Physics Module

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:

4.1.4 Structure of the Cathode Ray Oscilloscope

1. The cathode ray oscilloscope (C.R.O.) consists of the following components:

i. The electron gun ii. The deflecting plates iii. A fluorescent screen

Component Function

Electron gun

Filament When a current passes through the filament, the filament becomes hot and heats
Cathode up the cathode and release electrons.
Emits electrons when it is hot.
Control Grid When the potential is more negative, more electrons will be repelled from the grid.
 Control the number of electrons hitting the fluorescent screen.
Focusing Anode  Control the brightness of the spot on the screen.
Accelerating Anode To focus the electrons onto the screen.
To accelerate the electrons to high speed.

Deflecting system

Y-Plates To deflect the electron beam vertically.

X-Plates To deflect the electron beam horizontally.

- Glass surface coated with a fluorescent material (zinc sulphide)

Fluorescent screen - To convert the kinetic energy of the electrons to heat and light energy

- A bright spot will appear on the screen when the electrons hit it.

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 51

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:

2. The diagram shows a simple oscilloscope.

Component Function
Focus control - Adjusted to obtain a well-defined spot.
Brilliance control - Used to vary the number of electrons that hit the screen each second.
Time-base control - So, it changes the brightness of the spot.
- Used to vary the speed of the electron beam moving across the screen.
Y-gain control - The diagram shows that it takes 10 ms for the spot to move 1 cm across the screen.
X-shift || Y-shift - Used to vary the amplification of the input potential difference.
Input terminal - 5 V cm-1 means that the spot moves 1 cm up or down, for every 5 V applied across

the input terminals.
- Used to move the spot in X or Y directions.
- The input terminal is where the voltage signal from an external circuit is connected

(D.C. or A.C)

4.1.5 Use of the CRO
1. The uses of cathode-ray oscilloscope are:
i. To measure a D.C or A.C voltage.
ii. To measure a short time intervals.
iii. To display the waveform:
- Used in medical research such as in the study of heartbeats and brain waves.
- Used in the study of music to find out the quality and type of sounds those are made by
difference musical instruments.
2. **Television and computers use similar principal components as those in the C.R.O.

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 52

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
To measure a D.C voltage:

The unknown voltage, V = (Y-gain) × h
To measure a A.C voltage:

Peak-to-peak voltage, Vpp = (Y-gains) × h
Peak voltage, Vp = (Y-gains)  1 (h)

2
Short time intervals, t = n divisions between two pulses × time-base value.

3. Figure shows a trace on an oscilloscope for an A.C. source. If the Y-gain is set to 1.5 V cm-1 and
the time-base is 2 ms cm-1.

Calculate:

i.) The peak voltage, Vp of the a.c source.
ii.) The frequency, f of the A.C. source.

iii.) Sketch the trace displayed on the screen if the
settings are changed to 1 V cm-1 and 1 ms cm-1.

Solution:

i.) VP  1.5 V cm-1  1  4 cm iii.)
2

VP  3.0 V

ii.) T  4 cm 2 ms cm-1
T = 8 ms = 8 x 10-3 s

f = 1  125 Hz
T

4. An ultrasound signal is transmitted vertically down to the sea bed.

Transmitted and reflected signals are input into an oscilloscope with
a time base setting of 150 ms cm-1. The diagram shows the trace of

the two signals on the screen of the oscilloscope. The speed of
sound in water is 1200 ms-1. What is the depth of the sea?

Solution:

Time taken for ultrasonic waves to travel through a distance of
2 d  time betweenP and Q

 5 cm  150 ms sm-1  750 ms  0.75 s
Speed of ultrasonic waves, V  2d

t
Hance, d  1200  0.75  450 m

2

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 53

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.2 SEMICONDUCTOR DIODES

4.2.1 Properties of Semiconductors

1. Matters can be classified according to their electrical properties;

i. Electrical conductor ii. Electrical insulator iii. Semiconductor.

2. Electrical conductors such as copper are substances which have free electrons or mobile

charge carries. Examples of electrical conductors include all metals.

3. Insulators are materials which have little or no free charge carriers to conduct electricity under

application of electric field. Wood is considered as an electrical insulator.

4. Semiconductors are materials that have electrical conductivities which are higher that those of

insulator but less than those of conductors. Beside silicon and germanium, the common

material used as semiconductor are gallium arsenide, indium phosphide and mercury cadmium

telluride.

