Phy Notes Manhatten 3

Chapter 18 Electrostatics 　 18.1 Electric Charges 18.2 Different Charging Methods 18.3 Relation between Electric Current and Electric Charges 18.4 Electric Fields 18.5 Electrostatic Hazards and Applications Manhattan Press (H.K.) Ltd. © 2001

Section 18.1 Electric Charges • Charges in atom • Conservation of charges

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18.1 Electric charges (SB p. 2)

Electrostatics

Why c an t the hair he com s aft er c b attrac omb ing? t

??

3

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Charges in atom

18.1 Electric charges (SB p. 3)

Charges — charges in atom Atoms consist of nucleus and electrons electrons nucleus

++ + +

-

+

proton neutron

-

4

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-

electron

Charges in atom

18.1 Electric charges (SB p. 3)

Nucleus — consists of protons and neutrons

neutron

nucleus

++ + +

proton

-

+

proton neutron

-

5

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-

electron

Charges in atom

18.1 Electric charges (SB p. 3)

Protons, neutrons and electrons proton carries positive charge nucleus

-

neutron has no charge electron carries negative charge and circulates around nucleus ++ + +

-

+

proton neutron

-

6

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-

electron

Charges in atom

18.1 Electric charges (SB p. 3)

Charges • Unit: coulomb (C) ++ + + -

-

proton: 1.6 × 10–19 C neutron: 0 C electron: –1.6 × 10–19 C

7

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18.1 Electric charges (SB p. 3)

Charges in atom

Electrical neutrality

8

d n a e g r a ch e v i t i s o p A e g r a h c e a negativ er h t o h c a e cancel he T . t e e m y when the e b o t d i a atom is s l a r t u e n y electricall Normally, an atom has an equ al number of electrons an d protons (i.e . carries no n et Manhattan Press (H.K.) Ltd. © 2001 charge)

Charges in atom

18.1 Electric charges (SB p. 3)

Neutralization charges ++ + +

proton 4 × 1.6 × 10–19 C electron –4 × 1.6 × 10–19 C net charge 0

-

neutral

9

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Charges in atom

18.1 Electric charges (SB p. 3)

Positive ions charges ++ + + -

proton 4 × 1.6 × 10–19 C electron –3 × 1.6 × 10–19 C - net charge 1.6 × 10–19 C

carries positive charge

lost electron 10

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Charges in atom

18.1 Electric charges (SB p. 3)

Negative ions charges proton 4 × 1.6 × 10–19 C electron –5 × 1.6 × 10–19 C - net charge –1.6 × 10–19 C

++ + + -

carries negative charge

gain electron 11

-

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Conservation of charges

18.1 Electric charges (SB p. 4)

Conservation of charges When a body loses a certain amount of charge, another body will gain the same amount of charge at the same time transfer of electron

-

-

++ + +

++ + + -

12

-

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-

Section 18.2 Different Charging Methods • Charging insulators • Charging conductors

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18.2 Different charging methods (SB p. 4)

Conductors and insulators Insulators

Conductors

earth

silver

human bodies

copper water

aluminium

wood

iron

14

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18.2 Different charging methods (SB p. 4)

Free electrons and immobile electrons Insulators

Conductors free electrons

-

- - - + + + +

+

+

+

+

+ -

-

15

-

- + + + - -

electrons that are held tightly by nuclei

+ - + - + - + + - + - + - + + - + - + - + -

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Charging insulators

18.2 Different charging methods (SB p. 4)

Charging The process of converting an electrically neutral object to a charged object

charging

electrically neutral 16

+ + + + + + + + + +

or

positively charged Manhattan Press (H.K.) Ltd. © 2001

-

-

negatively charged

18.2 Different charging methods (SB p. 4)

Charging insulators

Discharging The process of converting a charged object to an electrically neutral object + + + + + + + + + +

or

-

- discharging -

-

positively charged negatively charged 17

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electrically neutral

Charging insulators

18.2 Different charging methods (SB p. 5)

Charging by friction - negatively charged polythene rod

Rubbing a polythene rod with a neutral dry cloth 18

Electrons are transferred from the cloth to the rod

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18.2 Different charging methods (SB p. 5)

Charging insulators

Charging by friction - positively charged acetate rod

Rubbing an acetate rod with a neutral dry cloth 19

Electrons are transferred from the acetate rod to the cloth

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Charging insulators

18.2 Different charging methods (SB p. 5)

Experiment 18A Charging by friction

20

Intro. VCD

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Expt. VCD

18.2 Different charging methods (SB p. 6)

Charging insulators

Experiment 18A Like charges repel two twostrips stripscarry carrylike likecharges charges and andrepel repeleach eachother other rubbed by fingers

21

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18.2 Different charging methods (SB p. 6)

Charging insulators

Experiment 18A Like charges repel rubbed with a piece of woollen cloth

22

two twoballoons balloonscarry carrylike like charges chargesand andrepel repeleach eachother other

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18.2 Different charging methods (SB p. 6)

Charging insulators

Experiment 18A Unlike charges attract two twoballoons balloonscarry carryunlike unlike two balloons charges chargesand andattract attracteach eachother other are rubbed against each other

23

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18.2 Different charging methods (SB p. 7)

Charging insulators

Class Practice 1 ：

If a polythene strip and an acetate strip are rubbed with a dry cloth, explain what happens when the strips are brought close together. attract The strips ____________ each other because the polythene strip and the acetate strip are charged negatively positively ____________ and _______________ respectively. Ans wer 24

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Charging insulators

18.2 Different charging methods (SB p. 7)

Why can a charged object attract a neutral object ? a positively charged ruler

+

+ + + + + + + + + - - - - - + + + + + +

neutral paper scrap

paper scrap is attracted upwards

- - - - - + + + + + +

25

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induced charges

18.2 Different charging methods (SB p. 8)

Charging insulators

Class Practice 2 ： What will happen to the water flowing from a tap if a charged rod is brought close to it? will be attracted The water ___________________ (will be attracted / will be repelled / will not be affected) by the charged rod. It is because water molecules become polarized. The ________________________________________ attractive force between the rod and water molecules is stronger than the repulsive force ______________________ Ans between them.

wer 26

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18.2 Different charging methods (SB p. 8)

Charging conductors

Experiment 18B Charging by EHT power supply Expt. VCD

insulating rods

E.H.T. power supply

metal strips 27

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18.2 Different charging methods (SB p. 9)

Unlike charges

-

28

_

Charging conductors

+ + + + + + + + +

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two twostrips stripsattract attract

18.2 Different charging methods (SB p. 9)

Charging conductors

Like charges _ -

29

_ -

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two twostrips stripsrepel repel

18.2 Different charging methods (SB p. 10)

Charging conductors

Experiment 18C Charging by sharing Expt. VCD

Van de Graaff generator

foam board 30

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Charging conductors

18.2 Different charging methods (SB p. 11)

Experiment 18C -

-

-

-

Van de Graaff generator

31

The hairs stand on their ends!

-

- -

-

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-

-

- - -

--

18.2 Different charging methods (SB p. 11)

Charging conductors

A metal sphere is charged by sharing of charges

32

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18.2 Different charging methods (SB p. 12)

Charging conductors

Sharing of charges between two spheres of different sizes conductor

33

less charges

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more charges

18.2 Different charging methods (SB p. 13)

Charging conductors

Charging by induction positively charged metal rod

_ _ _

_

_

_ metal sphere the sphere _ acquires a _ negative net charge _ _ insulated stand electrons electrons flow flow

34

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18.2 Different charging methods (SB p. 13)

Earthing

35

Charging conductors

All charges of the conductor will move to the earth

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Section 18.3 Relation between Electric Current and Electric Charges

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18.3 Relation between electric current and electric charges (SB p. 14)

Experiment 18D

Electric current and electrostatic charges Expt. VCD

earth socket

37

light beam galvanometer

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18.3 Relation between electric current and electric charges (SB p. 14)

Electric current in a m e r s n o Electr re e h t n e h w stationary to m e h t r o f is no path d e l l a c e r a flow. They s e g r a h c s c i t a t s o r t c ele The flow of electrons is called elect ric current 38

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Section 18.4 Electric Fields

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18.4 Electric fields (SB p. 15)

Experiment 18E

Different electric field patterns EHT power supply

Expt. VCD

– electrode

+ electrode

point electrode

castor oil semolina

40

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18.4 Electric fields (SB p. 15)

Electric field lines a s i d l e i f c i r t A n el ec n a h c i h w n i region e g r a h c c i r t c ele ce r o f a s e c n experie

The pattern s formed by semolina represent th e electric field lines

Electric f ield lines ind icates the direc tion of electri c field Manhattan Press (H.K.) Ltd. © 2001

41

18.4 Electric fields (SB p. 16)

Different electric field patterns

42

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18.4 Electric fields (SB p. 16)

Elec tric fi lines eld are direc from ted posit char ive ge

Positive charge

+

43

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18.4 Electric fields (SB p. 16)

Negative charge

–

44

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Elec tric fi lines eld are direc towa ted rds t nega he tive char ge

18.4 Electric fields (SB p. 16)

Direction of electric field lines

+

45

–

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18.4 Electric fields (SB p. 16)

Electric field lines

Do not cross one another

Do not have branches

Each point of the electric field has one direction only 46

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18.4 Electric fields (SB p. 17)

Class Practice 3 ： Sketch the electric field lines between the electrodes in the electric field apparatus shown below:

Ans wer 47

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Section 18.5 Electrostatic Hazards and Applications • Electrostatic hazards • Electrostatic applications

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18.5 Electrostatic hazards and applications (SB p. 17) Electrostatic hazards

Lightning -

-

-

-

-

+ +

+

-

-

-

+

+ +

49

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-

-

-

-

18.5 Electrostatic hazards and applications (SB p. 18) Electrostatic hazards

Lightning conductor -

-

-

-

-

+ +

+

+ +

50

-

+ -

-

-

-

install a lightning conductor

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-

-

-

18.5 Electrostatic hazards and applications (SB p. 20) Electrostatic hazards

Oil tanker

metal chain connected to the ground

51

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18.5 Electrostatic hazards and applications (SB p. 20) Electrostatic hazards

Aircraft landing

aircraft tyre made from conducting rubber

52

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18.5 Electrostatic hazards and applications (SB p. 21) Electrostatic hazards

Electrostatic nuisance The screens of TV sets, monitors and plastics attract dust particles

On dry days, touching metallic doors may get electric shock 53

When taking off woollen clothes, crackling sound will be heard CD, VCD attract dust particles easily

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18.5 Electrostatic hazards and applications (SB p. 21) Electrostatic applications

Electrostatic precipitator

54

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18.5 Electrostatic hazards and applications (SB p. 21) Electrostatic applications

Electrostatic precipitator

out high voltage source

clean gas positively charged smoke particles attract to side wall

negatively charged side wall exhaust gas

positively charged central part

in

55

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18.5 Electrostatic hazards and applications (SB p. 22) Electrostatic applications

Electrostatic spraying negatively negatively charged droplet charged droplet

positively charged metal surface

nozzle nozzle of of spray spray gun gun

56

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Chapter 19 Circuits

19.1 Electric Circuit 19.2 Electromotive Force and Potential Difference 19.3 Ohm’s law and Resistance 19.4 Simple Circuits 19.5 Electrical Power and Energy 19.6 Domestic Wiring and Electrical Safety Manhattan Press (H.K.) Ltd. © 2001

Section 19.1 Electric Circuit • Electric current • Circuit diagram

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19.1 Electric circuit (SB p. 30)

Electric circuit

Combinatio n of different electrical components

59

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19.1 Electric circuit (SB p. 30)

Open circuit

No electric current flows through the circuit. The light bulb doesn’t light the switch is opened

60

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19.1 Electric circuit (SB p. 30)

A closed circuit

the switch is closed

Electric current flows through the circuit. The light bulb lights 61

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Electric current

19.1 Electric circuit (SB p. 31)

Direction of electric current flow of electrons

conventional current

electrons

Conventional current 62

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Electric current

19.1 Electric circuit (SB p. 31)

Electric current Electric current (I)

Electric current = i.e. I =

Charge flow Time taken

Q t

- - - - -- - - - - - - - - - - - - -- -- - - - - - - - - - - -- - - - - - - - - -

time (t) 63

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quantity of charge (Q)

Electric current

19.1 Electric circuit (SB p. 32)

Unit of current: ampere (A) 1 A = 1 C s -1 1 mA (milliampe re) = 10 -3 C s -1

1μ A (microampe re) = 10 -6 C s -1

ammeter 64

milliammeter

microammeter

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Circuit diagram

19.1 Electric circuit (SB p. 33)

Circuit symbols of electric components

battery

65

light bulb

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switch

19.1 Electric circuit (SB p. 33)

Circuit diagram

Circuit symbols of electric components

ammeter

66

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voltmeter

Circuit diagram

19.1 Electric circuit (SB p. 33)

Circuit symbols of electric components

resistor

67

rheostat

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potential divider

19.1 Electric circuit (SB p. 33)

Circuit diagram

Class Practice 1 ： The following figure shows an electric circuit. Draw the circuit diagram for it. Ans wer

68

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Section 19.2 Electromotive Force and Potential Difference • Electromotive force • Potential difference • Relation between e.m.f. and p.d.

