Exercise 1.1 Electric Charges.doc

HKDSE Physics Chapter

1

Electricity and Magnetism

Electrostatics

Success is the ability to go from one failure to another with no lossContent of enthusiasm. Grad Comments Corrections e Sir Winston Churchill Exercise 1.1 Electric charges

1

1.

Electric Charges There are two types of charges: Positive charge & negative charge

Electrostatic forces between like charges and unlike charges

2.

Charge in an Atom

3.

Attraction of uncharged conductors and

insulations

2

 Conductor: The FREE electrons are loosely held to nuclei and can travel freely in the conductor.  Insulator:

The electrons are held tightly by nuclei and cannot move easily.

3

4

Electrostatic precipitation

Photocopying

Electrospray

5

HKDSE Physics Electricity and Magnetism – Chapter 1 Electrostatics Exercise 1.1: Electric Charges Name : Part A 1.

2.

3.

Class :

(

)

Date:

Multiple-choice Questions

A rubbed plastic rod attracts (1)

paper bits.

(2)

bits of aluminium foil.

(3)

a stream of running water.

A.

(1) and (2) only

B.

(1) and (3) only

C.

(2) and (3) only

D.

(1), (2) and (3)

Which of the following is/are correct? (1)

Like charges repel.

(2)

Unlike charges repel.

(3)

There are three types of charge: positive, negative and neutral.

A.

(1) only

B.

(2) only

C.

(1) and (3) only

D.

(2) and (3) only

A nucleus is made up of A.

atoms and electrons.

B.

electrons and neutrons.

C.

protons and electrons.

D.

protons and neutrons.

6

4.

5.

6.

Which of the following particles carries positive charge? A.

Atom

B.

Electron

C.

Proton

D.

Neutron

Which of the following is the unit of charge? A.

Kilogram

B.

Newton

C.

Coulomb

D.

Degree Celsius

Why are atoms neutral in normal cases? A. Atoms have equal numbers of protons and electrons. B. Atoms have equal numbers of protons and neutrons. C. Atoms have equal numbers of neutrons and electrons. D. Neutrons are neutral.

7.

When a glass rod is rubbed with a piece of silk cloth, the glass rod becomes positively charged. What happens in the process?

8.

A.

Protons are transferred from the silk cloth to the glass rod.

B.

Protons are transferred from the glass rod to the silk cloth.

C.

Electrons are transferred from the silk cloth to the glass rod.

D.

Electrons are transferred from the glass rod to the silk cloth.

Metal ball X carries charge of –1 C and metal ball Y carries charge of 0.6 C. Then the balls touch each other. What is the overall charge of the balls? A.

–1.6 C

B.

–0.4 C

C.

0.4 C

D.

1.6 C

7

9.

10.

11.

Metals are good conductors because they have movable A.

atoms.

B.

electrons.

C.

neutrons.

D.

protons.

Which of the following are insulators? (1)

Plastics

(2)

Wood

(3)

Copper

A.

(1) and (2) only

B.

(1) and (3) only

C.

(2) and (3) only

D.

(1), (2) and (3)

Two objects P and Q are 20 cm apart. P carries charge of 5  10–9 C and Q carries charge of –3  10–9 C. What is the force acting on P by Q? The permittivity of free space is 8.85  10–12 C2 N–1 m–2.

12.

A.

1.69  10–6 N

B.

6.74  10–7 N

C.

–3.37  10–6 N

D.

–4.24  10–5 N

Which of the following are applications of electrostatics? (1)

Electrostatic precipitation

(2)

Photocopying

(3)

Photocopying

A.

(1) and (2) only

B.

(1) and (3) only

C.

(2) and (3) only

D.

(1), (2) and (3)

8

13.

In the figure below, the balls are hung by nylon threads.

What kinds of charge may the balls carry?

14.

15.

X

Y

Z

A.

positive

negative

negative

B.

positive

positive

positive

C.

neutral

neutral

neutral

D.

positive

neutral

negative

Which of the following methods is the best to prevent charges from accumulating on a petrol tanker? A.