5. All semiconductors are insulators at low

temperatures. Their electrical resistivities

decrease with increasing temperature.

6. For an insulator, this conduction band and the

valance band are separated by a wide forbidden band.

7. For a semiconductor, the forbidden band is relatively small. Germanium has a smaller energy

gap separating its valence and conduction bands, s, less energy is required to cross the gap

between the bands.

8. For a conductor, the conduction and valence bands overlap and there is no forbidden band. It

means that a particular valence electron is not strongly associated with its own nucleus. It is

therefore free to move throughout the structure.

9. Materials that are insulators at room temperature can become conductors when the temperature

is raised to a high enough level. This causes some electrons to move to a higher energy band

(conduction band), where they become available for conduction.

10. Silicon and germanium are tetravalent, that is each has four electrons in its outermost shell.

Each atom is involved in four covalent bonds.

11. At 0 K, the valence electrons are bound to the covalent bonds and cannot conduct

electricity. When temperature is high enough, some of the valence electrons can escape from

the bonds and conduct electricity.

12. When an electron escapes from a bond, it leaves behind a vacancy in the lattice. The vacancy is

called a hole. A hole is a region in which there is an excess of positive charge.

13. All semiconductors are insulators at low temperatures. Their electrical resistivities decrease with

increasing temperature. Therefore, at high temperature, they are electrical conductors.

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 54

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.2.2 Doping

1. The electrical resistivity of semiconductors also can be decrease by adding small amount of

impurities.

2. The process is known as doping and it produces a considerable increase in conductivity.

3. The atoms (impurities) which are added are either pentavalent (e.g. antimony, arsenic and

phosphorus) or trivalent (e.g. boron, indium and gallium).

4. Pentavalent impurities, such as antimony and phosphorus which are used in doping donate
conduction electrons to the structure; they are also known as donors.

5. Trivalent impurities, such as boron and indium which are used in doping accept electrons from
the structure; they are also known as acceptors.

6. Doping germanium with pentavalent impurities will produce an n-type semiconductor, the
majority carriers of which are electrons.

7. Doping germanium with trivalent impurities will produce a p-type semiconductor, the majority
carries of which are holes.

4.2.3 Diodes
1. If a p-type semiconductor and n-type semiconductor are joined, we get a very useful device,
called a p-n junction diode.
2. A p-n junction diode allows a current to flow only one way. It is called a rectifier.
3. Under a forward-bias connection, the depletion layer of a diode will shrink, and the resistance
to current flow is low.

4. Under a reverse-bias connection, the depletion layer of a diode will expand, and the resistance
to current flow is high.

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 55

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
Depletion Layer

1. A junction is formed simply by putting a p-type semiconductor with an n-type semiconductor.
2. As soon as the junction is formed, electrons from the electron-rich n-type material on one side of

the junction diffuse into the p-type side and drop into the holes there.
3. At the same time, holes from the p-type side are filled by electrons. This means the hole and

electron cancel each other and vanish.
4. These exchanges take place in a narrow region known as the depletion layer.
5. The potential difference across the junction exerts a force on the free charge, driving it back to its

‘own side’ of the junction away from the depletion layer.
6. A free charge now requires some extra energy to overcome the forces from the donor or acceptor

atoms to be able to cross the layer.
7. Forward-bias connection

8. Reverse-bias connection

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 56

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.2.4 Rectification

1. Rectification is the process of turning or

converts an alternating current into a direct

current by using a diode.

2. Rectification is classified as either half-wave

rectification or full-wave rectification.

3. Half-wave Rectification (A single diode)

4. Full-wave Rectification (Two diodes)

5. Full-wave Bridge Rectification (Four diodes)

6. Capacitor acts as a current filter or smoother or regulator. A capacitor with 57
bigger capacitance will produce a smoother waveform for the output current.

7. In a half wave rectification circuit, the capacitor will
accumulate charge and energy during the positive half
cycle.

8. In the negative half cycle, when the diode is in reverse
bias, current cannot flow through the diode and the
capacitor will discharge and it is used to maintain the potential difference in the resistor.

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.3 TRANSISTOR

1. A transistor is a three-terminal device.

i. Base ii. Collector iii. Emitter
2. n-p-n transistor

i. A transistor consists of two n-type materials

separated by a p-type material.
3. p-n-p transistor

i. A transistor consists of two p-type materials
separated by an n-type material.