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19.2 Electromotive force and potential difference (SB p. 34) Electromotive force

Electromotive force

Electromotive force = e.m.f. =

Or

Energy supplied by the cell Charge through the cell E

Q

e.m.f.

70

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19.2 Electromotive force and potential difference (SB p. 35) Potential difference

Potential difference

Potential difference =

V =

Or

Energy converted into other forms between two points Charge through the points E Q

potential difference (V)

71

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19.2 Electromotive force and potential difference (SB p. 36) Potential difference

Connection of ammeter and voltmeter

A

I V V

72

ammeter in series

voltmeter in parallel

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19.2 Electromotive force and potential difference (SB p. 36) Relation between e.m.f. and p.d.

electromotive force = 6 V I

6V

2V

4V

V

V

2V

4V potential difference = 6 V

73

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Section 19.3 Ohm’s law and Resistance • Ohm’s law • Change of resistance with temperature • Change of resistance with dimensions of a wire • Resistor and rheostat Manhattan Press (H.K.) Ltd. © 2001

19.3 Ohm’s law and resistance (SB p. 37) Intro. VCD

Experiment 19A Ohm’s law rheostat

Ohm’s law Expt. VCD

battery switch

voltmeter ammeter

eureka wires

75

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19.3 Ohm’s law and resistance (SB p. 37)

Ohm’s law

Experiment 19A Results Potential 1 difference (V) / V Current (I) / A 0.1

2

3

4

5

6

0.2

0.3

0.4

0.5

0.6

potential difference

V ∝I current 76

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19.3 Ohm’s law and resistance (SB p. 38)

Ohm’s law

Ohm’s law The potential difference across a conductor is directly proportional to the current passing through it, provided that the temperature and other physical conditions remain unchanged

V ∝I 77

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19.3 Ohm’s law and resistance (SB p. 38)

Ohm’s law

Resistance (R) p.d. across conductor Resistance = current through conductor V i.e. R= (Unit : ohm, symbol : Ω) I R I V 78

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Change of resistance with temperature

19.3 Ohm’s law and resistance (SB p. 40)

Experiment 19B Change of resistance with temperature 12 V d.c. supply

switch

Expt. VCD

light bulb

ammeter

voltmeter

79

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Change of resistance with temperature

19.3 Ohm’s law and resistance (SB p. 41)

Experiment 19B Results Potential 1 difference (V) / V Current (I) / A 0.1

2

3

4.5

6

0.2

0.3

0.4

0.5

8

0.6

Potential difference

Do not obey Ohm’s law

80

Current

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Change of resistance with temperature

19.3 Ohm’s law and resistance (SB p. 42)

Temperature , resistance -------------

---------

When the temperature increases, the atoms of the conductor vibrate more violently and hinder the motion of the electrons. Hence, the resistance increases 81

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19.3 Ohm’s law and resistance (SB p. 43) Change of resistance with dimensions of a wire

Experiment 19C Change of resistance with dimensions of wire Expt. VCD battery voltmeter

ammeter

l

82

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eureka wires of different thickness

19.3 Ohm’s law and resistance (SB p. 44) Change of resistance with dimensions of a wire

Class Practice 2 ：

A uniform metal wire has a length of 2 m and a resistance of 10 . If the wire is cut into two halves, what would be the resistance of 1-m wire? 10 = 5Ω By R ∝ l, the resistance = ______________________ 2 If the two 1-m wires are twisted to form a thicker wire, what would be its resistance? The resistance of the wire would be in the range of _________ (0-5  / 5-10  / 10-20 ). Ans 0-5  wer 83

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19.3 Ohm’s law and resistance (SB p. 44)

Resistor and rheostat

Resistor and rheostat

resistors

84

rheostat

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19.3 Ohm’s law and resistance (SB p. 46)

Resistor and rheostat

Rheostat

A

C

85

B

By varying the position of the movable contact of the rheostat, the length of the uniform resistance wire in which the current flows is changed

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19.3 Ohm’s law and resistance (SB p. 47)

Resistor and rheostat

Class Practice 3 ： A student tries to find the resistance of a semiconductor. The data is given below.

Voltage / V 0.1 0.2 0.3 0.4 0.5 0.6 Current / mA 0.01 0.02 0.03 0.04 0.08 0.12 (a) Plot a graph of voltage

voltage

against current. (b) Does the semiconductor obey Ohm’s law? Explain briefly. No. Because the voltage is not proportional to the current 86

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Ans wer

current

Section 19.4 Simple Circuits • Combination of resistors

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19.4 Simple circuits (SB p. 48)

Combination of resistors

Resistors in series



88

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19.4 Simple circuits (SB p. 48)

Combination of resistors

I1 = I2 = I V1 + V2 = V As V = V1 + V2 IR = I1R1 + I2R2

R = R1 + R2 ( I 1= I2 = I )

89

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19.4 Simple circuits (SB p. 48)

Combination of resistors

Resistors in series

R = R1 + R2 + R3 + R4 + ...

90

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19.4 Simple circuits (SB p. 50)

Combination of resistors

Resistors in parallel



91

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19.4 Simple circuits (SB p. 51)

Combination of resistors

V = V1 = V2 As

I = I1 + I2 V V V = + R R1 R2 1 1 1 = + R R1 R2

or 92

R1R2 R= R1 + R2

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19.4 Simple circuits (SB p. 51)

Combination of resistors

Resistors in parallel

1 1 1 1 1 = + + + + ......... R R1 R2 R3 R4

93

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Section 19.5 Electrical Power and Energy • Electrical power • Electrical energy • Electric bill

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19.5 Electrical power and energy (SB p. 53)

Electrical power

Electrical power Electrical energy transferred Electrical power = Time taken E ( Unit : Watt, W ) P= t

95

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19.5 Electrical power and energy (SB p. 53)

Electrical power

Electrical power each second consumes 100 J

220 V 96

each second consumes 60 J

220 V Manhattan Press (H.K.) Ltd. © 2001

19.5 Electrical power and energy (SB p. 53)

Electrical power

Electrical power

E P= t QV ( E = QV ) = t Q  =  V t  ∴ P = IV 97

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19.5 Electrical power and energy (SB p. 54)

Electrical power

Electrical power

P = IV

= I ( IR )

( V = IR )

2

∴P = I R

98

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19.5 Electrical power and energy (SB p. 54)

Electrical power

Electrical power 2

P =I R 2

V  =  R R

V   I =  R 

V2 ∴P = R

99

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19.5 Electrical power and energy (SB p. 54)

Electrical power

P = VI or 2

P =I R or

2

V P= R

100

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Electrical power

19.5 Electrical power and energy (SB p. 54)

Electrical energy

Electrical energy

E = Pt = VIt 2

(P = VI )

(

2

= I Rt P = I R 2

V = t R

101

)

2  V  P =  R 

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   

19.5 Electrical power and energy (SB p. 54)

Electrical energy

Electrical energy

1 kWh = 1 kWh = 1 000 W × 1 h −1

= 1 000 J s × 3 600 s ∴1 kWh = 3.6 × 10 J 6

102

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19.5 Electrical power and energy (SB p. 55)

Electrical energy

Experiment 19D Electrical energy kilowatt-hour meter Expt. VCD rotating disc hair dryer

to mains

103

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19.5 Electrical power and energy (SB p. 55)

Electrical energy

Calculate electrical power Initial kWh meter reading : E1 Final kWh meter reading : E2 Electrical energy consumed by the appliance : E2 – E1 Time : t

E E2 − E1 Electrical Power (P ) = = t t 104

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19.5 Electrical power and energy (SB p. 56)

Electric bill

Class Practice 4 ： Complete the following table: Power Power Appliance /W / kW Kettle

1500

Light bulb 60 Vacuum 800 cleaner Iron 1000

105

Time /h

Energy consumed / kWh

1.5 0.06

3

4.5

100

6

0.8

5

4

1

3

3

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Ans wer

Class Practice 5 ： From the electric bill shown, find the cost of electricity per kWh.

$206.88 Cost for two months =__________________ 25974 − 25734 = 240 Energy consumed = ___________________________ kWh Ans 206 .88 = $0.862 Cost per kWh =________________________ wer 240 106

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Section 19.6 Domestic Wiring and Electrical safety • Electricity supply • Wiring of electric appliance • Domestic wiring • Electrical safety Manhattan Press (H.K.) Ltd. © 2001

19.6 Domestic wiring and electrical safety (SB p. 57) Electricity supply

DC and AC voltage / V

direct directcurrent current(d.c.) (d.c.)

voltage / V time / s time / s

alternating alternatingcurrent current(a.c.) (a.c.) 108

1 cycle

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19.6 Domestic wiring and electrical safety (SB p. 58) Electricity supply

Live wire (L) and neutral wire (N)

L

N

positive and negative voltages appear alternately

109

zero voltage

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19.6 Domestic wiring and electrical safety (SB p. 58) Electricity supply

Alternate change of positive and negative voltages in a live wire

0.01 s 0.01 s L: positive voltage N: zero voltage

110

L: negative voltage N: zero voltage

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19.6 Domestic wiring and electrical safety (SB p. 59) Wiring of electric appliance

Wiring of electric appliance

yellow-green wire is connected to the earth (E) pin blue wire is connected to the neutral (N) pin

111

plug

socket

E L N

cartridge fuse brown wire is connected to the live (L) pin

plastic coated wire

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E N

L

19.6 Domestic wiring and electrical safety (SB p. 60) Wiring of electric appliance

Switch

112

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19.6 Domestic wiring and electrical safety (SB p. 60) Wiring of electric appliance

Fuse

113

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19.6 Domestic wiring and electrical safety (SB p. 61) Wiring of electric appliance

Installing earth wire

fault occurs

electric current flows to the ground 114

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19.6 Domestic wiring and electrical safety (SB p. 61) Wiring of electric appliance

No earth wire 

fault occurs

electric current flows to the ground via the human body

115

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19.6 Domestic wiring and electrical safety (SB p. 62) Domestic wiring

Domestic wiring consumer unit (fuse box)

in parallel lightning circuit

kWh meter

N

main fuse at electric company L to water to aircable heater conditioner high power appliances 116

earth

ring mains

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Class Practice 6 ： On a Christmas tree, the bulbs are connected in series. Each bulb has rated value of “5 V, 5 W”.

(a) Calculate the resistance of one bulb. Ans wer

V 2 52 R= = =5Ω P 5

(b) If each bulb operates at rated value from the mains supply (220 V), how many bulbs can be connected? Number of bulbs =

Mains voltage Voltage of one bulb

=

220 5

= 44

(c) What is the total power output at that time? Total power output = 44 × 5 = 220 W 117

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19.6 Domestic wiring and electrical safety (SB p. 67) Electrical safety

Do not overload a socket

118

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19.6 Domestic wiring and electrical safety (SB p. 67) Electrical safety

Replace the worn leads and do not join wires

119

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19.6 Domestic wiring and electrical safety (SB p. 67) Electrical safety

Pull out the plug before filling an electric kettle

120

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19.6 Domestic wiring and electrical safety (SB p. 68) Electrical safety

Do not run extension leads into the bathroom

121

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19.6 Domestic wiring and electrical safety (SB p. 68) Electrical safety

Do not poke anything into sockets or appliances

122

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Chapter 20 Magnetic Effect of a Current 　 20.1 20.2 20.3 20.4

Magnetic Effect Electromagnet Force on Current-Carrying Conductor in Magnetic Field Moving-Coil Galvanometer Manhattan Press (H.K.) Ltd. © 2001

Section 20.1 Magnetic Effect • Permanent magnet • Magnetic field • Current-carrying conductors

Manhattan Press (H.K.) Ltd. © 2001

Permanent magnet

20.1 Magnetic effect (SB p. 87)

Permanent magnet t e n g a m y Ever e l o p h t r o has n and south pole

North pointing to Arctic 125

South

N

S

Manhattan Press (H.K.) Ltd. © 2001

pointing to Antarctic

Permanent magnet

20.1 Magnetic effect (SB p. 87)

Like poles repel

N S

S

126

SN

N

N

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N

S

NS

S

Permanent magnet

20.1 Magnetic effect (SB p. 87)

Unlike poles attrac t

N S

S

127

SN

N

S

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S

N

SN

N

Permanent magnet

20.1 Magnetic effect (SB p. 88)

The earth is like a magnet Arctic

Arctic

Antarctic The earth is like a large magnet 128

Antarctic N-poles of magnets point to Arctic

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20.1 Magnetic effect (SB p. 89)

Permanent magnet

Magnetic effect

S

N

Iron objects are magnetized 129

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iron rod

20.1 Magnetic effect (SB p. 89)

Permanent magnet

Which objects can be attracted by magnet? iron gold silver

N

copper

    

aluminium 130

Manhattan Press (H.K.) Ltd. © 2001

20.1 Magnetic effect (SB p. 89)

Permanent magnet

Class Practice 1 ： There are three metals X, Y and Z. X is attracted by a magnet no matter which pole of the magnet is facing it. Y may be attracted or repelled depending on the pole of the magnet. The magnet cannot attract Z at all. From these results, decide which of them is a piece of iron, a piece of aluminium and a Ans magnet. X is a piece of iron. wer Y is magnet. Z is a piece of aluminium. 131

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20.1 Magnetic effect (SB p. 89)

Magnetic field

Magnetic field i ll w t e n g a Am t ic e n g a m a produce e c a p s e h field in t a e k i l t s u j around it, ish l b a t s e l l i charge w ld e i f c i r t c e l an e

As magneti c fields can exert forces on each oth er , two magnet s can attrac t or repel over a distance 132

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Magnetic field

20.1 Magnetic effect (SB p. 90)

Experiment 20A Magnetic field of magnet

133

Intro. VCD Expt. VCD

Manhattan Press (H.K.) Ltd. © 2001

20.1 Magnetic effect (SB p. 91)

Magnetic field patterns bar magnet

134

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Magnetic field

20.1 Magnetic effect (SB p. 91)

Magnetic field patterns Two bar magnets with unlike poles facing each other

135

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Magnetic field

20.1 Magnetic effect (SB p. 91)

Magnetic field patterns Two bar magnets with like poles facing each other

136

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Magnetic field

20.1 Magnetic effect (SB p. 91)

Magnetic field patterns A slab-shaped magnet with unlike poles facing each other

137

Manhattan Press (H.K.) Ltd. © 2001

Magnetic field

20.1 Magnetic effect (SB p. 91)

Magnetic field

Class Practice 2 ：

For a bar magnet, the magnetic field is the strongest at its two _____________. The poles magnetic field lines are directed form one pole to another _____________ pole.

