chapter 9 exercise)

JPN Pahang Physics Module Form 5

Student’s Copy Chapter 9: Electronics

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1

9. 1: USES OF THE CATHODE RAY OSCILLOSCOPE (C.R.O)

9.1.1: Thermionic Emission

1. What is Thermionic Emission?

………………………………………………………………………………………………

2. (a) Label the figure of a vacuum tube:

(b) The figure shows ………… emitted are accelerated ………….. the anode by the high

…………………… between the cathode and anode.

(c) A beam of electrons moving at high speed in a vacuum is known as a ………………..

3. Factors that influence the rate of thermionic emission

Factor Effect on the rate of thermionic emission

Temperature of the cathode When the temperature of the cathode increases, the rate

of thermionic emission increases.

Surface area of the cathode A larger surface area of the cathode increases the rate of

thermionic emission.

Potential difference

between the anode and

cathode.

The rate of thermionic emission is unchanged, when the

potential difference increases, but the emitted electrons

accelerate faster towards the anode.

9.1.2 Properties of Cathode Rays

1. List the four characteristics of the cathode rays.

(i) ……………………………………………………………………..

(ii) …………………………………………………………………….

(iii) ……………………………………………………………………

(iv) ……………………………………………………………………

CHAPTER 9: ELECTRONICS

Figure 9.1



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Energy Change in A Cathode Ray

1. In a cathode ray tube, an electron with kinetic energy of 1.32 × 10-14

J is accelerated.

Calculate the potential difference, V between the cathode and the accelerating anode.

[ e = 1.6 x 10 -19 C]

Solution:

V 1025.8

106.110 1.32

2

1energy Kinetic

3

1914-

2

×=

×=×

==

−

V

V

eVmv

2. In a vacuum tube, a cathode ray is produced and accelerated through a potential

difference of 2.5kV. Calculate…

(a) The initial electric potential energy of the cathode ray.

(b) The maximum velocity of the electron.

[ e = 1.6 x 10 -19 C; m= 9 x 10 -31 kg]

Solution:

(a) J104105.2106.1energy potential Electric 16319 −− ×=×××== eV

(b) 142 104

2

1 −×== eVmv 2109

10431

142 ×

×

×=

−

−

v -1816 ms1098.21089.8 ×=×=v

3. If the potential difference between the cathode and the anode in a CRO is 3.5 kV,

calculate the maximum speed of the electron which hit the screen of CRO.

[ e = 1.6 x 10 -19 C; m= 9 x 10 -31 kg]

Solution:

163192 106.5105.3106.12

1 −− ×=×××== eVmv

15

31

162 1024.12

109

106.5×=×

×

×=

−

−

v -1615 ms1053.31024.1 ×=×=v

By using the principle of conservation of energy,

eVmv =2

2

1,

Maximum velocity of electron, m

eVv

2=

v = velocity of electron

V = Potential difference between Anode and

Cathode

e = Charge on 1 electron = 1.6 x 10 -19 C

m = mass of 1 electron = 9 x 10 -31 kg

Figure 9.2



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9.1.3 Structure of the Cathode Ray Oscilloscope

1. Label all parts of Cathode Ray Oscilloscope below.

2. Fill in the blank all components and its functions.

Main part Component Function

Electron gun

Deflecting

system

Fluorescen

t screen

Figure 9.3



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9.1.4 : The working Principle of the Cathode-Ray Oscilloscope.

1. Fill in the blank the structure of CRO.

9.1.5 Uses of the CRO.

1. The uses of cathode-ray oscilloscope are:

(a) ………………………………………..

(b) ……………………………………….

(c) ……………………………………….

2. If the CRO in figure uses Y-gains of 1.5 Vcm-1

, calculate the value of Vpp.

Solution:

0.30.25.1 =×=V V

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) 2

1× (h)

Effective voltage or root-mean-square voltage, Vr.m.s = pV2

1

Short time intervals, t = no. of divisions between two pulses × time-base value.

Figure 9.4



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3. The figure shows a trace on a CRO set at 5 Volt per division on the vertical axis.

(a) What is the maximum voltage (peak voltage)

indicated?

Solution:

Peak voltage, Vp = (Y-gains) 2

1× (h)

divs V/div VP 42

15 ××=

V VP 10=

4. Figure shows a trace on an oscilloscope for an a.c source.

If the Y-gain is set to 1.5 Vcm-1

and the time-base is 2 ms

cm-1

.

(a) Calculate the peak voltage,Vp of the a.c source.

Solution:

cm Vcm V1-

P 42

15.1 ××=

V VP 0.3=

(b) Calculate the frequency, f of the a.c source.

Solution:

24 ×= cmT ms cm-1

T = 8 ms ∴f = 1251

=T

Hz

(c) Sketch the trace displayed on the screen if the settings are changed to 1 Vcm-1

and 1

ms cm-1

.

