Ch13 Solution
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Ch13/P.1
13 Electrostatics 13.1 Electric Charge Warm-up 13.1 (p. 2) 1
2
The smoke is attracted by the ruler. It is because charged objects can attract neutral objects and objects with opposite charge. Since the ruler is charged after rubbing, it can attract neutral smoke particles and oppositely charged smoke particles. The ruler cannot attract the smoke if air is blown to it. When air is blown to the ruler, a thin layer of moisture will cover the ruler and provide a path of lower resistance for charges to flow. Therefore, it is easy for the charge on the ruler to be neutralized and the ruler cannot attract the smoke.
Check-point 1 (p. 7) 1 2 3
B A Negative charges, positive charges, attraction, repulsion
Check-point 2 (p. 13) 1 2 3
D A Friction, sparks, explosion, Earth/ground
Practice 13.1 (p. 14) 1 2 3 4 5 6 7
C D D C D D When rubbed, the plastic comb is charged by friction. Suppose the comb carries a positive charge. When the comb is put close to hairs, a negative charge is induced on hairs, on the side close to the comb; a positive charge is induced on hairs, on the side away from the comb. Since the negative induced charge on hairs is closer to the comb, the attraction between hairs and the comb is larger than the repulsion. Hence, the comb can attract hairs. The case is similar when the comb is negatively charged.
8
When we use a piece of cloth to clean TV and computer screens, the screens would be charged by friction. Suppose a screen is positively charged. When dust comes close to it, negative and positive charges are induced on dust, on the side close to the screen and the side away from the screen respectively. Since the negative induced charge on dust is closer to the screen, the attraction between the screen and dust is larger than the repulsion between them. Hence, dust is attracted to the screen. The case is similar when the screen is negatively charged.
Ch13/P.2
− 10 −9 = 6.25 × 109 − 1.6 × 10 −19
9
Number of extra electrons =
10
This is because as soon as the charges appear on the metal rod after rubbing, they are conducted away to the Earth via the human body. A metal rod can be charged by rubbing when it is insulated from the human body and the surroundings.
13.2 Electric Field Warm-up 13.2 (p. 15) The second statement.
Check-point 3 (p. 17) 1 2 3
B A Positive, stronger
Practice 13.2 (p. 17) 1 2 3 4
D D C
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.
Revision exercise 13 Multiple-choice (p. 21) Section A 1 B 2 C 3 C 4 C 5 D 6 D (HKCEE 2002 Paper II Q31)
Ch13/P.3
Section B 7 D 8 B 9 C 10 D 11 B (HKCEE 2001 Paper II Q28)
Ch13/P.4
Conventional (p. 23) Section A 1 (a) Sphere A carries a negative charge. (1A) Since like charges repel, the repulsion between spheres A and B implies that sphere A carries the same type of charge as sphere B, i.e. a negative charge. (1A) (b)
(c)
2
(a)
Correct weight (1A) Correct tension (1A) Correct electrostatic force (repulsion) (1A) (The forces should balance each other.) Since unlike charges attract, (1A) the attraction between spheres B and C may imply that they carry opposite charges, i.e. sphere C can be positively charged. (1A) Also sphere C can be neutral. (1A) It is because when sphere B gets close to a neutral object, positive and negative induced charges appear on the neutral object, on sides facing and away from sphere B respectively. (1A) Since the electrostatic force decreases with increasing distance between charges, the attraction between sphere B and the positive induced charge is greater than the repulsion between sphere B and the negative induced charge. (1A) This also results in attraction between spheres B and C. (i) The teeth become positively charged. (1A) (ii) The bristles of the toothbrush are negatively charged. (1A) (iii) Positively charged plaque leaves the tooth surface because the plaque and the teeth have the same type of charge and there is repulsion between them. (1A) Also since the bristles of the toothbrush and the plaque have opposite charges, the attraction between the plaque and the bristles makes the plaque leave the tooth surface. (1A)
