PHYSICSFORM 5

Cikgu DesikanCompiled by

Analysis of Past Year Questions

Learning Objectives :

Dear students,

1. Understanding the uses of the

Cathode Ray Oscilloscope (C.R.O.)

2. Understanding semiconductor diodes

3. Understanding transistors

4. Analysing logic gates

Look at the sky. We are not alone. The whole universe is friendly

to us and conspires only to give the best to those who dream

and work.

- Dr. Abdul Kalam

Chapter 9

Electronics

FO

RM

5 P

HY

SIC

S

2016

2007 2008 2009 2010 2011 2012 2013 2014 2015

P1 4 3 4 1 4 4 4 5

P2

A - 2 1 1 1 1 1 -

B 1 - - - - - - -

C - - - - - - 1 -

P3A - - - - - - - -

B - - - - - - - -

Concept Map

Dear brothers and sisters,

STOP searching for strength and willpower.

START creating it !!!

Electronics

Cathode ray

Oscilloscope

Semiconductor

diodes

Thermionic

emissionp-type and n-type

semiconductor

diodes

Transistors Logic gates

AND

OR

NOT

NAND

NOR

Truth Table

Controlling systems

Applications

Current

amplifier

Electrical

power

Half-wave

rectification

Half-wave

rectification

C.R.O

Structure

Applications

Chapter 9

Electronics

9.1 Cathode Ray Oscilloscope

Thermionic emission

How does thermionic emission occur?

1. Metal consists of a large number of electrons which are free to

move.

2. At room temperature, the electrons are free to move but

remain inside the metal.

3. The electrons cannot escape at the surface because they are

held back by the attractive forces of the atomic nucleus.

4. If the metal is heated at a high temperature, some of the free

electrons may gain sufficient energy to escape from the metal.

Cathode rays

Cathode Ray

Oscilloscope

4

e

e

e

e

Heated filament

cathode

- +Beam of

electrons

Fluorescent

Screen

Vacuum

Anode

6 V a.c

Cathode

EHT

Process of ______________ of electrons from a heated metal’s surface

1. Thermionic emissions can be used to produce a continuous flow of electrons in a cathode ray

tube.

2. When the cathode is connected to the anode by an extra high tension (EHT) voltage supply,

a narrow beam of fast electrons will move to the anode.

3. The beam of electrons moving from the cathode to the anode is called cathode rays.

4. Cathode rays can be used in picture tube of a television, a cathode ray oscilloscope and the

visual display on a radar screen.

Properties Of Cathode Rays

1.

2.

3.

4.

5.

5

Cathode rays

Investigate the properties of electron streams in a Maltese cross cathode ray tube

ProcedureObservation on the

fluorescent screen Explanation

Connect only

the 6.3 V power

supply to the

filament

A dark shadow of the Maltese

Cross is formed on the screen

When the 6.3 V power supply is switched on,

the filament is heated. The Maltese cross

shadow is formed on the screen due to the

light from the filament.

6

- +

Maltese

cross

Fluorescent

Screen

VacuumAnode

6 V a.c

Cathode

EHT

7

Connect the

6.3 V and EHT

to the

electrodes

A darker shadow of the

Maltese Cross is seen

on the screen. The

shadow is surrounded

by green light

1. When the EHT power supply is switched on, a

high voltage is applied between the cathode

and anode causing electrons to accelerate at

high speeds from cathode to anode. These

electrons are the cathode rays .

2. The cathode rays blocked by the Maltese Cross

causing a shadow to form on the screen. The

cathode rays travel in straight lines.

3. The green screen formed around the shadow

when the EHT power supply is switched on

shows that the kinetic energy of the electron is

converted into light energy when the electrons

hit the fluorescent screen.

Bring a pole of

a bar magnet

near to the

neck of the

tube

Two shadow are seen

on the screen. The light

shadow remains at the

centre of screen while

the dark one is shifted

1. When a strong magnet is placed at the side of

the Maltese Cross tube, the shadow formed is

moved and distorted.

2. This shows that cathode rays are deflected by a

magnetic field.