4. The function for each terminal in a transistor
i. The emitter
- It is a heavily doped, medium-sized layer designed to emit or inject electrons.
ii. The base
- It is a medium doped, mall layer designed to pass electrons.
iii. The collector
- It is a lightly doped, large layer designed to collect electrons.

4.3.1 Working principle of a transistor, n-p-n type

1. The base-emitter junction is normally forward-biased and
the base-collector junction is reverse-biased. The C-

electrode (collector) should be connected to the positive
terminal of the battery and the E-electrode (emitter) to the

negative terminal.

2. The potential difference between the emitter and the base is large enough to overcome the
contact potential.

3. At the base-emitter junction, electrons (the majority carriers in
the emitter) cross into the base region.

4. The emitter supplies electrons to the collector. The base

controls the flow of electrons from the emitter to the collector.

The collector receives electron from the emitter.
5. The current flows from the collector to the emitter. As a result: IE = IB + IC

4.3.2 Working principle of a transistor, p-n-p type 58
1. The emitter has more holes than the collector.
2. The current flows from the emitter to the collector and
IE = IB + IC

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:

4.3.3 Transistor as Current Amplifier Circuits

1. Transistor is used as a current amplifier.

2. The function of the resistor in the base circuit is to limit

the base current.

3. When the switch S is off, IB = 0, thus, IC = 0

4. When the switch is on, a small amount of current flows

in the base circuit, IB causing IC to increases.

i. Base current, IB controls the collector current, IC
ii. Thus if IB = 0, then IC = 0

iii. A small change in IB causes IC to increase significantly

4.3.4 Transistor as Automatic Switches

4.3.4.1 A Light Controlled Switch

1. A transistor and a LDR (Light Dependent

Resistor) as main components to form an

automatic switch circuit.

2. A relay switch S is used here to connect

the main circuit to another circuit that light the bulb.

3. A LDR functions as a sensor.
i. When there is light, its resistance is as low as 200 Ω.
ii. While in darkness its resistance can reach as high as 1 MΩ.

4. R2 acts as a potential divider between R2 and LDR.

i. VR2   R2 R2 V VLDR   RLDR V
 RLDR R2  RLDR

5. R1 limits the current that flows in the base circuit.
6. The LDR converts light into electrical signals.

Light on LDR Resistance of LDR VR2 Transistor Switch Lamp
Bright Low > VB ON ON
High < VB OFF OFF
Less Bright

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 59

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.3.4.2 A Heat Controlled Switch

1. A transistor and a thermistor as main components

to form an automatic switch circuit.

2. A relay switch is used here to connect the main

circuit to another circuit that on the alarm.

3. A thermistor functions as a sensor.

i. When it is cold, its resistance is very high.

ii. When it is heated, its resistance drops rapidly.

4. R2 acts as a potential divider between R2 and thermistor.

i. VR2   R2 R2 V Vthermistor   Rthermistor V
 Rthermistor R2  Rthermistor

5. RB limits the current that flows in the base circuit.

Temperature Resistance of Thermistor VR2 Transistor Switch Alarm
< VB OFF OFF
Room temperature High > VB ON ON

Fire Low

4.4 LOGIC GATES
1. Logic gates are electronic devices that have one or more inputs but can only produce one
output.
2. They are called gates because they control the flow of information from input devices to produce
output actions.
3. Truth table: (It is usually shown with numbers)

Input Output Input Output

AB Q ABQ

YES YES YES 111

YES NO NO 100

NO YES NO 010

NO NO NO 000

4. Logic gates operation can also be stated in expression form using the Boolean operators. The

Boolean expression is normally written under the truth table.

5. Q can define in the following ways:

i. Q is true if and only if A is true AND B is true.

Boolean algebra: Q = A AND B = A ● B

ii. Q is true if A is true OR B if true.

