Chapter 14

D Electricity and Magnetism

Chapter 14 Electrostatics

14 Electrostatics Practice 14.1 (p. 17)

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When rubbed, the plastic comb is charged by

1

D

friction. Suppose the comb carries a positive

2

D

charge. When the comb is put close to hairs, a

3

D

negative charge is induced on hairs, on the

4

D

side close to the comb; a positive charge is induced on hairs, on the side away from the

5

comb. Since the negative induced charge on tension

hairs is closer to the comb, the attraction between hairs and the comb is larger than the repulsion. Hence, the comb can attract hairs.

repulsion

The case is similar when the comb is

weight

negatively charged. 6

10

(a) True (b) False

computer screens, the screens would be

(c) True

charged by friction. Suppose a screen is

(d) True

positively charged. When dust comes close to

(e) True

it, negative and positive charges are induced on dust, on the side close to the screen and the

(f) True 7

Number of extra electrons =

−9

− 10 − 1.6 × 10 −19

side away from the screen respectively. Since the negative induced charge on dust is closer

= 6.25 × 10

9

8

When we use a piece of cloth to clean TV and

to the screen, the attraction between the screen

The small particles of polystyrene carry

and dust is larger than the repulsion between

electrostatic charge and repel one another.

them. Hence, dust is attracted to the screen.

Therefore they spread around and attach to the

The case is similar when the screen is

inside surface of the plastic bottle. When we

negatively charged.

rub the plastic bottle, the charges of the small

11

This is because as soon as the charges appear

particles and the hand repel each other, thus

on the metal rod after rubbing, they are

causing the small particles flying up and

conducted away to the Earth via the human

down.

body. A metal rod can be charged by rubbing when it is insulated from the human body and the surroundings.

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D Electricity and Magnetism

Chapter 14 Electrostatics

Practice 14.2 (p. 21) 1

D

2

D

3

C

4

(a)

(b)

(c)

(d)

5

For a small positive charge, it will move in the direction of electric field lines. For a small negative charge, it will move in the direction opposite to that of electric field lines.

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D Electricity and Magnetism

Chapter 14 Electrostatics

Revision exercise 14

(1A)

Multiple-choice (p. 24)

(The forces should balance each other.) (c) The net charge on sphere C can be either

Section A 1

B

positive or neutral.

(1A)

2

C

The student is incorrect.

(1A)

3

A

(a) A is negatively charged.

(1A)

4

C

C is positively charged.

(1A)

5

C

6

(HKCEE 2003 Paper II Q44)

7

(HKCEE 2004 Paper II Q27)

8

2

(b) tension force due to C −

(HKCEE 2005 Paper II Q17)

force due to B

A

Section B 9

D

10

B

11

C

12

D

13

(HKCEE 2001 Paper II Q28)

weight

(Correct free-body diagram for A.) (1A)

tension force due to A

Conventional (p. 26)

+

Section A 1

force due to C

B

(a) Sphere A carries a negative charge. (1A) Since like charges repel, the repulsion

weight

between spheres A and B implies that (Correct free-body diagram for B.) (1A)

sphere A carries the same type of charge as sphere B, i.e. a negative charge.

(1A)

(b)

tension +

force due to A C

force due to B

weight

(Correct free-body diagram for C.) (1A) (Correct weight.)

(1A)

(Correct tension.)

(1A)

(Correct electrostatic force: repulsion.)

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D Electricity and Magnetism

(c)

Chapter 14 Electrostatics

(i) If B is

(c) This can be prevented by earthing, i.e.

removed, A and C attract each

connecting the car and the Earth with a

other.

metal chain at the back of the car.

(1A)

(ii) If A and B are interchanged, A will not deflect,

3

5

(1A)

(a) When lightning hit the lightning rod, the

(1A)

metal ball would hit bell A first.

(1A)

B will deflect to the right and (1A)

When lightning hit the lightning rod, bell

C will deflect to the left.

A would be negatively charged. Positive

(1A)

(a) The teeth become positively charged.(1A)

induced charges would appear on the

(b) Negative charge is on the bristles of the

surface of the metal ball facing bell A,

toothbrush.