Reverse the

pole of the bar

magnet

The light shadow

remains at the centre of

screen while the dark

one is shifted to the

opposite direction

Investigate the properties of cathode rays in an electric field

8

-

-

+

+

Plates

VacuumAnode

6 V a.c

Cathode

EHT 1

EHT 2

No voltage connected to the

deflecting plates

Top plate is connected to

EHT (+) and lower plate is

connected to EHT (-)

Top plate is connected to EHT

(-) and lower plate is

connected to EHT (+)

Summary of Investigation

1.

2.

1. 2. 3.

Cathode Ray Oscilloscope

9

- +EHT

Accelerating

Anode

6 V a.c

Cathode

Focusing

Anode

Filament

Control grid Y-plate X-plate

Graphite

coating

Electron

beam

Fluorescent

Screen Vacuum

Bright spot

a. Uses a cathode ray tube that converts electronic and electrical signals to a visual display.

b. The graph produced consist of a horizontal axis which is normally a function of time, and a

vertical axis which is a function of the input voltage.

c. The components in a cathode ray tube consists of a vacuum glass tube with

i. an electron gun,

ii. a deflection system for deflecting the electron beam and

iii. a fluorescent coated screen.

10

Part Function

Filament Is heated when current flows through it. It is used to heat up the

cathode.

Cathode Heated cathode emits electrons through the process of thermionic

emissions.

Control grid Control the number of electrons in the electron beams.

The more negative the grid, the fewer the electrons are emitted from the

electron gun and the less the brightness of the bright spot on the

screen.

Focusing anode To focus the electrons into a beam and to attract electrons from the area

of the control grid.

Accelerating anode To accelerate the electron beam towards the screen.

Electron Gun

The electron gun is used to produce a narrow beam of electrons.

No input voltage.

The electron beam does not deflect

and the bright spot is at the centre

+ve voltage is applied.

The electron beam deflect upward.

The bright spot moves to the top.

Deflection System

1. The deflection system allows the electron beam to be deflected from its straight-line path when

it leaves the electron gun.

2. Y-plates is to move the electron beam vertically up and down the screen when an input voltage

is applied across it.

Y-plate X-plate

Electron

beam

Bright spot

Screen

No input voltage

is applied

+V

Electron

beam

A positive voltage

is applied Bright spot

Screen

-ve voltage is applied.

The electron beam deflect

downward.

The bright spot moves to the

bottom.

a.c voltage is applied.

The electron beam deflects up and

down.

The bright spot moves up and

down to form a bright vertical

trace on the screen

The function of the X-plates is to sweep the electron beam across the screen

horizontally from left to right at a steady speed. 12

Electron

beam

A positive voltage

is applied

Bright spot

-VScreen

Screen

A.C.voltage is

applied

Electron

beam

13

Fluorescent Screen

1. The fluorescent screen is coated on the inside surface with some fluorescent material such as

phosphor or zinc sulfide.

2. When electron beam strikes the screen, the material becomes glows. This enables a bright spot

to appear whenever an electron beam strikes the screen.

3. The moving electrons have kinetic energy. When this electrons strikes the screen, the

fluorescent coating on the screen converts the kinetic energy of the electrons into light energy.

Application of CRO

Working principle of the cathode ray oscilloscope, CRO

Y-Gain

Time base

Control knob Function

Power switch 1. Control the power supply

Focus 1. Control the sharpness of the bright spot

2. Connected to the focusing anode

3. The sharpness of the bright spot is also affected by the brightness

Brightness

1. To control brightness or intensity of the bright spot

2. Connected to the control grid

3. Brightness level should be set as low as possible to obtain a clear

and sharp trace

X-shift 1. To adjust the horizontal position of the bright spot on the screen

2. Connected to the X-plates

Y-shift 1. To adjust the vertical position of the bright spot or the trace displayed

2. Connected to the Y-plates

Y gain

(volts / div)

1. To control the magnitude of the vertical deflection of the bright spot or

the trace displayed on the screen by adjusting amplitude

2. Connected to the Y-plates

Time-base

(time/div)