Boolean algebra: Q = A OR B = A + B

iii. Q is true if A is false. Q = NOT A = Ā
Boolean algebra:

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 60

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.4.1 Logic Gates AND, OR, NOT, NAND, NOR
TRUTH TABLE
GATES SYMBOL

AND gate Input Output
AB Q
11 1
10 0
01 0
00 0

Q=A●B

OR gate Input Output
AB Q
11 1
10 1
01 1
00 0

Q=A+B

NOT gate Input Output
A Q
1 0
0 1

Q= A

NAND gate Input Output
NOR gate AB Q
11 0
10 1
01 1
00 1

Q = AB

Input Output
AB Q
11 0
10 0
01 0
00 1

Q = AB

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 61

Mr Ng Han Guan Form 5 Chapter 4: Electronics
Guru Cemerlang Physics MSAB
Date:
4.4.2 Combination Logic Gates
SYMBOL TRUTH TABLE

Input Output
AB
A C = AB
11 01
10 01
01 10
00 11

C = AB

Input Output

A BQ R

1 11 0

1 00 1

0 10 1

0 00 1

Q=A●B

R = Q = AB

ABCDPQS
1100000
1001011
0110101
0011000

C= A

D= B
P=C●B= A ●B
Q=D●A= B ●B

S=P+Q

3rd edition © 2011 | chp 4 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan 62

Mr Ng Han Guan Form 5 Chapter 5: Radioactive Date:
Guru Cemerlang Physics MSAB
CHAPTER 5: RADIOACTIVE

5.1 THE NUCLEUS OF AN ATOM

5.1.1 Proton Number, Nucleon Number and Isotopes

1. Proton number (atomic number), Z, of an atom is the number of protons in the nucleus.

2. Nucleon number (mass number), A, of an atom is the total number of protons and neutrons

in the nucleus.

3. The nuclide notation of an element X is written as [ A X ] where X is the chemical symbol of the
Z

element. Example: The nuclide of helium is 4 He
2

Atoms Hydrogen (H) Helium (He) Carbon (C)

Nuclide notation 11H 4 He 12 C
2 6

Proton number (atomic number), Z 1 2 6

Nucleon number (mass number), A 1 4 12

Number of neutrons = A - Z 0 2 6

5.2 RADIOACTIVE DECAY
5.2.1 Radioactivity

1. Radioactivity is the spontaneous and random disintegration of an unstable nucleus like
uranium accompanied by the emission of energetic particles or protons.

2. The process is said to be spontaneous because it is not influenced by any physical factors
such as temperature, pressure, time, etc……

3. It is said to be a random process because there is no way to tell which nucleus will decay,
nor is there any way to predict when it is going to decay.

4. Radioactive substances emit three types of radiation known as alpha particle ( ), beta particle
(  ) and gamma ray (  ).

5.2.2 Radiation Detectors
5.2.2.1 Photographic Film

1. All types of radiation will blacken the photographic film like a photographic film that is exposed to
ordinary light.

2. Dosimeters (with a piece of photographic film) are worn by workers exposed to radiation as a
safety measure.

3. The amount of blackening on the developed film shows the amount of radiation absorbed by
the worker.

63

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:
5.2.2.2 A Gold-Leaf Electroscope

1. The figure shows a positively-charged electroscope

with the gold leaf in a diverged position.

2. When the radium source is brought near to the cap,

the radiation emitted by the radium source ionises the

air molecules above the cap.

3. Since the cap is positively charged, the positive ions are

repelled while the negative ions are attracted to the cap.

4. When these negative ions reach the cap, they neutralise the

positively-charged electroscope. Therefore, the gold leaf

collapses.

5.2.2.3 Diffusion Cloud Chamber (Wilson Chamber)

1. The air inside is saturated with alcohol vapour, and

is cooled by means of dry ice.
2. The dry ice, which is solid carbon dioxide at -78oC,

is placed below a thin black metal base plate.

3. When a radioactive source is placed inside the

chamber, the radiation emitted by the source

passes through the supersaturated vapour, and ionises the air in the chamber.

4. The ions then serve as condensation centres for the vapour, which condenses around them.

5. The path of the radiation is thus shown by tracks of tiny liquid droplets in the supersaturated

vapour.

- The tracks are straight and thick because  -particles

have strong ionizing power.

Alpha Particle,  - The tracks have different lengths, which imply that the

 -particles have different amount of kinetic energy.

- The tracks are thinner and twisted because  -particle

has weaker ionizing power than  -particle.

Beta Particle,  - The tracks are twisted because  -particle is more easily

deviated by collisions with the vapour molecules.

- The tracks are thin, short and irregular because  -ray

has the weakest ionizing power.