138

Manhattan Press (H.K.) Ltd. © 2001

Ans wer

Magnetic field

20.1 Magnetic effect (SB p. 92)

Plotting a magnetic field line

compass

N

S A magn etic field lin e shows t directio he n of the magne tic field

139

Manhattan Press (H.K.) Ltd. © 2001

20.1 Magnetic effect (SB p. 92)

Magnetic field

Magnetic field lines Strength: higher density of magnetic field lines, greater magnetic field strength

Direction: from north to south 140

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20.1 Magnetic effect (SB p. 92)

Current-carrying conductors

Experiment 20B Magnetic effect of current

Expt. VCD

141

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20.1 Magnetic effect (SB p. 94)

Current-carrying conductors

Straight current-carrying wire

current points upwards 142

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20.1 Magnetic effect (SB p. 94)

Current-carrying conductors

Straight current-carrying wire

143

current points downwar d s Manhattan Press (H.K.) Ltd. © 2001

20.1 Magnetic effect (SB p. 94)

Current-carrying conductors

Right-hand grip rule for current-carrying straight wire field lines

current

right hand

144

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20.1 Magnetic effect (SB p. 94)

Current-carrying conductors

Current flows upwards field lines

current

right hand

145

Manhattan Press (H.K.) Ltd. © 2001

20.1 Magnetic effect (SB p. 94)

Current-carrying conductors

Current flows downwards

current

146

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20.1 Magnetic effect (SB p. 95)

Current-carrying conductors

Current-carrying flat coil

147

Manhattan Press (H.K.) Ltd. © 2001

20.1 Magnetic effect (SB p. 96)

Current-carrying conductors

Right-hand grip rule for current-carrying flat coil Magnetic field points upwards

148

Magnetic field points into the paper

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20.1 Magnetic effect (SB p. 96)

Current-carrying conductors

Current-carrying solenoid

149

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20.1 Magnetic effect (SB p. 97)

Current-carrying conductors

Right-hand rule for current-carrying solenoid

150

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Section 20.2 Electromagnet • Applications of electromagnets

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20.2 Electromagnet (SB p. 98)

Electromagnet consists of soft-iron core and solenoid low voltage d.c. power supply

soft iron U-cores

152

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20.2 Electromagnet (SB p. 98)

Experiment 20C Electromagnet

low voltage d.c. power supply

soft iron cores 153

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Expt. VCD

20.2 Electromagnet (SB p. 99)

Showing direction of magnetic field

When the power supply is turned on, the needle of the compass shows the direction of the magnetic field 154

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20.2 Electromagnet (SB p. 99)

Strength of magnetic field

number of turns of wire

Number of turns of wire ↑ Magnetic field strength ↑ 155

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20.2 Electromagnet (SB p. 99)

Direction of magnetic field S

Direction of magnetic field depends on direction of current 156

Manhattan Press (H.K.) Ltd. © 2001

N

20.2 Electromagnet (SB p. 99)

Applications of electromagnets

Applications of electromagnets

157

l l e b c i r t c e El

Telephone receiver

Tickertape timer

n i e n Cra d r a y p scra

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20.2 Electromagnet (SB p. 100)

Applications of electromagnets

Experiment 20D Model electric bell

158

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Expt. VCD

20.2 Electromagnet (SB p. 100)

Applications of electromagnets

A, B are in contact

Working principle of a model electric bell supporting block

B A

the circuit is closed

bladescrew

the electromagnet attracts the blade the blade bends downwards

to low voltage wire d.c. power supply

tape

A, B are not in contact

the electromagnet loses its magnetism the blade rebounds upwards 159

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20.2 Electromagnet (SB p. 101)

Applications of electromagnets

Telephone receiver

earpiece plate

mouthpiece

electromagnet

160

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20.2 Electromagnet (SB p. 101)

Applications of electromagnets

Structure of telephone receiver

sound wave

diaphragm carbon granules

speaker mouthpiece

161

iron electromagnet diaphragm

permanent magnet

Manhattan Press (H.K.) Ltd. © 2001

earpiece

20.2 Electromagnet (SB p. 102)

Applications of electromagnets

Ticker-tape timer Current

iron strip dipper

spring

coil 162

paper tape

Time diode

electromagnet Manhattan Press (H.K.) Ltd. © 2001

20.2 Electromagnet (SB p. 102)

Applications of electromagnets

Crane in scrapyard

163

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Section 20.3 Force on Current-Carrying Conductor in Magnetic Field • Fleming’s left hand rule • Moving-coil loudspeaker • Electric motors

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20.3 Force on current-carrying conductor in magnetic field (SB p. 103)

Experiment 20E Magnetic force on conductor 12 V d.c. power supply

Fleming’s apparatus 165

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Expt. VCD

20.3 Force on current-carrying conductor in magnetic field (SB p. 103)

When a current flows through the rider, the rider moves 12 V d.c. power supply the rider moves

rider

Fleming’s apparatus 166

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20.3 Force on current-carrying conductor in magnetic field (SB p. 104) Fleming’s left hand rule

Fleming’s left-hand rule force

field

current

167

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20.3 Force on current-carrying conductor in magnetic field (SB p. 104) Fleming’s left hand rule

Force acting currentcarrying conductor in B-field

force field

current current

168

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20.3 Force on current-carrying conductor in magnetic field (SB p. 104) Fleming’s left hand rule

Turning effect of a coil

field current force

169

Manhattan Press (H.K.) Ltd. © 2001

Class Practice 3 ： A copper rod is placed on an open circuit. When the switch is closed and the resistance of the rheostat is increased gradually, state and explain the motion of the rod when it is placed at positions AB and CD. At AB: The rod does not move because there is no magnetic field around it.

At CD:

The rod moves to the right according to Fleming’s left hand rule. It moves at a decreasing acceleration because the current is decreasing. 170

Manhattan Press (H.K.) Ltd. © 2001

magnetic field region

Ans wer

20.3 Force on current-carrying conductor in magnetic field (SB p. 106) Moving-coil loudspeaker

Moving-coil loudspeaker

171

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20.3 Force on current-carrying conductor in magnetic field (SB p. 106) Moving-coil loudspeaker

Structure of moving-coil loudspeaker paper cone

permanent magnet voice coil

back panel of receiver 172

speaker terminals

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20.3 Force on current-carrying conductor in magnetic field (SB p. 107) Moving-coil loudspeaker

Working principle of moving-coil loudspeaker magnetic field

force

voice coil

173

force

Manhattan Press (H.K.) Ltd. © 2001

paper cone

20.3 Force on current-carrying conductor in magnetic field (SB p. 107) Electric motors

Electric motors

a.c. drill

d.c. electric fans

d.c. drill

a.c. washing machine

174

Manhattan Press (H.K.) Ltd. © 2001

20.3 Force on current-carrying conductor in magnetic field (SB p. 108) Electric motors

Experiment 20F Model electric motor

Expt. VCD

175

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20.3 Force on current-carrying conductor in magnetic field (SB p. 109) Electric motors

Working principle of electric motor

rotation

commutator carbon brush

176

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20.3 Force on current-carrying conductor in magnetic field (SB p. 109) Electric motors

Rotation of a coil - at the beginning

177

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20.3 Force on current-carrying conductor in magnetic field (SB p. 109) Electric motors

Rotation of a coil - rotate 90°

178

Manhattan Press (H.K.) Ltd. © 2001

20.3 Force on current-carrying conductor in magnetic field (SB p. 109) Electric motors

Rotation of a coil - rotate 180°

179

Manhattan Press (H.K.) Ltd. © 2001

20.3 Force on current-carrying conductor in magnetic field (SB p. 109) Electric motors

Rotation of a coil - rotate 270°

180

Manhattan Press (H.K.) Ltd. © 2001

20.3 Force on current-carrying conductor in magnetic field (SB p. 110) Electric motors

Ways to increase the turning speed of the coil • Increase the current • Use a stronger magnet

• Increase the number of turns of the coil • Use a coil with larger surface area

181

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Section 20.4 Moving-Coil Galvanometer • Conversion of galvanometer to ammeter and voltmeter

Manhattan Press (H.K.) Ltd. © 2001

Electric motors

20.4 Moving-coil galvanometer (SB p. 111)

Moving-coil galvanometer

A galvanometer 183

Circuit symbol Manhattan Press (H.K.) Ltd. © 2001

20.4 Moving-coil galvanometer (SB p. 111)

Experiment 20G Model moving-coil galvanometer Expt. VCD

184

Manhattan Press (H.K.) Ltd. © 2001

20.4 Moving-coil galvanometer (SB p. 111)

Structure of a moving-coil galvanometer about

1 10 2

turns of insulated wire

wire wound in a loose spiral to form a spring thin rod

magnets iron yoke 185

Manhattan Press (H.K.) Ltd. © 2001

straw pointer

split pin

20.4 Moving-coil galvanometer (SB p. 112)

Conversion of milliammeter to galvanometer scale upper hairspring pointer

zero adjuster coil

fixed soft-iron cylinder

pointer counter balance cylinder support

permanent magnet to terminals

lower hairspring insulating support

186

Manhattan Press (H.K.) Ltd. © 2001

20.4 Moving-coil galvanometer (SB p. 113)

Ways to increase the sensitivity of moving-coil galvanometer: • Use a stronger magnet • Use weaker hairsprings • Increase the number of turns of the coil • Increase the surface area of the coil

187

Manhattan Press (H.K.) Ltd. © 2001

20.4 Moving-coil galvanometer (SB p. 114) Conversion of galvanometer to ammeter and voltmeter

Full-scale deflection (f.s.d.)

to terminals 188

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20.4 Moving-coil galvanometer (SB p. 114) Conversion of galvanometer to ammeter and voltmeter

Conversion to ammeter

shunts

189

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20.4 Moving-coil galvanometer (SB p. 114) Conversion of galvanometer to ammeter and voltmeter

Structure of ammeter

ammeter r Ig

I

shunt Rs I − Ig

190

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20.4 Moving-coil galvanometer (SB p. 114) Conversion of galvanometer to ammeter and voltmeter s

Resistance of shunt (R )

Voltage across the galvanometer = Voltage across the shunt

Ig r = (I − Ig ) Rs Rs = Rs = 191

Ig r I − Ig Ig r I

Manhattan Press (H.K.) Ltd. © 2001

(∴ I >> Ig )

20.4 Moving-coil galvanometer (SB p. 115) Conversion of galvanometer to ammeter and voltmeter

Conversion to voltmeter

multipliers

192

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20.4 Moving-coil galvanometer (SB p. 115) Conversion of galvanometer to ammeter and voltmeter

Structure of a voltmeter

voltmeter multiplier r Ig

Rm

Ig

V

193

Manhattan Press (H.K.) Ltd. © 2001

20.4 Moving-coil galvanometer (SB p. 115) Conversion of galvanometer to ammeter and voltmeter

Resistance of a multiplier

Voltage across the voltmeter = Voltage across the galvanometer + Voltage across the multiplier

V = Ig r + Ig Rm = Ig (r + Rm ) Rm = 194

V Ig

−r

Manhattan Press (H.K.) Ltd. © 2001

Class Practice 4 ： A milliammeter of resistance 100  and full-scale deflection current 10 mA is converted to an ammeter by using a resistor of resistance 11 . (a) Draw a circuit to show how the milliammeter can be converted to the ammeter.