Keep the petrol tanker dry.

B.

Use insulating materials to make the tanker.

C.

Use insulating materials to make the tyres.

D.

Use conducting materials to make the tyres.

Three point charges P, Q and R are arranged at the vertices of an equilateral triangle as shown. P and Q carry positive charges while R carries negative charges of the same magnitude as P and Q. What is the direction of the net electric force acting on P?

A.

→

B.

←

C.

↑

D.

↓

9

16.

17.

The magnitude of the electrostatic force between two charged objects is related to (1)

the signs of the charge carried by the objects.

(2)

the amount of charge carried by the objects.

(3)

the distance between the objects.

A.

(1) only

B.

(2) only

C.

(1) and (3) only

D.

(2) and (3) only

In the figure, two rubbed acetate strips repel each other when brought together. Which of the following is the reason?

18.

19.

A.

The strips carry like charges.

B.

The strips carry unlike charges.

C.

One strip is charged and the other is neutral.

D.

Both strips are neutral.

If a neutral atom contains 3 protons and 4 neutrons, how many electrons does it have? A.

1

B.

3

C.

4

D.

7

Which of the following figures correctly shows the electric force between two charges? A.

B.

C.

D.

10

20.

Two charged objects X and Y are 10 cm apart. The charge on X is 2 × 10−6 C. The electric force acting on it by Y is 7.2 N. What is the charge on Y? (Given  8.85 × 10−12 C2 N−1 m−2)

21.

22.

A.

3 × 10−7 C

B.

4 × 10−6 C

C.

6 × 10−4 C

D.

5 × 10−3 C

Which of the following are caused by electrostatics? (R11) (1)

When you brush your hair with a plastic comb, the comb attracts your hair.

(2)

We are attracted to the earth.

(3)

Lightning strikes a building.

A.

(1) and (2) only

B.

(1) and (3) only

C.

(2) and (3) only

D.

(1), (2) and (3)

The balls shown below carry the same amount of charge of the same sign.

What is the direction of the electric force acting on ball Y? A.

→

B.

←

C.

↘

D.

↖

□ 11

23.

Metal ball X carries charge of 0.6 C and metal ball Y carries charge of −0.3 C. If they touch each other, which of the following statements is correct? A.

X still carries positive charge.

B.

Y becomes neutral.

C.

Electrons flow from X to Y.

D.

To separate the balls, a force is needed to overcome the electrostatic force between them.

□

24.

When a metal sphere is earthed in the absence of other charged objects, which of the following statements must be correct? (1)

The metal sphere is positive-charged initially.

(2)

The metal sphere is neutral finally.

(3)

Electrons flow form the earth to the metal sphere.

A.

(1) only

B.

(2) only

C.

(3) only

D.

(1), (2) and (3)

□ 25.

Two charged spheres are hung from the ceiling by insulating threads and keep balanced as shown below.

12

Which of the following statements must be correct? (1)

The spheres carry the same type of charge.

(2)

The spheres carry the same amount of charge.

(3)

The spheres have the same mass.

A.

(1) only

B.

(3) only

C.

(1) and (3) only

D.

(1), (2) and (3)

□ 26.

The dome of a Van de Graaff generator is earthed and a positively-charged metal rod is brought near the dome. What is the charge on the dome just before and after the rod touches the dome? Before

After

A.

negative

positive

B.

negative

neutral

C.

neutral

positive

D.

neutral

neutral

□

13

Part B

Structural Questions

1.

Give a method that can charge a plastic object.

2.

Two acetate strips are rubbed by dusters. What will happen when the two rubbed strips are brought together? Explain briefly.

(2 marks)

3.

Give two ways that can reduce the electric force between two charged objects.

4.

Two neutral objects A and B are rubbed against each other and objects A becomes negatively charged.

5.