1. To control the magnitude of the horizontal deflection of the bright spot

or the trace displayed on the screen by adjusting the frequency

2. Connected to the X-plates

15

Control knob Function

X-input 1. A terminal to connect the voltage to the X-plates

Y-input 1. A terminal to connect the voltage to the Y-plates

AC/DC switch

1. To select the type of input received

2. When the switch is at DC position, the a.c and d.c voltages will be

displayed

3. When the switch is at AC position, only the a.c voltage will be

displayed. Any signals of d.c voltage will be blocked by a capacitor in

the CRO

Earth 1. To disconnect the input voltage at the Y-plates and to earth the input

terminal

Life affords no higher pleasure than that of surmounting

difficulties, passing from one step of success to another,

forming new wishes, and seeing them gratified. He that labors

in any great or laudable undertaking has his fatigues first

supported by hope, and afterwards rewarded by joy...

To strive with difficulties, and to conquer them, is the highest

human felicity.

Samuel Johnson

16

Display wave forms and measuring voltage from a DC source using a CRO

17

Type of power

supply

connected to

Y-input of CRO

Time-base

switched off

Time-base

switched on

No input

DC power

supply

AC power suppy

a.c d.cY-input

CRO

Battery

Measuring Potential Difference using the CRO

The selected range of

the Y-gain control

Displacement of the bright

spot from the zero position X

time-base off time-base on

DC voltage =

What is the value of the dc voltage in

figure (a) and (b) if the Y-gain control is

1 V/div ?

a) b)

18

a.c d.cY-input

a.c d.cY-input

Measuring Potential Difference using the CRO

The selected range

of the Y-gain control

Height of vertical trace from

the zero position XPeak ac voltage =

Y-gain = 2 V/div

Height of vertical trace from zero position =

Peak ac voltage =

19

a.c d.cY-input

a.c d.cY-input

Measure short time intervals

1. The time-base is set to 1 ms/div

2. It means I div = 0.001 s

3. The number of div is counted between

two crests of a wave

4. The short time interval between pulses =

Multiplying the number of division by the

time-base .

Length between 2 signals = ____ div

Time base is set = 10 ms/div

Time taken, t =

Solve problems based on the CRO display

Example 1

Diagram 1 shows a trace produced by an ac

power supply which is connected to Y-input of an

CRO setting at 20 V/div and 5 ms/div.

Calculate:

(a) Period

(b) Frequency

(c) Peak voltage

20

a.c d.cY-input

Example 3

Diagram 3 shows a wave produced by an audio generator displayed

the screen of a CRO. The length between the two crests is 3 cm.

(Given 1 division = 1cm)

(a) What is the period of the wave?

(b) If the time-base is set to 5 ms/div, find the frequency.

(c) When the frequency of the wave is double, what is the length

between the two crests?

Example 2

21

3 cm

Diagram 2 shows a trace produced by an a.c power supply connected to a

CRO with the time base is switched of. The Y-gain is set to 20 V/div. Find

the peak voltage.

4.2 Semiconductor Diodes

Metal Insulators Semiconductors

Good conductors of

electricity because

they have free

electrons that can

move easily between

atoms

The resistance of

metals is generally

very low.

Poor conductors of

electricity because

they have too few

free electrons to

move about.

The resistance of

insulators is very

high.

1. A material that has an electrical conductivity that

is between that of a conductor and an insulator.

2. The resistance of semiconductors is between

that of conductors and insulators.

3. Semiconductors can be pure element such as

silicon or germanium.

4. At 0 Kelvin it behaves as an insulator. When the

temperature increases, the conductivity of the

electricity will increase because its resistance

will be lowered.

Semiconductors in terms of resistance

Charge Carriers

Electricity conductivity in semiconductors occurs because there is two type of charge carriers:

1. _______________ which is negatively charged

2. _____________which is positively charged.

22

Characteristics of a silicon atom

1. There are four electrons in the outermost shell of a

silicon atom and they are shared between four other

neighbouring atoms to form four covalent bonds.

2. Each of the covalent bonds has a pair of electrons.

Every atoms shares one electron with each of its

neighbours.