Ans wer

(b) Find the maximum current that can be measured by the ammeter.

I g r = ( I − I g ) Rs (10 × 10− 3 )(100) = ( I − 10 × 10− 3 )(11) 1 = 11I − 0.11

195

Manhattan Press (H.K.) Ltd. © 2001

I = 0.101A

Class Practice 5 ： A milliammeter of resistance 1 000  and full-scale deflection current 10-3 A is converted to a voltmeter of full-scale deflection voltage 10 V. (a) Draw a circuit to show the connection.

(b) Find the resistance of the multiplier and the voltmeter. V = I g r + I g Rm = I g ( r + Rm ) 10 = (10− 3 )(1 000 + Rm ) ∴ Rm = 9 000 Ω

196

Resistanceof the voltmeter= Resistanceof milliammeter + Resistanceof multiplier = 1 000 + 9 000 Manhattan Ltd. © 2001 = 10 000 ΩPress = 10 k(H.K.) Ω

Ans wer

Chapter 21 Electromagnetic Induction 　 21.1 21.2 21.3 21.4

Induced EMF and Induced Current Generators Transformer Transmission of Electrical Energy

Manhattan Press (H.K.) Ltd. © 2001

Section 21.1 Induced EMF and Induced Current • •

Lenz’s law Induced e.m.f. and induced current in a conducting wire • Applications of induced e.m.f. in coils Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 133)

Experiment 21A Electromagnetic induction

Intro. VCD Expt. VCD

light-beam galvanometer

magnet 199

coil

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 133)

Tests: 1. 　 Insert a bar magnet into the coil. current flows

2. 　 Hold the magnet still inside the coil.

3. 　 Withdraw the magnet from the coil. current flows

200

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 133)

Ways to increase the induced e.m.f.:

• Move the magnet faster • Use a stronger magnet • Increase the number of turns in the coil

201

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 133)

Faraday’s law of electromagnetic induction

conductor + change of magnetic field

e.m.f.

strength of e.m.f.  rate of change of magnetic field 202

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 134)

Lenz’s law

Electromagnetic induction - induced e.m.f.

S

N

induced e.m.f.

203

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 134)

Lenz’s law

Electromagnetic induction - induced current

S

N

induced current 204

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21.1 Induced EMF and induced current (SB p. 134)

Lenz’s law

Lenz’s law

Lenz’s law states that the induced current always flows in a direction such that it opposes the change producing it.

205

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 135)

To prove Lenz’s law repulsive repulsive force force

N

206

Manhattan Press (H.K.) Ltd. © 2001

Lenz’s law

21.1 Induced EMF and induced current (SB p. 135)

To prove Lenz’s law attractive attractive force force

S

207

Manhattan Press (H.K.) Ltd. © 2001

Lenz’s law

21.1 Induced EMF and induced current (SB p. 137) Induced e.m.f. and induced current in a conducting wire

Fleming’s right hand rule motion

field

current 208

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 137) Induced e.m.f. and induced current in a conducting wire

Fleming’s right hand rule

motion current

field 209

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 137) Induced e.m.f. and induced current in a conducting wire

Fleming’s right hand rule

field

current motion

210

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21.1 Induced EMF and induced current (SB p. 138) Induced e.m.f. and induced current in a conducting wire

Ways to increase the induced e.m.f.

e r i w e h t e Mov r e t s a f 211

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 138) Induced e.m.f. and induced current in a conducting wire

Ways to increase the induced e.m.f.

e h t e s Increa f o r e b num e r i w f turns o 212

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 138) Induced e.m.f. and induced current in a conducting wire

Ways to increase the induced e.m.f.

r e g n o tr s a e Us t e n g a m 213

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 139)

Class Practice 1 ： In which direction will the induced current flow (if any) when the conductor or magnet is moved in the ways shown below? Ans wer

no current

214

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 140)

Class Practice 2 ：

When a copper rod is moving along a metal frame, an induced current flows as shown. (a) In which direction is the copper rod moving? By using Fleming’s right hand rule, the copper rod is moving to the left.

induced current metal frame

(b) Due to the induced current, the rod is experiencing a force inside the magnetic field. In which direction does this force act on the rod?

215

By using Fleming’s left hand rule or Lenz’s law, the force on the rod acts to Manhattan Press Ltd. © 2001 the right. It opposes the(H.K.) motion of the rod.

Ans wer

21.1 Induced EMF and induced current (SB p. 140) Applications of induced e.m.f. in coils

Applications of induced e.m.f. in coils Moving-coil microphone diaphragm magnet to amplifier moving coil

induced induced e.m.f. e.m.f. 216

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21.1 Induced EMF and induced current (SB p. 141) Applications of induced e.m.f. in coils

Moving-coil microphone

induced current

induced e.m.f.

moving coil

An alternating current is induced in the coil when it vibrates inwards and outwards 217

Manhattan Press (H.K.) Ltd. © 2001

21.1 Induced EMF and induced current (SB p. 141) Applications of induced e.m.f. in coils

Applications of induced e.m.f. in coils Magnetic tape recording and playback current

coil

magnet

magnetic tape 218

Manhattan Press (H.K.) Ltd. © 2001

tape motion

Section 21.2 Generators • A coil moving in a magnetic field • AC generator (alternator) • DC generator (d.c. dynamo) • Bicycle alternator • Alternators in power stations and cars Manhattan Press (H.K.) Ltd. © 2001

21.2 Generators (SB p. 142)

A coil moving in a magnetic field

A coil moving in a magnetic field

220

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21.2 Generators (SB p. 142)

A coil moving in a magnetic field

When the coil starts to turn

the current flows from the right to the left

221

When the plane of the coil is horizontal,  the rate of cutting the magnetic field lines is the highest the induced current is the maximum

Manhattan Press (H.K.) Ltd. © 2001

21.2 Generators (SB p. 142)

A coil moving in a magnetic field

The coil is turned 90°

222

When the plane of the coil is vertical,  no field lines are cut no current is induced After passing the vertical position, no current the induced current recurs, but the direction Manhattan Press (H.K.) Ltd. © 2001is reversed

21.2 Generators (SB p. 143)

A coil moving in a magnetic field

The coil is turned 180° When the plane of the coil is horizontal,

the rate of

the direction of induced current is reversed

223

cutting the magnetic field lines is the highest

the induced current is the maximum Manhattan Press (H.K.) Ltd. © 2001

21.2 Generators (SB p. 143)

A coil moving in a magnetic field

The coil is turned 270°

224

When the plane of the coil is vertical, no field lines are cut  no current is induced

After passing the vertical position, no current  the induced current recurs, but the direction Manhattan Press (H.K.) Ltd. © 2001is reversed

21.2 Generators (SB p. 143)

A coil moving in a magnetic field

The coil is turned 360° The coil is turned to the starting position The coil is turned continuously An alternating current is produced 225

Manhattan Press (H.K.) Ltd. © 2001

21.2 Generators (SB p. 144)

AC generator (alternator)

AC generator (alternator)

rotation

carbon brush

226

a current is induced

slip rings external circuit

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21.2 Generators (SB p. 144)

AC generator (alternator)

Induced e.m.f. and the number of revolutions

227

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21.2 Generators (SB p. 144)

AC generator (alternator)

Ways to increase the e.m.f. • Rotate the coil at a higher speed • Increase the number of turns of the coil • Wind the coil on a soft-iron core (armature) • Use a stronger magnet

228

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21.2 Generators (SB p. 145)

DC generator (d.c. dynamo)

Experiment 21B Model dynamo Expt. VCD

229

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21.2 Generators (SB p. 146)

DC generator (d.c. dynamo)

DC generator (d.c. dynamo)

a current is induced

rotation

commutator carbon brush

230

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21.2 Generators (SB p. 146)

DC generator (d.c. dynamo)

Induced e.m.f. and the number of revolutions

231

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21.2 Generators (SB p. 147)

Bicycle alternator

Bicycle alternator cylindrical magnet (rotor)

driving wheel axle soft iron

coil output terminals

232

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21.2 Generators (SB p. 148)

Experiment 21C Model bicycle alternator Expt. VCD

233

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Bicycle alternator

21.2 Generators (SB p. 149)

Bicycle alternator

Display of output of a bicycle alternator

234

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21.2 Generators (SB p. 150)

Class Practice 3 ： The graph of output voltage against time for an a.c. generator is shown below: (a) Find the peak voltage and the frequency of the output voltage. 5V Peak voltage = ___________ 0.02 s Period = ________________ 1 = 50 Hz 0.02

Frequency = _____________ 235

Manhattan Press (H.K.) Ltd. © 2001

Ans wer

21.2 Generators (SB p. 150)

Class Practice 3 (Cont’d): (b) Draw the new graph when the number of turns of the coil is trebled.

Ans wer 236

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21.2 Generators (SB p. 150) Alternators in power stations and cars

Alternator in power station

237

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Section 21.3 Transformer • Efficiency of a transformer

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21.3 Transformer (SB p. 151)

Transformer

239

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21.3 Transformer (SB p. 151)

Mutual induction of a transformer

at the instant when the switch is closed

240

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21.3 Transformer (SB p. 151)

Mutual induction of a transformer at the instant when the switch is closed

at the instant when the switch is opened

241

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21.3 Transformer (SB p. 152)

Experiment 21D Simple transformer

242

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Expt. VCD

21.3 Transformer (SB p. 153)

Transformer

Vp

Np

Ns

Vs Ns = Vp N p 243

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Vs

21.3 Transformer (SB p. 154)

A step-up transformer soft-iron core

input voltage

primary secondary coil coil

output voltage

(Ns > Np) circuit symbol 244

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21.3 Transformer (SB p. 154)

A step-down transformer soft-iron core

input voltage

primary coil secondary coil

output voltage

(Ns < Np) circuit symbol 245

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21.3 Transformer (SB p. 154)

Find the number of turns in the secondary coil 1 000

:

2 500

Vs Ns = Vp N p 220 V

Vs 2500 = 220 1000 Vs = 550

246

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21.3 Transformer (SB p. 155)

Experiment 21E Winding a transformer Expt. VCD

247

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21.3 Transformer (SB p. 155)

Efficiency of a transformer

Efficiency of a transformer

Efficiency of a transformer =

Output electrical power Input electrical power

Vs Is e= × 100% Vp Ip

248

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× 100%

21.3 Transformer (SB p. 156)

Efficiency of a transformer

Ideal transformer

Vs I s = Vp I p Vs I p = Vp I s Vs I p N s = = Vp I s N p 249

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21.3 Transformer (SB p. 156)

Efficiency of a transformer

Ideal transformer VS Practical transformer

input power

output power transformer

input power

output power transformer power loss

ideal

250

practical

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21.3 Transformer (SB p. 156)

Efficiency of a transformer

Reason for energy loss (1) iron core

input voltage

primary coil secondary coil

output voltage

: s s o l gy r e n e i ze f m o i n s i e rthe coil produce m current i o w t g the coils have s in n y ti c Wa u d n e o c c n e a t s resistance s iheating effect •u s e r r malle icker wire s electrical energy th a e s energy is lost u • converts to heat 251

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21.3 Transformer (SB p. 156)

Efficiency of a transformer

Reason for energy loss (2)

eddy current

iron core

input voltage

primary coil secondary y g r e coil n ee

output ss: lovoltage

f o k c e z a i t s m i a n i m m r o o f t m o s e r f y d a a d e m W t a e l r u o s c n i e n c e o r r n i a a t resis eddy use an l slices that se theinduce a ta primaryincoil currentininmthe t e e r n c e r r u o t c th current in the , y r d e d h t e o produce magnetic field e n h a t e e c on u iron core d e r d an heating effect of energy is lost the iron core 252

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21.3 Transformer (SB p. 157)

Efficiency of a transformer

Reason for energy loss (3) Alternating current through the transformer : s s continuous magnetization and o l gy r e n e an ize of ironiccore c demagnetization m i h n i h m w o t , e s d r y e o z c i t Wa n e o ir is heated gn t a f o m the iron core up s e use a tized and d s reduced i e s n s g o a l y g e m is lost r benergy e n e o s , easily

253

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21.3 Transformer (SB p. 158)

Class Practice 4 ： A transformer is used to step down the 220 V mains supply to run an appliance of 110 V, 550 W. (a) If the primary coil has 1 000 turns, what is the number of turns in the secondary coil? By

N s Vs = N p Vp

Ns 100 = 1 000 220 N s = 500 turns

(b) What is the current drawn by the appliance? 254

P 550 I = = =5 A V Press110 Manhattan (H.K.) Ltd. © 2001

Ans wer

21.3 Transformer (SB p. 158)

Class Practice 4 (Cont’d): (c) It is found that the current drawn from the mains is 2.8 A. What is the efficiency of the Ans transformer? wer

Output power = 550 W

Input power = VI = 220 × 2.8 = 616 W 550 Efficiency = ×100% 616 = 89% 255

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Section 21.4 Transmission of Electrical Energy • Transmission system in Hong Kong

Manhattan Press (H.K.) Ltd. © 2001

21.4 Transmission of electrical energy (SB p. 159)

Experiment 21F Transmission of electrical power

Expt. VCD

transmission line

step -up transformer

257

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step-down transformer

21.4 Transmission of electrical energy (SB p. 160)

Transmission of electricity

258

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21.4 Transmission of electrical energy (SB p. 160)

Power loss of cables

P =I R 2

I 12 V

12 V

cable

259

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21.4 Transmission of electrical energy (SB p. 160)

Power loss of cables Ip

Is

Vp

Vs

Np : Ns

step-up transformer increases the voltage power loss (P) = Is2 R 260

Vs increases, Is decreases energy loss is reduced

Manhattan Press (H.K.) Ltd. © 2001

Class Practice 5 ： A light bulb is connected to a 10 V a.c. supply as shown. The total resistance of the wires is 90  . The current drawn from the supply is 0.1 A. (a) What is the power input in this transmission system? Input power = VI = 10 × 0.1 = 1W (b) What is the power loss in the wires? Power loss = I 2 R = (0.1) 2 × 90 = 0.9W

(c) What is the efficiency of power transmission?