Gamma Ray, 

64

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:

5.2.2.4 Geiger-Muller Tube (G-M Tube)

1. The figure shows a G-M tube connected to a rate-meter and loudspeaker.
2. Radiation enters the G-M tube through its thin mica window.
3. It then ionises the argon gas in the tube.
4. The potential difference across the tube accelerates the positive ions to the cathode and the

electrons to the anode.
5. Thus, a pulse current is produced, amplified and finally recorded on the rate-meter.

65

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: R
Guru Cemerlang Physics MSAB

5.2.3 Characteristics of the Three Types of Radioactive Emissions

Radioactive Alpha particles
emissions

Nature Positive charged, helium atom

Symbol 4 He
Charge 2

+2 electric charges

Slightly deflected towards the negative Strongl
Effect of Electric Field

plate

Effect of Magnetic Slightly deflected because it has a big Stron

Field mass.

Ionising Power Strongest Interm
Penetrating Power Low Af

Stopped by A thin sheet of paper

Range in air A few centimetres
Detectors
Spark counter, Gold-leaf electroscope
Photograph

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Radioactive

Date:

Beta particles Gamma rays

Electron Neutral electromagnet ray

-01e γ

-1 electric charge 0
ly deflected towards the positive Does not bend , showing that it is

plate neutral
ngly deflected because it has a Does not bend showing that it is

small mass. neutral.

mediate. Lower than  -particle. Weakest
High
Intermediate

ew millimetres of aluminium A few centimetres of lead or concrete

A few metres A few hundred metres

.- -
hic film, cloud chamber and G-M tube

66

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:
5.2.4 Radioactive Decay

1. Radioactive decay is the process where unstable radioactive nuclei emit  - particles,  -

particles and/or  -rays in order to form more stable nuclei of the same element or other

elements.
2. Radioactive decay is a spontaneous, uncontrollable and random process. It is also not

affected by the physical or chemical properties of the atom.

Alpha Decay

Example:
Beta Decay

Example:
Gamma Decay

Example:

Decay Series
Plutonium-241 29441Pu is an unstable nucleus. In order to form the more stable protactinium-233

nucleus, 23931Pa it decays according to the following series: 29441Pu29451Am29337 Np23931Pa

67

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:

Can you guess which type of decay (alpha or beta) takes place in each stage?

5.2.5 Half – Life

1. Half-life is the time taken for half of the radioactive nuclei to decay.

2. On the graph you can see that after 1 half-life, half of the nuclei have disintegrated and half of

them survived.

3. After a further half-life, the activity is half again so that only 1 of the nuclei survived.
4

4. After 3 half-lives, it is half again and only 1 of the nuclei survived.
8

5. Example: Pa takes 20.8 hours to shrinks from 80 g to 5 g. How many half –lives are there

80 g  40 g  20 g  10 g  5 g

T½ T½ T½ T½

(This decay process has taken the time of 4 half – lives)

20.8 hours = 4 T½  T½ = 20.8 / 4 = 5.2 hours.

6. A radioactive of gamma rays has a half – life of 4 days. A Geiger counter placed 3 m from the

source initially has a count – rate of 21600 per minute. After 8 days, the counter is moved back to

a distance of 3 m from the source and its rate, in counts per minute is then 5400 per minute.

7. The number of radioactive nuclides in two different samples P and Q are initially 4 N and N
respectively. If the half – life of P is t and that of Q is 3 t, the number of radioactive nuclides in P

will be the same as the number of radioactive nuclides in Q after a time of 3 t.

68

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:
5.2.5.1 Uses of Half – Life

1. The most common tracers is called Technitium-99.
 It is very useful and safe because Technitium-99 has a short half-life (6 hours). So, after
one day, only 6.25% of Technitium-99 will remain in a patient’s body.
 This is to ensure that it remains long enough to be detected but not so long that it becomes a

safety problem.

2. Smoke alarm
 The smoke alarm contains a weak source made of Americium-241, which emits alpha-

particles. This will ionise the air so that it conducts electricity. This results in a small current

flow.

 If smoke enters the alarm, the  - particles will be

absorbed. The current is thus reduced. The alarm is

then sounded.
 Americium-241 has a half-life of 460 years.