(1 mark)

(2 marks)

(a)

Is object B negatively charged, positively charged or neutral after rubbing?

(1 mark)

(b)

Explain briefly why object A becomes negatively charged after rubbing.

(1 mark)

(a)

What are conductors?

(1 mark)

(b)

Give two examples of conductors.

(2 marks)

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6.

Two balls X and Y are 40 cm apart. Ball X carries charge of 2 × 10−6 C and ball Y carries charge of −3 × 10−6 C. The permittivity of free space is 8.85 × 10−12 N−1 m−2.

7.

(a)

What is the electric force between them?

(2 marks)

(b)

Is the force attractive or repulsive?

(1 mark)

(c)

If the balls are moved closer to each other, how will the electric force between them change?

(1 mark)

In the figure, a negatively-charged metal sphere is earthed.

(a)

What is earthing? (1 mark)

15

(b)

8.

Why does the sphere become neutral after earthing?

(1 mark)

Ball S hung by a nylon thread is attracted towards object T as shown. The charge of S and T are 6 × 10−6 C and −5 × 10−6 C respectively. Take −C2 N−1 m−2.

(a)

Find the electric force between S and T.

(2 marks)

(b)

Find the mass of S. The acceleration due to gravity is 9.81 m s−2.

(2 marks)

16

9.

(a)

In the figure, a neutral metal-coated polystyrene ball is hung on an insulated rod. It is brought near the dome of device X slowly and is attracted by the dome.

(b)

(i)

Name device X.

(ii)

What will happen if the ball touches the dome? Explain briefly.

(1 mark)

(3 marks)

Bring an earthed metal sphere instead of the insulated ball near the dome. What will happen? Explain briefly. (2 marks)

17

10. Three balls are positioned on a smooth horizontal surface as shown. Both balls X and Z carry charge of 0.6 C and ball Y carries charge of −0.5 C. Ball Y is at the mid-point between balls X and Z.

(a)

The balls are released at the same time. How will Y move just after it is released? Explain briefly. (4 marks)

(b)

Z is removed and X and Y are released at the same time, how will the force acting on Y by X change? How will Y move just after it is released?

11.

(3 marks)

In the figure, a negatively-charged rod is brought near a piece of uncharged aluminium foil from the top.

(a)

Draw the induced charge on the foil in the figure.

(2 marks) 18

(b)

12.

Why does the negatively-charged rod attract the aluminium foil?

(2 marks)

The figure below shows a simplified structure of an atom.

(a)

What is structure X? What is it made up of?

(3 marks)

(b)

What is particle Y? What type of charge does it carry?

(2 marks)

(c)

Briefly explain why an atom is neutral in normal cases.

(2 marks)

19

13.

In the figure, a negatively-charged rod is brought near a neutral insulated metal sphere.

(a)

(b)

(i)

What happens to the protons and electrons in the metal sphere when the rod is brought near?(2 marks)

(ii)

What is the use of the insulating support?

(1 mark)

Then the metal sphere is connected to the earth via a conducting wire when the negatively-charged rod is still next to it. (i)

What particles will flow between the metal sphere and the eartg? In what direction do the particles flow? (2 marks)

(ii)

What will be the charge on the metal sphere?

(1 mark)

(iii) If we want to retain the charge on the metal sphere after the negatively rod is removed, what should we do first, removing the rod or disconnecting the sphere from the earth?

(1 mark) 20

14.

In the figure, two metal spheres are hung from the ceiling by insulating threads.

(a)

Briefly explain why the two metal spheres can keep balanced in this way.

(b)

Suppose that A and B have the same mass of 0.5 kg and the charge of A is 3 × 10−6 C.

(3 marks)

(Given: g = 9.81 m s−2, −C2 N−1 m−2) (i)

Find the charge of B.

(3 marks)

21

(ii)

(c)

Find the tension in the string hanging B.

(2 marks)

If A is neutral and B carries positive charge, can they keep balanced like the situation shown in the figure finally? Why?

(4 marks)

22