Ans wer

Output power = Input power − Power loss = 1 − 0.9 = 0.1W Output power 0.1 Efficiency = ×100 % = ×100 % = 10 % Input power 1 261

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Class Practice (Cont’d): (d) The same light bulb is connected to the same a.c. supply through transformers. Assume the transformers are ideal ones and the current drawn from the a.c. remains 0.1 A as before. What is the efficiency of the power transmission? Ans  Vp  wer 1    The current in the transmiss ion wire =

I =  0.1  V  p  10   s

= 0.01 A Power loss in wires = I 2 R = (0.01) 2 × 90 = 0.009 W Efficiency = 262

Manhattan Press (H.K.)

1 − 0.009

1 = 99 .1% Ltd. © 2001

× 100 %

21.4 Transmission of electrical energy (SB p. 163) Transmission system in Hong Kong

Power station

263

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21.4 Transmission of electrical energy (SB p. 164) Transmission system in Hong Kong

Transmission system in Hong Kong

400 kV or 275 kV

132 kV

users

33 kV 11 kV step-up transformer at power station

264

380 V

step-down step-down transformers transformer at zone substations Manhattan Press (H.K.) Ltd. © 2001

220 V

Chapter 22 Electronics 　 22.1 22.2 22.3 22.4

Cathode Ray Oscilloscope (CRO) Electronic Devices Logic Gates Applications of Electronic Devices and Logic Gates Manhattan Press (H.K.) Ltd. © 2001

Section 22.1 Cathode Ray Oscilloscope (CRO) • Use of CRO

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22.1 Cathode ray oscilloscope (CRO) (SB p. 180)

Cathode ray oscilloscope (CRO)

267

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22.1 Cathode ray oscilloscope (CRO) (SB p. 181)

Structure of a cathode ray tube deflecting plates

electron gun cathode grid

anode

Y-plates

fluorescent screen

X-plates

filament

vacuum E.H.T. (d.c.)

268

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spot

22.1 Cathode ray oscilloscope (CRO) (SB p. 181)

The panel of CRO

Use of CRO Y-shift Y-input

Y-gain earth terminal time base

X-shift 269

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22.1 Cathode ray oscilloscope (CRO) (SB p. 182)

Display of CRO

to Y-input

no connection to earth terminal

no connection

270

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Use of CRO

22.1 Cathode ray oscilloscope (CRO) (SB p. 182)

Display of CRO

to Y-input

to earth terminal

potential of Y-input > potential of earth terminal 271

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Use of CRO

22.1 Cathode ray oscilloscope (CRO) (SB p. 182)

Display of CRO

to earth terminal

to Y-input

potential of Y-input < potential of earth terminal 272

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Use of CRO

22.1 Cathode ray oscilloscope (CRO) (SB p. 183)

Thermionic tube electric field is applied

273

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Use of CRO

22.1 Cathode ray oscilloscope (CRO) (SB p. 183)

Use of CRO

Principle of thermionic tube positive terminal cathode ray (electron beam)

negative terminal

274

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22.1 Cathode ray oscilloscope (CRO) (SB p. 183)

Use of CRO

Class Practice 1: A bright spot is brought to the centre of the screen of a CRO. Then the following circuit is connected to the two terminals of the CRO as shown.

to Y-input

to earth terninal 275

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22.1 Cathode ray oscilloscope (CRO) (SB p. 184)

Use of CRO

Class Practice 1 (Cont’d): (a) If the Y-gain is 2 V cm-1, sketch the position of the bright spot on the screen. Ans wer

276

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22.1 Cathode ray oscilloscope (CRO) (SB p. 184)

Use of CRO

Class Practice 1 (Cont’d): (b) If now the earth terminal is connected to point A while the Y-input remains at point B, sketch the new position of the spot. Ans wer

277

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22.1 Cathode ray oscilloscope (CRO) (SB p. 184)

Use of CRO

Alternating current (a.c.) peak to peak voltage of a.c.

Voltage across Y-plates

peak peak voltage

peak to peak voltage

Time

peak

278

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22.1 Cathode ray oscilloscope (CRO) (SB p. 185)

Time base circuit

279

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Use of CRO

22.1 Cathode ray oscilloscope (CRO) (SB p. 185)

Use of CRO

Turn on time base

d.c. d.c.voltage voltage

280

a.c. a.c.voltage voltage

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22.1 Cathode ray oscilloscope (CRO) (SB p. 185)

Use of CRO

Peak voltage Amplitude of waveform = 2 div

Peak voltage = Amplitude of waveform (div) × Y-gain (V div–1 ) 281

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22.1 Cathode ray oscilloscope (CRO) (SB p. 186)

Use of CRO

Period and frequency wavelength = 4 div

Period = Wavelength (div) × Time base (s div–1 ) 1 Frequency = Period 282

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22.1 Cathode ray oscilloscope (CRO) (SB p. 186)

Experiment 22A Cathode ray oscilloscope

283

Use of CRO

Intro. VCD Expt. VCD

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22.1 Cathode ray oscilloscope (CRO) (SB p. 188)

Use of CRO

Class Practice 2: The figure shows the waveform on a CRO screen. The time base and Y-gain are set to 1 ms cm-1 and 2 V cm-1 respectively. Sketch the new waveforms in the following figures when (a) the Y-gain is changed to 1 V cm−1 ,

Ans wer 284

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22.1 Cathode ray oscilloscope (CRO) (SB p. 188)

Use of CRO

Class Practice 2 (Cont’d): (b) the time base is further changed to 2 ms cm−1 . Ans wer

285

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Section 22.2 Electronic Devices • • • • • • •

Diode Light emitting diode (LED) Light dependent resistor (LDR) Thermistor Reed switch Reed relay Potential divider Manhattan Press (H.K.) Ltd. © 2001

22.2 Electronic devices (SB p. 189)

Different computers

AAmodern modernlap-top lap-topcomputer computer

The Thefirst firstcomputer computer occupied occupiedan anentire entire room roomwas wasbuilt builtinin1946 1946

287

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Diode

22.2 Electronic devices (SB p. 189)

Diode

circuit circuitsymbol symbol appearance appearance 288

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22.2 Electronic devices (SB p. 190)

Diode

Diode in forward bias

current flows, and the light bulb glows 289

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the resistance is so small that can be neglected

22.2 Electronic devices (SB p. 189)

Diode

Diode in reverse bias

no current flows, the light bulb does not glow 290

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the resistance is very high

Diode

22.2 Electronic devices (SB p. 190)

Half-wave rectification rectification

a.c.

d.c.

to Y-input a.c. power supply

to earth terminal

291

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Diode

22.2 Electronic devices (SB p. 191)

Adaptor adaptor

220 V a.c.

292

3 - 9 V d.c.

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22.2 Electronic devices (SB p. 191)

Light emitting diode (LED)

Light emitting diode (LED)

appearance appearance

293

circuit circuitsymbol symbol

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22.2 Electronic devices (SB p. 191)

Light emitting diode (LED)

Operation of LED

current flows, LED lights up 294

no current flows, LED does not light up

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22.2 Electronic devices (SB p. 192)

Light emitting diode (LED)

Applications of LEDs

295

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22.2 Electronic devices (SB p. 192)

Light emitting diode (LED)

Polarity indicator

current flows clockwise red LED lights 296

current flows anti-clockwise green LED lights

Manhattan Press (H.K.) Ltd. © 2001

22.2 Electronic devices (SB p. 193)

Class Practice 3: A student uses four diodes to construct a rectifying circuit as shown below. The a.c. voltage, supplied by a signal generator, is shown in Fig. a. Sketch the output voltage.

to Yinput

Fig. a 297

to earth terminal

Ans wer Manhattan Press (H.K.) Ltd. © 2001

22.2 Electronic devices (SB p. 193)

Light dependent resistor (LDR)

Light dependent resistor (LDR)

circuit circuitsymbol symbol appearance appearance

298

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22.2 Electronic devices (SB p. 194)

Light dependent resistor (LDR)

Operation of LDR light

resistance

299

no light

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resistance

Thermistor

22.2 Electronic devices (SB p. 194)

Thermistor

circuit circuitsymbol symbol

appearance appearance 300

Manhattan Press (H.K.) Ltd. © 2001

22.2 Electronic devices (SB p. 194)

Thermistor

Operation of thermistor

301

low temperature

high resistance

high temperature

low resistance

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Thermistor

22.2 Electronic devices (SB p. 195)

Overheat warning circuit When the temperature of thermistor rises,  resistance decreases  brightness of the bulb increases 302

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Reed switch

22.2 Electronic devices (SB p. 195)

Reed switch

circuit circuitsymbol symbol

appearance appearance 303

Manhattan Press (H.K.) Ltd. © 2001

22.2 Electronic devices (SB p. 195)

Reed switch

Reed switch

current flows

magnet 304

Manhattan Press (H.K.) Ltd. © 2001

Reed switch

22.2 Electronic devices (SB p. 195)

Operation of reed switch no magnet, the buzzer does not sound

305

a magnet is present, the buzzer sounds

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Reed relay

22.2 Electronic devices (SB p. 196)

Reed relay

circuit circuitsymbol symbol appearance appearance 306

Manhattan Press (H.K.) Ltd. © 2001

Reed relay

22.2 Electronic devices (SB p. 196)

Potential divider

Vin

Vin Vout

307

Vin Vout

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Vout

Reed relay

22.2 Electronic devices (SB p. 197)

Potential divider

l2 Vout = × Vin l1 + l2

Vin

Vout

308

Manhattan Press (H.K.) Ltd. © 2001

22.2 Electronic devices (SB p. 197)

Reed relay

Potential divider

R2 Vout = ×Vin R1 + R2

Vin

Vout

309

Manhattan Press (H.K.) Ltd. © 2001

Section 22.3 Logic Gates • • • • •

NOT gate AND gate NAND gate OR gate NOR gate

Manhattan Press (H.K.) Ltd. © 2001

22.3 Logic gate (SB p. 199)

Logic gates

NOT gate

OR gate

311

AND gate

NAND gate

NOR gate

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22.3 Logic gate (SB p. 199)

Operation of logic gates +5 V

input A output

input B

0V 312

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22.3 Logic gate (SB p. 200)

Operation of logic gates positive supply rail

+5 V

negative supply rail

0V

The inputs for a logic gate are provided by the positive and negative supply rails When the output is “high”, the LED lights up

+5 V

0V 313

Manhattan Press (H.K.) Ltd. © 2001

22.3 Logic gate (SB p. 201)

Experiment 22B Logic gates

314

Manhattan Press (H.K.) Ltd. © 2001

Expt. VCD

22.3 Logic gate (SB p. 201)

NOT gate high voltage supply rail (+5 V)

input output

NOT

LED

low voltage supply rail (0 V) 315

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22.3 Logic gate (SB p. 201)

Class Practice 4:

Ans wer In the circuit on the left, when the sliding contact B is being moved increases from C to A slowly, the brightness of the LED _______________ ( gradually ( suddenly / gradually ). increases / decreases ) _______________ In the circuit on the right, when the sliding contact B is being moved form A to C slowly, the brightness of the LED suddenly increases _______________ ( increases / decreases ) _______________ (suddenly / gradually). Manhattan Press (H.K.) Ltd. © 2001 316