5.3 Radioisotope
1. Isotopes are atoms of the same element that have the same number of protons but different
number of neutron. Example: Hydrogen [ 11H ], Deuterium [ 21H ], Tritium [ 31H ]
2. The physical properties of isotopes of an element are different due to the difference in their
masses.
3. The chemical properties of the isotopes of an element are usually the same because these
isotopes have the same number of electrons.
4. Radioisotope is an isotope with unstable nucleus.
5. Radioisotopes are unstable isotopes that go through radioactive decay to attain greater stability.

5.3.1 Applications of Radioisotopes

1. Radioisotope in Medical Treatment – Sterilising, Radioactive Tracer and Cancer Treatment

 Radioactive cobalt, Co-60, decays with the emission of beta-particles and high energy

gamma-rays.

- Gamma-rays can be used to detect and treat deep cancerous growths in a cancer patient.

- The radiation kills the cells of the malignant tumour in the patient.

- The machine built for this purpose is gammatron which is very useful in radiotherapy.
 Iodine-131 is used to detect problems of the thyroid gland.

- Iodine accumulates readily in the thyroid gland.

- By using radioisotope iodine-131 and finding out the rate at which it accumulates in the

thyroid. The thyroid function can be monitored. 69

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:
2. Radioisotope in Industry

 Detecting defects in welding

- Gamma source can be used to penetrate deep into welding to reveal faults which normally

cannot be detected.
 Thickness controller

- In the area of manufacturing, suitable

radioactive sources are used to check the

thickness of rolled sheets of metal, paper or

plastic.

- The figure shows a typical arrangement to

control the thickness of a rolled sheet of metal.

- Gamma source and G-M tube are used to check the thickness of the metal sheet.

- Lower activity indicates that the sheet is thicker and higher activity indicates that the

sheet is thinner.

- Beta sources are used to check the thickness of paper or plastic sheets.

- Gamma radiations are used to check the thickness of metal sheets.
 Detecting Leakage in Pipes

- A small amount of radioisotope (sodium) that

has a short half-life is dissolved in water and

allowed to flow in the pipe.

- If high activity is detected by the G-M tube,

there is leakage in that area.

3. Radioisotope in Agriculture

 Sterilisation

- Strong gamma rays are used to kill bacteria in pre-packed frozen food.

- This will sterilise the food and prevent food poisoning.

4. Archaeological Dating

- Carbon-14 exists in plants and animals.

- When living things die, they stop taking in carbon and the activity of carbon-14 begins to

decrease with a half-life of 5700 years.

- Therefore, the age of the dead plants and animals can be determined by comparing the

activity of carbon-14 in the relics with that of living trees and animals.

70

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:
5.4 Nuclear Energy

1. Nuclear energy is energy released from the nucleus due to a reduction in its mass.

2. The release of nuclear energy occurs in processes like radioactive decay, nuclear fission and

nuclear fusion.

5.4.1 Atomic Mass Unit (a.m.u.) and Energy

1. Nuclear reaction or radioactivity decay produces new nuclei and emits radioactive particles. The

total mass of the products is less than the total initial mass.

2. This mass difference is called mass defect.

3. The energy released is due to this mass defect.

Mass defect  energy

4. This energy is calculated by Einstein’s mass-energy equation:

E = m c2

E = energy (J) m = mass difference (kg) c = velocity of light = 3 x 108 m s-1

5. The unit for nucleus mass is unified atomic mass unit (a.m.u.)

1 a.m.u. = 1 x mass of a carbon-12 atom = 1.66 x 10-27 kg
12

6. Calculate the energy released when a radium-226 nucleus decays as shown below.

28286Ra 222 Rn  4 He  energy
86 2

Given: - mass of radium-226 = 226.02540 a.m.u.

- mass of radon-222 = 222.01757 a.m.u.

- mass of alpha particle = 4.00260 a.m.u.

Solving:

Total of mass after decay = 222.01757 + 4.00260 = 226.02017 a.m.u.

Before decay = 226.02540 a.m.u.

Mass defect = 226.02540 u – 226.02017 a.m.u. = 0.00523 a.m.u.
= 0.00523 x 1.66 x 10-27 kg = 8.68 x 10-30 kg

E = m c2
Energy released = 8.68 x 10-30 x (3 x 108)2 = 7.81 x 10-13 J

71

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:
5.4.2 Nuclear Fission

1. Nuclear fission is a process whereby a heavy unstable nucleus of an atom splits into two or

more lighter nuclei with the release of energy.

2. Uranium-235 is bombarded by a slow moving neutron. After being bombarded by the neutron,

uranium-235 forms uranium-236.