Section B 3 (a) Charges are accumulated on the car. (1A) When the car runs on a road, its tyres and body are charged by friction with road and air respectively. (1A) (b) (i) When the driver steps on the ground with his hand touching a car door, the car is earthed through his body. (1A) If the car body is negatively charged, electrons flow through the driver from the 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)
Ch13/P.5
(ii) This can be prevented by connecting the car and the Earth with a metal chain at the back of the car. (1A)
Ch13/P.6
4
(a)
(b)
(c)
5
(a)
(b)
(i)
When the plastic ruler is rubbed with a duster, electrons are transferred (1A) from the duster to the ruler. (1A) Since the ruler gains extra electrons, it becomes negatively charged. (1A) (ii) A positive charge is left on the duster. (1A) (i) Since like charges repel, (1A) the positively charged paint droplets repel each other (1A) and spread out much more than uncharged paint droplets. (ii) Since unlike charges attract, (1A) the positively charged paint droplets would stick to a negatively charged car door better. (1A) (i) Charges accumulate on a fuel truck. (1A) (Or other reasonable answers, like lightning.) (ii) Charges on the fuel truck can be discharged safely by connecting the truck and the Earth (1A) with a metal chain. (1A) (Or other reasonable answers. For lightning, charges on the cloud can be discharged by using lightning conductor on the roof of a building. Lightning conductor conducts electrons on a cloud to the Earth.) (i) Electrons (ii) When the charge built up is large enough, sparks can be formed. If sparks ignite petrol vapour, explosion results. Therefore, it is very dangerous. (iii) A metal chain can be used to connect the pipe and the Earth to prevent the build-up of charge. The granules gain the same type of charge when they pass the funnel. Since like charges repel, the granules repel each other. Hence, some of them are pushed out sideways and miss the container.
(1A) (1A) (1A)
(1A) (1A) (1A)
Physics in articles (p. 24) (a) (b)
(c)
Electrons, protons and neutrons Any one of the following: − Quarks have fractional elementary charge. − Protons and neutrons are composed of quarks. A proton is composed of one A quark and two B quarks.
(3 × 1A) (1A)
(1A)
13 Electrostatics 13.1 Electric Charge Warm-up 13.1 (p. 2) 1
2
The smoke is attracted by the ruler. It is because charged objects can attract neutral objects and objects with opposite charge. Since the ruler is charged after rubbing, it can attract neutral smoke particles and oppositely charged smoke particles. The ruler cannot attract the smoke if air is blown to it. When air is blown to the ruler, a thin layer of moisture will cover the ruler and provide a path of lower resistance for charges to flow. Therefore, it is easy for the charge on the ruler to be neutralized and the ruler cannot attract the smoke.
Check-point 1 (p. 7) 1 2 3
B A Negative charges, positive charges, attraction, repulsion
Check-point 2 (p. 13) 1 2 3
D A Friction, sparks, explosion, Earth/ground
Practice 13.1 (p. 14) 1 2 3 4 5 6 7
C D D C D D When rubbed, the plastic comb is charged by friction. Suppose the comb carries a positive charge. When the comb is put close to hairs, a negative charge is induced on hairs, on the side close to the comb; a positive charge is induced on hairs, on the side away from the comb. Since the negative induced charge on hairs is closer to the comb, the attraction between hairs and the comb is larger than the repulsion. Hence, the comb can attract hairs. The case is similar when the comb is negatively charged.
8
When we use a piece of cloth to clean TV and computer screens, the screens would be charged by friction. Suppose a screen is positively charged. When dust comes close to it, negative and positive charges are induced on dust, on the side close to the screen and the side away from the screen respectively. Since the negative induced charge on dust is closer to the screen, the attraction between the screen and dust is larger than the repulsion between them. Hence, dust is attracted to the screen. The case is similar when the screen is negatively charged.
Ch13/P.2
− 10 −9 = 6.25 × 109 − 1.6 × 10 −19
9
Number of extra electrons =
10
This is because as soon as the charges appear on the metal rod after rubbing, they are conducted away to the Earth via the human body. A metal rod can be charged by rubbing when it is insulated from the human body and the surroundings.