NOT gate

22.3 Logic gate (SB p. 202)

Truth table of NOT gate

input

output

Input

Output

0

1 00

1

317

Manhattan Press (H.K.) Ltd. © 2001

AND gate

22.3 Logic gate (SB p. 202)

Truth table of AND gate Input A Input B Output Z input A input B

318

output Z

0

0

0

0

1

0

1

0

0

1

1

1

Manhattan Press (H.K.) Ltd. © 2001

NAND gate

22.3 Logic gate (SB p. 203)

Truth table of NAND gate Input A Input B Output Z input A input B

319

output Z

0 0 1 1

Manhattan Press (H.K.) Ltd. © 2001

0 1 0 1

1 1 1 0

NAND gate

22.3 Logic gate (SB p. 203)

Truth table of the combination of AND gate and NOT gate input A input B

320

C

output Z

Input A Input B

C

Output Z

1 1 1 0

0

0

0

0

1

0

1

0

0

1

1

1

Manhattan Press (H.K.) Ltd. © 2001

22.3 Logic gate (SB p. 203)

Class Practice 5: Complete the truth table for the following combinations of logic gates. input A output Z input B

Input A Input B Output Z 0 0 1 0 1 0 1 0 0 1 1 0 321

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Ans wer

22.3 Logic gate (SB p. 203)

Class Practice 5 (Cont’d): Complete the truth table for the following combinations of logic gates. input A

output Z

input B

Input A Input B Output Z 1 unconnected 0 unconnected 1 0

Ans wer 322

Manhattan Press (H.K.) Ltd. © 2001

22.3 Logic gate (SB p. 203)

Class Practice 5 (Cont’d): Complete the truth table for the following combinations of logic gates.

input A input B

output Z

input C

Input A Input B Input C Output Z 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 0

Ans wer 323

Manhattan Press (H.K.) Ltd. © 2001

OR gate

22.3 Logic gate (SB p. 204)

Truth table of OR gate Input A Input B Output Z input A input B

324

output Z

0

0

0

0

1

1

1

0

1

1

1

1

Manhattan Press (H.K.) Ltd. © 2001

NOR gate

22.3 Logic gate (SB p. 204)

Truth table of NOR gate Input A Input B Output Z input A input B

325

output Z

0 0 1 1

Manhattan Press (H.K.) Ltd. © 2001

0 1 0 1

1 0 0 0

22.3 Logic gate (SB p. 205)

Class Practice 6: Match the circuit to the logic gate which it serves. Press the key for a “high” input and the bulb will light up when the output is “high”.

Ans wer NOT gate 326

AND gate

NAND gate

OR gate

Manhattan Press (H.K.) Ltd. © 2001

NOR gate

Section 22.4 Applications of Electronic Devices and Logic Gates • Fire alarm circuit • Thermostat circuit • Car doors warning signal circuit • Burglar alarm circuit • Elevator door controller circuit Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 206)

Electric circuit is divided into three parts Switches and sensors are installed in this part. This triggers the logic level of the input signal to a logic gate

Input circuit

Processing circuit

Output circuit 328

A logic gate is placed in this part

The output signal given by the logic gate operates the device

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 206)

Input circuit Vin

to input of logic gate

0V

329

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22.4 Applications of electronic devices and logic gates (SB p. 207)

Experiment 22C Applications of devices and logic gates Expt. VCD

Fire alarm circuit 330

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22.4 Applications of electronic devices and logic gates (SB p. 210) Fire alarm circuit

Fire alarm circuit

+5V variable resistor “low”

“high” sound

thermistor

temperature

buzzer

resistance 0V

331

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 210) Fire alarm circuit

Fire alarm circuit

+5V variable resistor

A

B alarm

thermistor

buzzer

temperature

0V

332

Temperature

A

B

low high

high low

low high

Manhattan Press (H.K.) Ltd. © 2001

Alarm not sound

sound

22.4 Applications of electronic devices and logic gates (SB p. 211) Thermostat circuit

Thermostat circuit

333

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 211) Thermostat circuit

Thermostat circuit

+5 V

water detector variable resistor

contacts

“high” thermistor

“high” reed relay

“high”

on heater 0V

temperature resistance 334

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 211) Thermostat circuit

Thermostat circuit

+5 V

water level

variable resistor

contacts

temperature

reed relay

heater

thermistor

0V

335

Water Temperature Heater level off low low

high

low

off

low

high

off

high

high

on

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 211) Car doors warning signal circuit

Car doors warning signal circuit

336

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 211) Car doors warning signal circuit

Car doors warning signal circuit “high”

left door is opened

“high”

“low” switches right door is closed

glows

337

Manhattan Press (H.K.) Ltd. © 2001

22.4 Applications of electronic devices and logic gates (SB p. 211) Car doors warning signal circuit

Car doors warning signal circuit

left door

right switches door

338

Left door open close open close Manhattan Press (H.K.) Ltd. © 2001

Right door open open close close

LED glows glows glows not glow

22.4 Applications of electronic devices and logic gates (SB p. 212) Burglar alarm circuit

Burglar alarm circuit

339

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22.4 Applications of electronic devices and logic gates (SB p. 212) Burglar alarm circuit

Burglar alarm circuit

“low”

“high”

“high” buzzer press

340

darkness

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22.4 Applications of electronic devices and logic gates (SB p. 212) Burglar alarm circuit

Burglar alarm circuit alarm

press

light intensity

341

buzzer

Press switch press

Light intensity dark

press not press press

dark light light

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Alarm

not sound sound sound sound

22.4 Applications of electronic devices and logic gates (SB p. 212) Elevator door controller circuit

Elevator door controller circuit

342

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22.4 Applications of electronic devices and logic gates (SB p. 212) Elevator door controller circuit

Elevator door controller circuit

+5 V press “high”

turn off

“low”

“high”

motor light 0V

343

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22.4 Applications of electronic devices and logic gates (SB p. 212) Elevator door controller circuit

Elevator door controller circuit

press switch

+5V

motor light intensity

0V Press switch press

Light intensity dark

not press

dark light light

press not press 344

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Motor

off off off on

Chapter 23 Radioactivity I 23.1 Discovery of Radioactivity 23.2 Detection of Radiation 23.3 Properties of Radiation and Background Radiation 23.4 Biological Hazards and Safety Precautions Manhattan Press (H.K.) Ltd. © 2001

Section 23.1 Discovery of Radioactivity • Antoine Henri Becquerel • Different types of radiation

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23.1 Discovery of radioactivity (SB p. 230)

Antoine Henri Becquerel

Antoine Henri Becquerel He placed some uranium salt on a black paper with a photographic plate inside. Uranium salt make the photographic Conclusion:

plate blacken.

• Uranium salt emits invisible rays spontaneously. • This phenomenon is called radioactivity.

347

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Different types of radiation

23.1 Discovery of radioactivity (SB p. 231)

Different types of radiation Radiation — substances emitted from the radioactive elements 3 common types of radiation

 radiation  radiation  radiation

Note: Radiations are emitted from the nuclei of atoms, so they are also called nuclear radiations.

348

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23.1 Discovery of radioactivity (SB p. 231)

Different types of radiation 1.  radiation

 particle

Note:Symbol is 349

Different types of radiation

radiation  radiation  radiation

Nature —  particle 2 protons and 2 neutrons (helium nucleus) proton Speed — 1/10 of the speed of light Charge — 2 positive charges neutron Initial speed — same (emitted from same source)

4 or 2

4 2 He

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23.1 Discovery of radioactivity (SB p. 231)

 radiation  radiation  radiation

Different types of radiation 2. 

radiation

 particle

Note : Symbol is 350

electron

Different types of radiation

Nature — β particle (fast moving electrons) Speed of the speed of light Charge — 1 negative charge Initial speed the same

0 0 β or e -1 -1 Manhattan Press (H.K.) Ltd. © 2001

— 9/10

— not

23.1 Discovery of radioactivity (SB p. 232)

Different types of radiation 3. 

Different types of radiation

 radiation  radiation  radiation

radiation (gamma rays)

 radiation

Nature — electromagnetic waves Speed — speed of light Note : Symbol is Charge — neutral Initial speed — same (speed of light) 351

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0  0

Section 23.2 Detection of Radiation • Methods of detections

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23.2 Detection of radiation (SB p. 232)

Methods of detections

Methods of detections Nuclear radiation • cannot be seen • cannot be heard • cannot be tasted • cannot be smelled • cannot be touched

353

Methods of detections 1. Photographic film 2. Diffusion cloud chamber 3. Geiger-Muller counter

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23.2 Detection of radiation (SB p. 233)

Methods of detections

Experiment 23A: Blackening of photographic films Lifting tool

Intro. VCD

Expt. VCD

sealed radium source

sealed photographic film 354

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key

23.2 Detection of radiation (SB p. 233)

Methods of detections

Methods of detections 1. Photographic film — radiation can penetrate the wrapping of the film to blacken the film — but it cannot penetrate metal objects — advantage: cheaper and more convenient — disadvantage: cannot distinguish the types of radiation Photographic plate badge 355

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23.2 Detection of radiation (SB p. 234)

Methods of detections

X-ray — is one type of radiation — is for viewing the inner parts of bodies — is electromagnetic wave — has higher energy and penetrating power than  radiation

X-ray film 356

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23.2 Detection of radiation (SB p. 234)

Methods of detections

Methods of detections 2. Diffusion cloud chamber light felt ring soaked with alcohol transparent lid radioactive source insulator

foam 357

upper compartmen t lower compartment

base lid

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dry ice

Methods of detections

23.2 Detection of radiation (SB p. 235)

Experiment 23B: Cloud chamber tracks of alpha particles Expt. VCD

felt ring

transparent lid

radium source

dry ice compartment 358

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rubber wedge

23.2 Detection of radiation (SB p. 235)

Methods of detections

Operation principle

upper lower compartment compartment

1. Upper compartment — the felt light ring is soaked with alcohol felt ring soaked — filled with alcohol vapour and air 2. with alcohol Lower compartment — dry ice is put on it — cool down the alcohol vapour radioactive to a lower temperature source 3. Upper compartment — radiations emitted from the source ionize the air — the alcohol vapour condenses on the ions — white tracks are formed dry ice

359

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Methods of detections

23.2 Detection of radiation (SB p. 235)

Cloud chamber tracks Tracks of  particles Tracks — thick and straight — about the same length

360

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23.2 Detection of radiation (SB p. 235)

Methods of detections

Cloud chamber tracks Tracks of  particles Tracks — thin and twisted — different lengths

361

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Methods of detections

23.2 Detection of radiation (SB p. 235)

Cloud chamber tracks Tracks of  radiation Tracks — seldom leave tracks Note: The shape of tracks characterizes the types of radiation.

362

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Methods of detections

23.2 Detection of radiation (SB p. 236)

Class Practice 1 : The following diagrams show the cloud chamber tracks when the radioactive sources cobalt-60, radium-226 and americium-241 are used.

cobalt-60

radium-226

americium-241

Write Writedown downthe thetype typeof ofradiation radiationemitted emittedby byeach eachsource sourcein in the thefollowing followingtable. table.

Source

Type(s) of radiation em itted

C obalt-60 R adium226 A m ericium -241 363

 , 

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Ans wer

23.2 Detection of radiation (SB p. 237)

Methods of detections

Methods of detections 2. Geiger-Muller counter (or GM counter) — can measure the amount of radiation — consists of two components: Geiger-Muller tube (or GM tube) scaler or ratemeter

364

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23.2 Detection of radiation (SB p. 237)

Methods of detections

GM tube — a metal tube filled with argon gas at low pressure The argon gas is ionized by radiation radiation insulator Electron and positive ion pair is produced

thin mica window

By the action of electric field, the positive ion moves towards the cathode Electric pulse is produced By the action of electric field, the electron moves towards the anode argon gas cathode (−) anode (+) An electric field is set up inside by the d.c. voltage

365

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23.2 Detection of radiation (SB p. 237)

Methods of detections

GM tube is connected to — Scaler : record the total number of counts (pulses) within a fixed time interval — Ratemeter : record the average number of counts per second (count rate) 1 count = one α or β particle that entered the GM tube Note : Due to the weak ionizing power of  rays, every 100  rays can give only one count. Note : GM counter cannot distinguish the types of radiation, but only measure the intensity of radiation. 366

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Methods of detections

23.2 Detection of radiation (SB p. 238)

Class Practice 2 : Fill in the table with the appropriate radiation detector that should be used in each situation. Situation

Detector

To test the exposure to radiation over one week.

Photographic film

To test the presence of α-particles.

Cloud chamber

To locate the position in a building with maximum radiation intensity. To measure the total number of β -particles emitted by a radioactive source in 5 minutes.

GM tube connected to a ratemeter

367

GM tube connected to a scalerAns

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wer

Section 23.3 Properties of Radiation and Background Radiation • Ionizing power • Range in air • Cloud chamber track • Electric deflection • Magnetic deflection • Penetrating power • Background radiation Manhattan Press (H.K.) Ltd. © 2001

23.3 Properties of radiation and background radiation (SB p. 238) Ionizing power

Properties of radiation Ionizing power

— this property is known as the ionizing power

Note : But the chance of α particles having head-on collisions with Electron electrons small.is knocked out of is airvery molecules

An ion pair is formed

 particle collides with air molecules nucleus



369

proton neutron electron

particle

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23.3 Properties of radiation and background radiation (SB p. 239) Ionizing power

The positive charges of α particle — set up an electric field — attract the outer electrons of air molecules — the electrons are pulled off, and form ion pairs

 particle

electron 370

positive ion

No. of ion pair produced per cm 105 αparticle

βparticle

103

γradiation

seldom

Due to the electrically neutral of  radiation

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23.3 Properties of radiation and background radiation (SB p. 240) Ionizing power

Class Practice 3: Two positively charged aluminium strips are held by two insulating rods as shown in the following figure. Since they carry same type of charge, they repel as shown.