3. Figure on the left shows the outer electrons in a silicon

crystal which all are involved in perfect covalent bonds,

leaving no free electrons to conduct electricity.

4. At very low temperature, pure silicon crystal is an

insulator and has a high resistance to current flow.

5. As the temperature of pure silicon crystal increases, the energy of the vibrating atoms in the

silicon crystal causes some electrons to break free.

6. For every electron that is broken free, there is a hole in the bonding structure between the

atoms of the crystal. (atom X)

7. These holes are said to be carriers of positive charge

8. One outer electron from the neighboring atom (Y) will fill the hole and at the same time will

produce a hole at Y.

9. When the valence/outer electron moves to the left, the hole ‘move’ to the right

10. This is the physical origin of the increase in the electrical conductivity of semiconductors with

temperature

23

Valence electron

Covalent

bond

Pure semiconductor at 0 Kelvin

Doping process

Doping is a

n-type semiconductor

1. n-type doping is to produce an abundance of electrons in the semiconductor

2. A silicon atom has four valence / outer electrons which each electron is covalently bonded with

one of four adjacent silicon atoms

24

Hole

X Y

Hole

Hole ( + )

Electron ( - )

3. If atoms with five valence electrons (pentavalent atoms) are doped into the pure

semiconductor, then each of the pentavalent atoms will have four covalent bonds and one extra

electrons.

4. It takes only a very small quantity of the impurity to create enough free electrons to allow

electric current to flow through silicon.

5. The free electrons are the majority carriers and the holes are the minority carriers

6. Since the pentavalent atom donates an extra electron it is therefore called the donor atom.

7. Example: phosphorus, arsenic, or antimony

Phosphorus atom

25

Phosphorus

atom

1. p-type doping is to create an abundance of holes in the material.

2. If atoms of three valence electrons (trivalent atoms) are doped into the pure semiconductor, one

electron is missing from one of the four covalent bonds. The deficiencies of valence electrons

are called holes.

3. When current passes, a ‘hole’ is filled by an electron from a neighbouring atom. In this way the

hole moves from one atom to another.

4. The holes are the majority carriers and the free electrons are the minority carriers.

5. Since the trivalent atom accepts an electron, it is therefore called the acceptor atom.

6. Examples: boron, aluminium, gallium

Electronic Structure of

Aluminium (Al)

p-type semiconductor

26

Aluminium

(Al)

Comparison between the n-type and p-type semiconductor

Aspect n-type

Semiconductor

p-type

Semiconductor

Pure

semiconductor

Dopants material

Function of the

dopants material

Valens electrons

of the dopant

material

Majority charge

carriers

Minority charge

carriers

27

Semiconductor diodes

1. The simplest semiconductor device is a diode.

2. A diode is made by joining a p-type and n-type

semiconductors

3. A diode is a device that allows current to flow in one

direction only but blocks it in the opposite directions.

p-n junction

1. A p-n junction is formed when a n-type and p-

type semiconductors are joined together.

2. The boundary between the p-type and n-type

regions is called the ________________.

3. At the p-n junction, electrons from the n-side

move to the p-side and recombine with the

holes.

4. Holes from the p-side similarly move into the n-

side, where they recombine with electrons.

5. As a result of this flow, the n-side has a net

positive charge, and the p-side has a net

negative charge.

28

p n

Structure

Symbol

Cathode (-)Anode (+)

Junction electronhole

p n

Depletion Layer

1. The region around the junction is left with

neither holes nor free electrons.

2. This neutral region which has no charge carriers

is called the ______________________.

3. This layer which has no charge carrier is a poor

conductor of electricity.

Forward bias

1. The p-type of the diode is connected to the

positive terminal and the n-type is connected to

the negative terminal of a battery.

2. The diode conducts current because the holes

from the p-type material and electrons from the

n-type material are able to cross over the

junction.

3. A light bulb will _______________.

29

--

++

Junction electronhole

p n

Depletion layer

Narrow depletion layer

What is reversed bias?

1. The n-type is connected to the positive terminal and

the p-type is connected to the negative terminal of

the battery.

2. The reversed polarity causes a very small current to

flow as both electrons and holes are pulled _______

from the junction.