(1A)

while induced negative charges would

(c) Positively-charged plaque leaves the tooth

appear on the other side of the metal ball.

surface because the plaque and the teeth

(1A)

have the same type of charge and there is

Since unlike charges attract and like

repulsion between them.

charges repel

(1A)

(1A)

Also since the bristles of the toothbrush

and induced positive charges would be

and the plaque have opposite charges, the

closer to the bell A, the metal ball would be

attraction between the plaque and the

attracted towards bell A and hit bell A first.

bristles makes the plaque leave the tooth surface.

(1A)

(1A)

(For effective communication.) (b)

(i)

Sparks are produced as charges

Section B 4

jump from bell A to the ball. (1A)

(a) Charge is accumulated on the body of the car.

(ii) A −

(1A)

− − − −

When the car runs on a road, its tyres and body are charged by friction with road and air respectively.

(1C)

B +− +−

+ +

(1A)

(b) When the driver steps on the ground with his hand touching a car door, the car is earthed through his body.

(Correct shape of electric field lines.)

(1A)

(1A)

If the car body is negatively charged,

(Correct direction of electric field lines.)

electrons flow through the driver from the

(1A)

car to the Earth. (The direction of electron flow reverses if the car is positively charged.)

(1A)

The flow of electrons through the driver causes electric shock.

(1A)

(For effective communication.)

(1C)

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D Electricity and Magnetism

6

(a) (i)

Chapter 14 Electrostatics

When the plastic ruler is rubbed with

7

a duster, electrons are transferred (1A)

enough, sparks can be formed. (1A)

Since the ruler gains extra electrons,

If sparks ignite petrol vapour,

it becomes negatively charged. (1A)

explosion results.

(iii) A metal chain can be used to connect

(1A)

the pipe and the Earth to prevent the

the positively-charged paint droplets repel each other

build-up of charge.

(1A)

(1A)

(b) The granules gain the same type of charge

and spread out much more than

when they pass the funnel.

uncharged paint droplets.

Since like charges repel, the granules repel

(ii) Since unlike charges attract,

(1A)

(1A)

each other. Hence, some of them are

the positively-charged paint droplets

pushed out sideways and miss the

would stick to a negatively-charged

container.

car door better. (c) (i)

(1A)

Therefore, it is very dangerous.

(1A) Since like charges repel,

(1A)

(ii) When the charge built up is large

(ii) A positive charge is left on the duster. (b) (i)

Electron is transferred from one material to the other.

(1A) from the duster to the ruler.

(a) (i)

(1A)

(1A)

Physics in articles (p. 28)

Charges accumulate on a fuel truck. (1A)

1

(a) The droplets of spray with the same charge

(Or other reasonable answers, like

repel each other. Thus tiny droplets will

lightning.)

not accumulate as a large droplet of spray.

(ii) Charges on the fuel truck can be

(1A)

discharged safely by connecting the

The skin is originally neutral. Charge is

truck and the Earth

(1A)

induced on the skin, which is opposite to

with a metal chain.

(1A)

the charge of the droplets.

(1A)

(Or other reasonable answers. For

Droplets are attracted to the skin and stick

lightning, charges on the cloud can be

on it because of the induced charge. (1A)

discharged by using lightning

(For effective communication.)

conductor on the roof of a building.

(1C)

(b) Only a little amount (about 20 millilitres)

Lightning conductor conducts

of new spray is wasted in each spray.(1A)

electrons on a cloud to the Earth.)

(c) If the droplets and the skin have the same charge, the droplets and the skin will repel each other.

(1A)

The droplets will not stick to the skin. (1A)

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D Electricity and Magnetism

2

(a) (i)

Chapter 14 Electrostatics

When a positively-charged rod is put near the electroscope, negative charge is induced on the metal plate. (1A) Positive charge is induced in the metal support and the gold leaf so that the positive charge is further from the positively-charged rod. (1A) Because of the repulsion between like charges, the gold leaf will rise. (1A) (For effective communication.) (1C)

(ii) The gold leaf will drop. (b) (i)

The gold leaf will rise.

(ii) The gold leaf will drop.

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(1A) (1A) (1A)

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