Electrostatics
16. Electric field line (p. 20) Fig. 14.15 (p. 21)
(a) The method to represent an electric field pattern:
- By electric field lines
(b) Properties of electric field lines:
(i) They show the directions of electrostatic
forces experienced by a small positive charge in
an electric field.
(ii) They are directed from positive charges to
negative charges.
(iii) They do not have branches.
(iv) They do not cross one another.
(v) The closer the electric field lines, the
stronger the force.
(c) Example:
A small positive charge (at position X)
experiences an electrostatic force that:
(i) is tangential to the electric field line.
(ii) points towards the negative charge.
Class Practice 4 (p. 21),
STS Corner 1 Applications of electrostatic (p. 22)
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06-07 CE Physcis/Ch 14 Electrostatics
14.5 Electrostatic hazard (p. 24)
STS Corner 2 What to do when caught in a storm? (p. 24)
17. Lightning (p. 25) Fig. 14.16 (p. 25)
(a) Formation of lightning: Fig. 14.17 (p. 25)
(i) Storm clouds gather and rub with air, and the
water and ice particles inside them bump 撞擊
together.
(ii) An enormous 大量 static charge builds up
on the clouds.
(iii)Negative charges gather at the bottom of the
clouds and induce positive charges on the roofs
of buildings.
(iv) They form a very strong electric field.
(v) A flash of lightning 電層與地面之間產生
強大而短暫的電流 is released as a result of
discharging.
(b) Formation of thunder 雷聲:
(i) The most dangerous lightning bolt may go
through the air to the ground at one third of the
speed of light.
(ii) Each flash instantly heat the surrounding air
up to 28 000 C (five times the temperature of
the sun’s surface).
(iii)The air expands very quickly, causing
deafening shock waves (thunder).
18. Lightning conductor 避雷針(p. 26) Fig. 14.18 (p. 26)
(a) Aim:
Prevent damaging to buildings and loss of lives.
(b) Principle:
(i) It is installed on the roof of a building.
(ii) It is a sharp edged metal connected to metal
pipes buried in the ground.
(iii)It provides a conducting path of least
resistance to the charges.
(iv)When lightning does occur, it allows charges
to pass to the ground safely, leaving the building
undamaged.
STS Corner 3 Electrostatic hazards and precautions (p. 26)
STS Corner 4 A brief history of electrostatics (p. 28)
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