3. When the potential difference due to the widen

depletion region equals the voltage of the battery,

the current will cease. Therefore the bulb ________

light up.

30

Wide depletion layer

Half-wave rectification

Half-wave rectification by using one diode

1. When a diode is connected in series with the resistor, any current that passes through the

resistor must also pass through the diode.

2. Since diode can only allow current to flow in one direction, therefore the current will only flow in

the first half-cycle when the diode in forward bias.

3. The current is blocked in the second half-cycle when the diode is in reverse bias.

Output

(varying dc)

31

Input

ac current

Diodes as rectifiers

1. A rectifier is an electrical device that converts alternating current to direct current.

2. Rectification is a process to convert an alternating current into a direct current by using a diode.

3. Two type of rectification:

a. ___________________________________________

b. ___________________________________________

Full-wave rectification

1. A process where both halves of every cycle of an alternating current is made to flow in the

same direction.

2. In the first half, the current flows from A to P to TU to R to B

3. In the second half, the current flows from B to S to TU to Q to A.

4. The direction of the ac current passing through the resistor for each half cycle is the same ie T

to U.

Full-wave rectification by using four diodes

Input

ac current

Output

(varying dc)

32

Use of a capacitor to smooth out output current and output voltage in a rectifier circuit

1. When the current pass through the resistor and capacitor, the capacitor is charged and stores

energy.

2. When there is no current pass through the resistor and capacitor, the capacitor discharge and

the energy from it is used to produce voltage across the resistor. As a result it produces a

smooth dc output.

33

Circuit Diagram

Output ( V – t Graph )

Input

ac current

9.3 Transistor

1. A transistor has three leads connected to the emitter, base and

collector.

2. The emitter emits or sends charge carriers through the thin base

layer to be collected by the collector.

3. There is two-type of transistor: npn transistor and pnp transistor.

4. In an npn transistor the emitter sends negative electrons to the

collector.

5. In an pnp transistor, the p-type emitter sends positive holes to the

collector.

6. In both cases, the arrow on the emitter shows the direction of

current flow.

7. The output current, of a transistor flows between the emitter and

the collector.

8. The current in the collector lead is called collector current, IC.

9. The base current, IB is used to control the collector current

through the transistor. The base current can be used to switch the

collector current on or off.

34

npn transistor

pnp transistor

n

p

n

p

n

p

B

Base current is too small compared to the

collector current. The unit of base current

is μA while the unit for the collector

current is mA.

Emitter current, IE is equal the sum of

base current and collector current :

Therefore,

If there is no current flow in the base circuit, then there is also no current flow in the collector

circuit.

IB = 0 then IC = 0 ►

IB ≠ 0 then IC ≠ 0 ►35

R1

R2

E1

E2B

C

E

IB

IC

Transistor as a current amplifier

Transistor as an automatic switch

A small change in the base current, results in a big change in the collector current.

Transistor as an automatic switch

1. Choose a suitable resistor R1 and a variable resistor R2. The voltage at base terminal can be

adjusted to switch the transistor on or off.

2. If the variable resistor = 0, base voltage = 0 and the transistor remains off.

3. If the variable resistor is increased, the base voltage increases.

4. When the base voltage reaches certain minimum value, the base current switches the

transistor on.

5. The large collector current flows through the transistor causing the bulb to light up.

Potential divider circuit

If the variable resistor in the

transistor is replaced by a

device such as light

dependent resistor (LDR), a

thermistor or a microphone,

the transistor can be used as

an automatic switch

controlled by light, heat or

sound.

36

IB

10 kΩ

R2

6 V

10 kΩ

R1

IC

IE

Battery

Voltage

Base

Voltage

Transistor as a Current Amplifier

1. A transistor functions as a current amplifier by allowing a small current to control a larger

current.

2. The magnitude of the ________________________, IC is primarily determined by the _______

_____________, IB.

3. A __________ change in the base current, IB will cause a ________ change in the collector

current, IC.

4. The current amplification can be calculated as follows :

37

Current Amplification =

1. The LDR has a very resistance in darkness and a resistance in light. R is a

fixed resistor.