3. Uranium-236 is unstable and splits into two nearly equal radioactive nuclei, often being barium

and krypton together with the production of three neutrons.

236 U 15461Ba  92 Kr  301n  energy
92 36

4. The total mass of the product particles on the right side of the equation is less than the mass

of the initial nucleus on the left side of the equation.

5. The loss in mass is accounted for by the gain in energy. The energy released is in the form of an

increase in the kinetic energy of the product particles.

6. The two fast moving fission fragments collide with the surrounding atoms and raise their kinetic

energy and thus their temperature. This process will produce heat.

5.4.2.1 Chain Reaction
7. The three fast moving neutrons are made to slow down. They will collide with other uranium-235.
8. The uranium nucleus again undergoes fission and generates more fission fragments; produce
more neutrons and more energy.
9. This nuclear fission process is repeated. As a result, more new nuclei, neutrons and energy are
released.

72

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:

10. The minimum mass of uranium-235 for a chain reaction to occur is called critical mass.

11. If the mass of U-235 is more than the critical mass, the chain reaction will continue to

progress and release a large amount of energy.

12. In a nuclear reactor, the number of neutrons producing nuclear fission is controlled. Thus the

energy is generated at a steady, controllable rate.

13. In an atomic bomb explosion, the chain reaction is not under control. Thus a gigantic amount

of energy is released in a short time.

5.4.3 Nuclear Fusion

21H  31H 42 He  01n  energy

21H  11H 32 He    energy

3 He  3 He 42 He  211H  energy
2 2

1. Nuclear Fusion is the process whereby two light nuclei combine to form a heavier nucleus

with the release of energy.

2. The energy released is due to the loss of mass.

3. Fusion gives out more energy per kilogram of fuel than fission.
4. For fusion to take place, a temperature of about 100 million oC is required. This process is

difficult to control.

5. In the sun, more than 500 million tonnes of hydrogen fuse together to form helium at an extremely

high temperature.

73

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:

5.4.4 Differences between Nuclear Fusion and Nuclear Fission

Nuclear Fusion Nuclear Fission

Definition Process whereby lighter nuclei fuse Process whereby a heavy unstable

together to form a single heavier nucleus of an atom splits into lighter

nucleus with the release of energy. nuclei with the release of energy.

Where did the The reduction in mass, when two light The reduction in the total mass of

energy come nuclide fuse together, is converted into fragments compared to the mass of the

form? energy. original nuclide is converted into energy.

Process that Light nuclei at high speeds and very Moving particles, e.g. neutrons, hit and

takes place high temperatures overcome the break up heavy nucleus and produce

repulsion force and fuse to form a single enough neutrons to break up other

nucleus. nuclei (chain reaction).

Can the rate Difficult to control. Can be controlled.

of reaction be

controlled?

Examples Fusion is the process that powers the Fission is the process used in a nuclear

Sun. reactor.

74

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan

Mr Ng Han Guan Form 5 Chapter 5: Radioactive
Guru Cemerlang Physics MSAB
Date:

5.5 Management of Radioactive Substances

1. Radiation from radioactive substances can be very harmful to humans and proper safety

precautions must be taken to avoid overexposure.

2. Radiation from radioactive sources can cause radiation burn, radiation sickness, leukaemia,

cataract, genetic mutations or death. These may sometimes appear many years later.

5.5.1 Precautions against Radiation Hazards
1. Workers dealing with radioactive sources must wear film badges or pocket dosimeters to keep
track of the accumulated dosage of radiation they have been exposed to. The badges have
“windows” made of different materials to enable us to determine the type of radiation received.

2. The radioactive sources must be kept in prominently 75
labelled lead-line boxes.

3. The wall of the storage rooms of nuclear laboratories are
built with lead bricks that are 1 m thick.

4. Radiation symbol is prominently displayed
to warn people of the location or presence
of radioactive sources.

5. Persons doing radiation experiments must wear special lead-line suits
and use tweezers or remote control equipment (robots) to handle
radioactive sources.

6. Food and drinks are strictly prohibited in the laboratory to minimise the
possibility of consuming radioactive dust together with the food.

7. All work areas, equipment and clothing should be routinely checked for
contamination.

3rd edition © 2011 | chp 5 | Maktab Sultan Abu Bakar | Sekolah Kluster Kecemerlangan