13.2 Electric Field Warm-up 13.2 (p. 15) The second statement.
Check-point 3 (p. 17) 1 2 3
B A Positive, stronger
Practice 13.2 (p. 17) 1 2 3 4
D D C
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.
Revision exercise 13 Multiple-choice (p. 21) Section A 1 B 2 C 3 C 4 C 5 D 6 D (HKCEE 2002 Paper II Q31)
Ch13/P.3
Section B 7 D 8 B 9 C 10 D 11 B (HKCEE 2001 Paper II Q28)
Ch13/P.4
Conventional (p. 23) Section A 1 (a) Sphere A carries a negative charge. (1A) Since like charges repel, the repulsion between spheres A and B implies that sphere A carries the same type of charge as sphere B, i.e. a negative charge. (1A) (b)
(c)
2
(a)
Correct weight (1A) Correct tension (1A) Correct electrostatic force (repulsion) (1A) (The forces should balance each other.) Since unlike charges attract, (1A) the attraction between spheres B and C may imply that they carry opposite charges, i.e. sphere C can be positively charged. (1A) Also sphere C can be neutral. (1A) It is because when sphere B gets close to a neutral object, positive and negative induced charges appear on the neutral object, on sides facing and away from sphere B respectively. (1A) Since the electrostatic force decreases with increasing distance between charges, the attraction between sphere B and the positive induced charge is greater than the repulsion between sphere B and the negative induced charge. (1A) This also results in attraction between spheres B and C. (i) The teeth become positively charged. (1A) (ii) The bristles of the toothbrush are negatively charged. (1A) (iii) Positively charged plaque leaves the tooth surface because the plaque and the teeth have the same type of charge and there is repulsion between them. (1A) Also since the bristles of the toothbrush and the plaque have opposite charges, the attraction between the plaque and the bristles makes the plaque leave the tooth surface. (1A)
Section B 3 (a) Charges are accumulated on the car. (1A) When the car runs on a road, its tyres and body are charged by friction with road and air respectively. (1A) (b) (i) When the driver steps on the ground with his hand touching a car door, the car is earthed through his body. (1A) If the car body is negatively charged, electrons flow through the driver from the 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)
Ch13/P.5
(ii) This can be prevented by connecting the car and the Earth with a metal chain at the back of the car. (1A)
Ch13/P.6
4
(a)
(b)
(c)
5
(a)
(b)
(i)
When the plastic ruler is rubbed with a duster, electrons are transferred (1A) from the duster to the ruler. (1A) Since the ruler gains extra electrons, it becomes negatively charged. (1A) (ii) A positive charge is left on the duster. (1A) (i) Since like charges repel, (1A) the positively charged paint droplets repel each other (1A) and spread out much more than uncharged paint droplets. (ii) Since unlike charges attract, (1A) the positively charged paint droplets would stick to a negatively charged car door better. (1A) (i) Charges accumulate on a fuel truck. (1A) (Or other reasonable answers, like lightning.) (ii) Charges on the fuel truck can be discharged safely by connecting the truck and the Earth (1A) with a metal chain. (1A) (Or other reasonable answers. For lightning, charges on the cloud can be discharged by using lightning conductor on the roof of a building. Lightning conductor conducts electrons on a cloud to the Earth.) (i) Electrons (ii) When the charge built up is large enough, sparks can be formed. If sparks ignite petrol vapour, explosion results. Therefore, it is very dangerous. (iii) A metal chain can be used to connect the pipe and the Earth to prevent the build-up of charge. The granules gain the same type of charge when they pass the funnel. Since like charges repel, the granules repel each other. Hence, some of them are pushed out sideways and miss the container.
(1A) (1A) (1A)
(1A) (1A) (1A)
Physics in articles (p. 24) (a) (b)
(c)
Electrons, protons and neutrons Any one of the following: − Quarks have fractional elementary charge. − Protons and neutrons are composed of quarks. A proton is composed of one A quark and two B quarks.
(3 × 1A) (1A)
(1A)