Ans wer When an -source is placed near to the strips,

insulating rod

When an  -source is placed near to the strips, the thestrips strips__________ __________(collapse (collapse//repel repelfurther). further).

collapse

This Thisisisbecause because -particles -particles__________ __________(ionize (ionize //neutralize) neutralize)air airmolecules moleculesand andthe thepositive positive

ionize the strips are neutralized by the charges chargeson on the strips are neutralized by the __________ __________(( -particles -particles//electrons electrons//positive positive ions). ions).

371

electrons

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aluminium strips

 -sourc e

23.3 Properties of radiation and background radiation (SB p. 241) Range in air

Range in air

Experiment 23C: Range of alpha particles scaler

Expt. VCD

 -source (americium)

GM tube

372

metre ruler

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23.3 Properties of radiation and background radiation (SB p. 241) Range in air

Range in air

 particle — ionizes air molecules,  particle’s kinetic energy decreased — it will stop until it loses all its kinetic energy — totally absorbed by the air — range in air means the distance it has travelled before it is totally absorbed by the air Range

αparticle βparticle γradiation 373

Ionizing power

few cm

strongest

several m

weak

several 100 m

weakest

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23.3 Properties of radiation and background radiation (SB p. 243) Cloud chamber track

Cloud chamber track Tracks of  particles

Tracks — strong ionizing 1. thick power, produce a large number of ion pairs

2. straight

3. about same length 374

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— large mass, their paths are deflected hardly — their kinetic energies are about the same

23.3 Properties of radiation and background radiation (SB p. 243) Cloud chamber track

Cloud chamber track

Tracks of 

Tracks 1. thin

particles

—

weaker ionizing power, produce a smaller number of ion pairs

2. twisted

—

smaller mass, their paths are deflected easily

3. different lengths

— their kinetic energies are different

375

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23.3 Properties of radiation and background radiation (SB p. 243) Cloud chamber track

Cloud chamber track Tracks of 

radiation Tracks seldom leave tracks — due to the lowest of their ionizing power, seldom ionize air molecules

376

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23.3 Properties of radiation and background radiation (SB p. 243) Cloud chamber track

Cloud chamber track

Tracks of  particles in a cloud chamber filled with helium gas Right-angled fork track the mass of an  particle is about the same as that of a helium atom

an  particle is a helium nucleus 377

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23.3 Properties of radiation and background radiation (SB p. 243) Electric deflection

Electric deflection

positive metal plate

negative metal plate



     

 

—deflect towards positive metal plate (negative charge) —does not deflect (electrically neutral) —deflect towards negative metal plate (positive charge)

β particle is deflected more than α particle, as the mass of β particle is smaller

378

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23.3 Properties of radiation and background radiation (SB p. 244) Magnetic deflection

Magnetic deflection

Experiment 23D: Deflection of beta particles in magnetic field Expt. VCD

scaler

lead plate  -source (strontium)

GM tube to scaler magnet 379

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23.3 Properties of radiation and background radiation (SB p. 245) Magnetic deflection Magnetic deflection β particle is deflected more than α particle, as the mass of β particle is smaller 





  and and — —directions directionsof of deflection deflectioncan canbe bepredicted predictedby by the theFleming’s Fleming’sleft lefthand handrule rule

radioactive source that emits three types of   — —does doesnot notdeflected deflected radiation radiation Note : Since α particles have a short range (electrically (electricallyneutral) neutral)

of few cm in air, the experiment must be done lead in a vacuum. 380

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23.3 Properties of radiation and background radiation (SB p. 246) Penetrating power

Penetrating power

Experiment 23E: Penetrating power of radiation and background radiation scaler Expt. VCD

radioactive source

GM tube

381

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stopping material

23.3 Properties of radiation and background radiation (SB p. 247) Penetrating power

Penetrating power — the ability to pass through a material without being absorbed  particle (the weakest)  particle  radiation (the strongest)

paper 382

5 mm aluminium sheet

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thick lead plate

23.3 Properties of radiation and background radiation (SB p. 247) Penetrating power

Class Practice 4 ： A student uses a GM counter to determine the type(s) of radiation emitted by a radioactive source in the laboratory. He inserts different materials between the source and the GM tube. The table below summarizes the results.

Stopping material

Count rate (min−1)

3 cm

nil

520

10 cm

nil

300

20 cm

3 mm aluminium sheet 3 mm aluminium sheet + 20 mm lead plate

200

Distance between radioactive source and GM tube

20 cm

200

From the above From theand abovedata, data,we wecan canconclude concludethat thatthe thesource sourceemits emits Ans ____________ radiation(s).  ____________ radiation(s). wer 383

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23.3 Properties of radiation and background radiation (SB p. 248) Background radiation Background radiation

— from the radioactive substances in our environment 12% through eating, drinking and breathing 14%  rays from rocks and soil 10% cosmic rays from outer space

51% radon inside our home

0.2% from industrial uses < 0.1% waste from the nuclear industry 12% medical − mainly 0.4% fallout from weapons from X-rays tests and nuclear accidents 0.4% miscellaneous − mainly from air travel and luminous watches 384

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Section 23.4 Biological Hazards and Safety Precautions • Biological hazards • Safety precautions

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23.4 Biological hazards and safety precautions (SB p. 249) Biological hazards

Biological hazards

When our human bodies

absorb

a large dose

— causes death immediately a small dose

— disrupts the DNA, causes malfunction and cancer

386

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23.4 Biological hazards and safety precautions (SB p. 250) Biological hazards

Protection from radiation

protective screen

wear radiation protective clothing stand behind a protective screen 387

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23.4 Biological hazards and safety precautions (SB p. 250) Biological hazards

Damage of radiation to human body Radiation

Outside the body

Inside the body

Damage

Damage

α

smaller

the greatest

β

smaller

smaller

γ

the greatest

smaller

The range is the longest and the penetrating power is the greatest 388

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The ionizing power is the highest

23.4 Biological hazards and safety precautions (SB p. 251) Safety precautions

Safety precautions

(i)

A radioactive source should be stored in a sealed lead box with a warning label for radiation.

(ii) Use forceps and gloves to handle radioactive sources. (iii) Always keep the source at a distance. Never direct the source towards anybody and try to look into the source with your eyes close to it

389

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23.4 Biological hazards and safety precautions (SB p. 251) Safety precautions

Safety precautions

(iv) Do not break the wrapping of the sealed radioactive source. (v) After a radiation experiment, wash your hands with soap immediately and thoroughly.

390

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Chapter 24 Radioactivity II 24.1 Models of Atom 24.2 Atomic Structure 24.3 Radioactive Decays 24.4 Uses of Radioisotopes 24.5 Nuclear Energy Manhattan Press (H.K.) Ltd. © 2001

Section 24.1 Models of Atom • Thomson’s atomic model • Rutherford’s atomic model

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Thomson’s atomic model

24.1 Models of atom (SB p. 263)

Thomson’s atomic model — Under normal condition, atoms are neutral — positively charged sphere + negatively charged electrons positive charge

puddin g plums immersed in the pudding Thomson

electrons

Note : The mass of an atom was mainly contributed from the positively charged sphere. 393

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24.1 Models of atom (SB p. 264)

Rutherford’s atomic model

Rutherford’s atomic model Geiger-Marsden scattering experiment — experiment of  particles striking a gold foil  -source (radium) lead box beam of  -particles

fluorescent screen

thin gold foil

viewing microscope

Ernest Rutherford

Note :  particle is heavy and has more energy, so it can penetrate deep into atoms. 394

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Rutherford’s atomic model

24.1 Models of atom (SB p. 265)

Set-up of the Geiger-Marsden scattering experiment  -particles  -source (radium) scattered in little deviation scintillations

beam of  -particles

most  -particles are undeflected 395

a few  -particles scattered in a large angle (≈ 180o)

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24.1 Models of atom (SB p. 266)

Scattering of 

Rutherford’s atomic model

particles gold atom

nucleus of gold atom (positive charge and mass of the atom are confined here)

Most of the space inside an atom is empty no deflection

particles the  -particle collides head-on with the nucleus

small deflection

large deflection 396

positively charged nucleus repels the -particle

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24.1 Models of atom (SB p. 266)

Rutherford’s atomic model

Rutherford’s atomic model atom nucleus

Note : The ratio of the diameters of nucleus to the atom is just like that of a ping-pong ball to a football field. 397

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Section 24.2 Atomic Structure • Electron, proton and neutron • Atomic number and mass number • Isotopes

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24.2 Atomic structure (SB p. 267)

Atomic structure atom nucleus neutro n

electron

399

proton

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24.2 Atomic structure (SB p. 267)

Electron, proton and neutron

Electron, proton and neutron atom nucleus

proton

electron

Particle

Mass / kg

Electron Proton Neutron

9.11 × 10−31

400

1.67 × 10−27 1.68 × 10−27

neutro n

Relative mass

Charge / C

1 −1.6 × 10−19 The is mainly 1833mass +1.6 × 10−19 contributed from the 1844 0 proton and neutron Manhattan Press (H.K.) Ltd. © 2001

Location in atom

negative moves around the nucleus charge positive incharge nucleus inelectrically nucleus neutral

24.2 Atomic structure (SB p. 268)

Atomic number and mass number

Atomic number and mass number Atomic number (Z)

=

Number of protons in the atom

Note : Atomic number determines the

=

Number of electrons in the atom

chemical properties of the element.

Mass number (A)

= =

no. of protons + no. of neutrons no. of nucleons in the atom

Note : Mass number Protons and neutrons determines the physical

electron

nucleus

are called nucleons

properties of the element.

neutron proton 401

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24.2 Atomic structure (SB p. 268)

Atomic number and mass number

Nucleus symbol of an element X Mass number = no. of protons + no. of neutrons

A ZX Atomic number = no. of protons = no. of electrons

Number of neutrons (N) = Mass number (A) − Atomic number (Z) 402

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Atomic number and mass number

24.2 Atomic structure (SB p. 269)

Common elements Element

Atomic Mass Nucleus No. of No. of No. of number number symbol protons neutrons electrons (Z) (A) 1 1H

1

Helium

4 2 He

2

no. of 0 protons =26

Carbon

12 6C

6

Oxygen

16 8O

8

Copper

63 29 Cu

29

Lead

208 82 Pb

82

Hydrogen

403

1

1

1

2

2

4

6

6

6

12

8

8

8

16

no. of 34 = no. of29protons + 29

neutrons 126 = 6 + 682 = 12

Manhattan Press (H.K.) Ltd. © 2001

82

63 208

Isotopes

24.2 Atomic structure (SB p. 269)

Isotopes — same proton number — different neutron number

same atomic number, but different mass number

12 6C

14 6C

6 protons (Z = 6) 6 neutrons (N = 6)

6 protons (Z = 6) 8 neutrons (N = 8)

A = 6 + 6 = 12

A = 6 + 8 = 14

Note : Isotopes have same chemical properties, but different physical properties. 404

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Isotopes

24.2 Atomic structure (SB p. 270)

Isotopes of hydrogen 1 1H

2 1H

(hydrogen) (deuterium)

3 1H

(tritium)

Proton number (Z)

1

1

1

Neutron number (N)

0

1

2

Mass number (A)

1

2

3

405

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Isotopes

24.2 Atomic structure (SB p. 270)

Class Practice 1 : Complete the following table by filling in the values of Z, A and N.