2. The LDR and R form a potential divider in the circuit.

A light controlled switch

Circuit switches on the light at daytime and

switches off the bulb at night automatically

Circuit switches on the bulb at night and

switches off the bulb at day time automatically

38

Z

X

Y

50 kΩ

5 kΩ

LDR6 V

Bulb

Z

X

Y

50 kΩ

5 kΩ

LDR

6 V

Bulb

1. In daylight, the LDR has a very low

resistance as compared to R.

1. In daylight, the LDR has a very low

resistance as compared to R.

“ Your diamonds are not in far distant mountains or in yonder

seas; they are in your own backyard, if you but dig for them.”Russell H. Conwell

39

2. Therefore the base voltage is _______

enough to switch the transistor on and to

light up the bulb.

3. In darkness, the LDR has a very ________

resistance and therefore the base voltage is

too _______ to switch the transistor on. The

bulb light off.

2. Therefore the base voltage is too _______

to switch the transistor on

3. In darkness, the LDR has a very _______

resistance and the base voltage is ______

to switch the transistor on to light on the

bulb.

Heat-Controlled Switch

1. Figure shows a transistor-based circuit that function as a heat controlled switch.

2. A _______________ is a special type of resistor. Its resistance becomes very __________

when it is cold.

3. When the thermistor is heated, its resistance __________ rapidly.

4. At room temperature, the thermistor has a __________ resistance compared to R.

Therefore, the base voltage of the transistor is too low to switch on the transistor.

5. When the thermistor is heated, its __________ drops considerablely compared to R.

6. Therefore, the ______________ , VB is high enough to switch ______ the transistor.

7. When the transistor is switch on, the relay switch is activated and the relay is switched ____.

8. The circuit can also be used in a fire alarm system.

9. Function of a diode in this circuit : protect transistor from being damaged by the large

e.m.f in relay coil when IC drops to zero.40

50 kΩ

40

kΩ

6 V

Loud speaker

10

kΩ

Sound amplifier

Microphone

1. Microphone converts audio (sound) signal into ______________signal (varying current).

2. Capasitor allows the varying current flow it ( and prevent direct current to flow from

the battery to the transistor and the loudspeaker).

3. Base current is changed and causes large change in the collector current, IB.

4. Collector current flows in loudspeaker.

5. Sound waves with higher amplitude is produced.

41

9.4 Logic Gates

Logic gates as switching circuits in electronic systems

1. Security lamps, alarm systems, and washing machines can make some simple decisions.

2. The switching on and off operations are controlled by electronic switches made up of logic

gates.

3. Logic gates work using tiny transistors as switches. They are manufactured as integrated circuit

(IC), with each chip holding several gates.

4. A logic gate is a circuit that has one or more input signals but only one output signal.

5. For each gate, the input or inputs are on the left of the symbol. The output is on the right

6. Each input and output can be either high (logic 1) or low (logic 0).

7. A binary “0” represents 0 V, and a binary “1” represents a non zero voltage.

Truth table

A truth table lists all input possibilities and the corresponding output for each input.

Gates Truth Table Action

AND gate For the input to be ON, both

inputs must be ON.

Output in ON only when both

inputs A and B are ON.

42

A

BX

INPUT OUTPUT

A B X

Gates Truth Table Action

OR gate For the output to be ON at

least one of the inputs must be

ON.

Output Q is ON when input A

or B or both is ON

NOT gate The output is ON when the

input is OFF, and vice versa

43

A

BX

INPUT OUTPUT

A B X

A X

INPUT OUTPUT

A X

“Believe it is possible to solve your problem. Tremendous things happen

to the believer. So believe the answer will come. It will.”Norman Vincent Peale

Gates Truth Table Action

NAND gate It is equivalent to an AND gate

with its output inverted by a

NOT gate.

Output Q is OFF when inputs A

and B are both ON

NOR gate It is equivalent to an OR gate

with its output inverted by a

NOT gate.

Output Q is ON when both

input A and input B are OFF

44

A

BX

INPUT OUTPUT

A B X

A

BX

INPUT OUTPUT

A B X