Nucleus

Z

A

N

35 ( Chlorine−35 17 Cl)

17

35

18

37 ( Chlorine−37 17 Cl)

17

37

20

23 ( Sodium−23 11 Na)

11

23

12

1 ( Hydrogen−1 1 H)

1

1

0

Ans wer isotopes Chlorine−35 and chlorine−37 are called ____________. 406

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Section 24.3 Radioactive Decays • Alpha decay • Beta decay • Gamma emission • Decay series • Random nature of decay

Manhattan Press (H.K.) Ltd. © 2001

24.3 Radioactive decays (SB p. 271)

Radioactive decays — protons and neutrons inside a nucleus held together by strong nuclear forces — cancel out the repulsive electric forces stable nucleus between protons (not radioactive)

+++ + + + ++ + + + + + ++ + + + ++ + + ++ + + 408

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are

24.3 Radioactive decays (SB p. 271)

Radioactive decays the ratio of neutron number to proton number is not within the range of 1 ≤ ≤ 1.5 N

Z

unstable nucleus

emit

+++ + + + ++ + + + + + ++ + + + ++ + + ++ + + isotopes are Note : Radioactive called radioisotope.

the nucleus is unstable

4

 -particle( 2 He)  -particle (electron)  -rays (electromagnetic wave)

form a more stable nucleus

This process is called radioactive decay or disintegration 409

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24.3 Radioactive decays (SB p. 272)

Alpha decay

Alpha decay Example:

radium nucleus

radon nucleus

226 222 4 88 Ra→ 86 Rn+ 2 He

410

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 -particle

Alpha decay

24.3 Radioactive decays (SB p. 272)

Alpha decay

A Z X

A- 4 Z - 2Y

4 2 He

Mass number is decreased by 4

A A−4 4 Z X → Z − 2Y + 2 He Atomic number is decreased by 2 411

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24.3 Radioactive decays (SB p. 272)

Beta decay

Beta decay Example:

thorium nucleus

protactinium nucleus

234 234 0 90Th→ 91 Pa + -1e

412

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 -particle

Beta decay

24.3 Radioactive decays (SB p. 273)

Beta decay a neutron inside the nucleus

A Z X

a proton an electron

the electron is emitted

A Z +1Y

1 proton more, 1 electron less

0 -1 e

Mass number remains unchanged

A A 0 Z X → Z +1Y + - 1e 413

Atomic number is increased Manhattan Press (H.K.) Ltd. © 2001 by 1

Gamma emission

24.3 Radioactive decays (SB p. 272)

Gamma emission — After  or  decay, the daughter nucleus often possesses excess energy — The excess energy is emitted in the form electromagnetic wave

Mass number and atomic number remain unchanged

A A 0 Z X →Z X + 0 γ 414

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of

Gamma emission

24.3 Radioactive decays (SB p. 274)

Changes of nucleus during the decays

αdecay βdecay γdecay Change in A

− 4

nil

nil

Change in N

− 2

− 1

nil

Change in Z

− 2

+1

nil

415

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Decay series

24.3 Radioactive decays (SB p. 274)

( r eb mun nort ue N

The daughter nucleus undergoes subsequent decays until the nucleus is stable

N)

Decay series

decay decay

The decay series of Pu-241 416

Atomic number (Z)

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Decay series

24.3 Radioactive decays (SB p. 275)

N−Z graph (neutron no. vs atomic no.)

A−Z graph (mass no. vs atomic no.)

β α 225 225 221 Ra  → Ac  → 88 89 87 Fr

Neutron number (N) Ra Ra(radium) (鐳)

Mass number (A)

βAc ( 錒 )

Ra Ra(radium) (鐳) β

Ac (Ac 錒)

(actinium)

Ac (actinium)

α

α Fr (francium)

Fr (francium)

Atomic number (Z) 417

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Atomic number (Z)

Decay series

24.3 Radioactive decays (SB p. 275)

Atomic number (Z)

418

( r eb mun ss a M

( r eb mun nort ue N

A)

N)

Class Practice 2 : Refer to Fig. 24.15, plot N–Z graph and A–Z graph to show the decay series from Np−237 to U−233 in the following figures.

Atomic number (Z)

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Ans wer

24.3 Radioactive decays (SB p. 276)

Random nature of decay

Random nature of decay Activity • — Number of disintegrations per second of a element • —

Unit : becquerel (Bq)

• —

1 Bq = 1 disintegration per second = 1 s−1

419

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radioactive

24.3 Radioactive decays (SB p. 276)

Random nature of decay

Experiment 24A: Dice decay analogue Intro. VCD

Expt. VCD

420

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24.3 Radioactive decays (SB p. 276)

Random nature of decay

Radioactive decay is a random process — We cannot predict which nucleus will decay •

— But we can predict that a certain portion of the nuclei will undergo disintegrations after a certain time

421

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Random nature of decay

24.3 Radioactive decays (SB p. 277)

Decay curve

Corrected count rate / s-1

Radioactive nuclei decay continuously

Decay curve

No. of radioactive nuclei reduces Activity falls

Activity

∝

Time / s

A graph of corrected count (activity) against time is No. ofrate radioactive nuclei called decay curve Note : Corrected count rate = measured count rate − count rate (background radiation) 422

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Random nature of decay

24.3 Radioactive decays (SB p. 278)

Half-life The time that the radioactive substance takes for half of its nucleus to decay Radon-222 3.8 days

2 000 000 radioactive radon nuclei

423

3.8 days

1 000 000

3.8 days

500 000

250 000

The half-life of radon-222 is 3.8 days Manhattan Press (H.K.) Ltd. © 2001

24.3 Radioactive decays (SB p. 278)

Random nature of decay

Decay curve of radon-222 No. of nuclei remained

Time / day half-life 424

half-life

half-life

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24.3 Radioactive decays (SB p. 280)

Random nature of decay

Half-lives of different radioisotopes

425

Radioisotope

Half−life

Polonium−214

0.000 164 second

Radon−222

3.82 days

Radium−226

1 600 years

Carbon−14

5 600 years

Uranium−238

4.5 × 109 years

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24.3 Radioactive decays (SB p. 280)

Random nature of decay

Class Practice 3 : A student uses a GM counter to measure the count rate of actinium−228 nuclei at every 30 minute interval. The background count rate is found to be 5 counts per second. •• (a) (a)Complete Completethe thefollowing followingtable. table.

Time / min

0

30 60 90 120 150

1 410 299 219 160 118 Count rate / count s −

90

Corrected count rate / 405 294 214 155 113 85 1 count s−

Ans wer 426

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24.3 Radioactive decays (SB p. 281)

Random nature of decay

Class Practice 3 (Cont’d) : (b) Complete the following graph showing the corrected count rate against time due to actinium−228. Corrected count Corrected count rate / count s−1-1 rate / count s

Time /

Time / min ••From the graph, the half− life of actinium− 228 is ___________ minutes. From the graph, the half−life of actinium−228 is ___________ minutes. min

Ans wer

64 427

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Section 24.4 Uses of Radioisotopes • • • •

Thickness gauge Smoke detector Sterilization Medical, industrial and agricultural uses • Carbon dating

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Thickness gauge

24.4 Uses of radioisotopes (SB p. 281)

Thickness gauge

The count rate detected by the detector will also change Radiation passes thickness through the metal indicator sheet to the detector

roller control wheels

radiation detector

rollers metal sheet

The rollers The thickness of the and wheels metal sheet changes are then monitored

429

 - source

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Put the  source on one side of the metal sheet

24.4 Uses of radioisotopes (SB p. 282)

Smoke detector

Smoke detector

Since smoke particles move slower, the current will reduce

current-detecting alarm-triggering circuit

ions

radioactive source

air

alarm The ions produced set up a current

If smoke enters the detector, the ions will attach to the smoke particles α -particles ionize the air molecules 430

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The detector detects the current change and gives an alarm signal

24.4 Uses of radioisotopes (SB p. 283)

Sterilization — γ radiation has high energy and high penetrating power — It can pass through the packing of food and instruments to undergo sterilization

431

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Sterilization

24.4 Uses of radioisotopes (SB p. 283)

Medical, industrial and agricultural uses

Medical, industrial and agricultural uses Radiotherapy — High energy γ rays — High penetrating power — Travel deep into the body — Kill the cancer cells

radioactive source



cancer cell

432

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24.4 Uses of radioisotopes (SB p. 284)

Tracer

Medical, industrial and agricultural uses

— Eat a small amount of radioactive substance — Their paths inside human bodies can be traced by a GM counter outside

Detect the position of tumour — After the human organs absorb the radioisotopes Note : γ radiation is used due to its weak ionizing power, so — The radiations emitted by a it causes less damage to bodies. cancer cell and a healthy cell are different Note doctor : Use thecan radioactive nuclei with a short half−life of several hours — find out the position ofthe thetime tumour or days to reduce remain in bodies. 433

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24.4 Uses of radioisotopes (SB p. 284)

Industrial uses

Medical, industrial and agricultural uses

Some radioactive tracers are added to — the liquid waste to find out the flow of liquid waste in pipes

Agricultural uses Some radioactive tracers are added to — the fertilizers to trace how the fertilizers circulate in the plants 434

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24.4 Uses of radioisotopes (SB p. 285)

Carbon dating

Carbon dating — In the atmosphere, there are two carbon isotopes (stable carbon−12 and radioactive carbon−14) — Carbon reacts with oxygen to form carbon dioxide (CO2) — Living bodies take up CO2 — The C−14 to C−12 ratio remains constant in bodies 435

12 6 CO 2

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14 6 CO 2

24.4 Uses of radioisotopes (SB p. 285)

Carbon dating

Carbon dating

— Once the bodies die, they stop CO2 intake — C−14 decays continuously and thus reduces 14 6C

→147 N+ -01e

— The carbon−14 to carbon−12 ratio decreases — Compare this value Note : Carbon dating works well for organic with that found in a2 000 and 20 000 materials that are between 14 living 6C years old. material — The ages of the sample can be isdetermined Note : C-14 used because it has a long half-life (5 570 years) and can be found in all organic compounds. 436

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24.4 Uses of radioisotopes (SB p. 285)

Carbon dating

The ratio of C−14 to C−12 remains constant in the atmosphere n

n

14 7N

cosmic rays yield neutrons

neutron strikes nitrogen nucleus

14 1 N + 7 0n

437

14 6C

to form carbon-14

→146 C + 11H

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14 6 CO 2

carbon dioxide formed throughout the atmosphere

Section 24.5 Nuclear Energy • Nuclear fission and nuclear fusion • Nuclear power plant • Nuclear debate

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24.5 Nuclear energy (SB p. 287)

Nuclear fission and nuclear fusion

Nuclear fission and nuclear fusion Nuclear energy is released from nuclei through nuclear fission and nuclear fusion

Nuclear fission

235 92 U

fission fragment

From a heavier nucleus Split into lighter nuclei

heavy nucleus fission fragment

1 0n 439

+

235 141 → 56 Ba 92 U 

+

92 36 Kr

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+

1 30 n

24.5 Nuclear energy (SB p. 288)

Chain reaction Neutron emitted

Nuclear fission and nuclear fusion

The neutron causes other nuclei to undergo fission

The heavy nucleus undergoes fission

A slow neutron hits a heavier nucleus 235 92 U

fission fragment 440

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24.5 Nuclear energy (SB p. 290)

Nuclear fission and nuclear fusion

Nuclear fusion From two or more light nuclei

light nucleus

a heavier nucleus

Join to form a heavier nucleus 2 1H

441

light nucleus

2 3 + 1H  →2 He

1 +0 n

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24.5 Nuclear energy (SB p. 290)

Nuclear power plant

Nuclear power plant Pressurized water reactor (PWR) control rods

Heat up the water

Boron and cadmium absorb neutrons containment structureto reduce the reaction rate pressurized Water changes to steam water The steam turns the turbine steam line that operates a generator electricity

steam

heat exchanger and steam generator

fuel rods

electrical turbine energy generator

pump water

condenser condenser cooling water is cooled and Fission The As water pressurized of uranium is pressurized, water heats it willupnot pump is reThe steam The water reactor core

yieldsthe nuclear water change energy in other to steam system 442

cycled Manhattan Press (H.K.) Ltd. © 2001

condensed into water

24.5 Nuclear energy (SB p. 291)

Nuclear power plant

Safety facilities of a nuclear power plant The whole system is well monitored. If The reactor is built inside the there is any abnormality in the core system, containment to stand high pressure the reactor will be shut down immediately by inserting control rods

The fuel is immersed in water to The nuclear power plant must stand remove the heat from nuclear fission. fire, earthquake, high winds, flooding Back-up systems are provided and even aircraft crashing 443

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24.5 Nuclear energy (SB p. 293)

Nuclear debate

Nuclear debate Nuclear accidents Year 1979

Leakage Leakageof of radiation radiationin inthe theThree Three Mile MileIsland Island nuclear nuclear power power plant, plant, U.S. U.S. 444

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24.5 Nuclear energy (SB p. 293)

Nuclear debate

Nuclear accidents Year 1986

Fire Fire in inthe theChernobyl Chernobyl nuclear nuclearreactor, reactor, Russia Russia

445

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24.5 Nuclear energy (SB p. 294)

Nuclear debate

Nuclear accidents Year 1999

Leakage Leakageof of radiation radiation from from aauranium uranium processing processingplant plant in in Tokaimura, Tokaimura, Japan Japan

446

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24.5 Nuclear energy (SB p. 294)

Nuclear debate

Are there alternatives to nuclear energy? — We can also use solar energy, tidal energy, wind energy or energy from coal-burning — But a large amount of pollutants is produced during the burning of coal — The supply is limited

447

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24.5 Nuclear energy (SB p. 295)

Nuclear debate

About Operating cost Nuclear power plant

Conventional power plant

Construction cost

high

low

Fuel cost

low

high

It is cheaper to operate the nuclear power plant in the long turn 448

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24.5 Nuclear energy (SB p. 295)

About radioactive waste Radioactive waste is stored underground

449

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Nuclear debate

The End

450

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