5. The diagram shows the trace on the screen of a CRO when an

a.c voltage is connected to the Y-input. The Y-gain control is

set at 2 V/div and the time base is off.

Calculate the value of :

(a) Peak-to-peak voltage, Vpp

(b)Peak voltage, Vp.

Figure 9.5

Figure 9.6

Figure 9.7



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5 divs

(c)Root-mean-square voltage, Vr.m.s

Solution:

(a) Peak-to-peak voltage, Vpp = (Y-gains) × h

= 2V/div × 6 divs

= 12 V

(b) Peak voltage, Vp = 6 V

(c) Vr.m.s = pV2

1= 24.46

2

1=× V

6. When two claps are made close to a microphone which is

connected to the Y-input and earth terminals, both pulses

will be displayed on the screen at a short interval apart as

shown in figure below. Measure the time lapse between the

two claps.

Solution:

7. Figure shows the trace displayed on the screen of a

CRO with the time-base is set to 10 ms/div. What is the

frequency, f of the wave?

Solution:

8. 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:

Length between two pulses = 5 divs

Time taken, t = 5 divs × 10 ms/div

= 50 ms

∴Time interval = 0.05 s

m 1502

0.251200 d Hance,

t

2d V waves,ultrasonic of Speed

s 0.25 ms 250sm ms 50 cm 5

Q and Pbetween time d 2

of distance a through travel to wavesultrasonicfor taken Time

1-

=×

=

=

==×=

=

Distance for two complete wave = 2 divs

∴ Time taken = 2 divs ×10ms/div

= 20 ms

∴frequency, f =T

1=

ms 20

1= 50 Hz

Figure 9.8

Figure 9.9

Figure 9.10



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9.2 SEMICONDUCTOR DIODES

9.2.1 Properties of Semiconductors

1. Semiconductor is

……………………………………………………………………………..

…………………………….

2. Give the examples of pure semiconductor:

(a) ……………………………

(b) …………………………...

(c) ……………………………

9. What is the “doping” process?

………………………………………………………………………………………………

………………………………………………………………………………………………

10. Base on the figure, complete the statement below.

(a) n-type semiconductors

Silicon like Silicon doped with ………………atoms such as …………… or

phosphorus …………. the number of free electron. The phosphorus atoms have

…….. valence electrons, with …… being used in the formation of covalent bonds.

The fifth electron is free to move through the silicon. The silicon has

….………………….. as majority charge-carriers and it thus known as an n-type

semiconductor.

a group of materials that can conduct better than insulators but not as

good as metal conductors.

Silicon

Germanium

Selenium

Doping is a process of adding a certain amount of other substances called dopants

such as Antimony and Boron to a semiconductor, to increase its conductivity.

pentavalent antimony

increases

negative electrons

five four

Figure 9.11

Figure 9.12



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(b) p-type semiconductors

Semiconductor like Silicon doped with ……………….. atoms such as …………

or indium has more positive holes. The Boron atoms have only ………….

valence electrons; hence ………. of the covalent bonds has a missing electron.

This missing electron is called a ‘positive hole’. The majority charge-carriers in

this semiconductor are the ………………. and this semiconductor is thus known

as a p-type semiconductor.

9.2.2 The p-n junction (Semiconductor diode)

1. What is the function of semiconductor diode?

………………………………………………………………………………………………

………………………………………………………………………………………………

2. Label the p-n junction below and draw a symbol of the diode.

trivalent Boron

positive holes

three

one

The function of semiconductor diode is to allow current to flow through it in one direction

only.

Figure 9.13

p-type n-type

Positive hole Negative electron

Symbol

p-n junction



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+ -

- +

3. (a) Forward-biased

(i) In forward-bias, the p-type of the diode is connected to …………………. and the n-

type is connected to the …………………… of the battery.

(ii) Complete the diagram below to show the diode is in forward-bias.

(iii) Draw arrows to show the current, electrons and holes flow in the diagram.

(b) Reverse-biased

(i) In reverse-bias, the p-type of the diode is connected to …………………. , and the n-

type is connected to the …………………… of the battery.

(ii) Complete the diagram below to show the diode is in reverse-bias.

4. Draw arrows to show the current, electrons and holes flow in the diagram.

5. What the meaning of rectification?

………………………………………………………………………………………………

negative terminal

positive terminal

positive terminal

negative terminal

The bulb is light up

The bulb does not light up

The bulb is light up

The bulb does not light up

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

Figure 9.14

Figure 9.15

Figure 9.13



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………………………………………………………………………………………………

6. The figure shows a half-wave rectifier circuit that is connected to CRO.

(i) Sketch waveform of the voltages observed on the CRO screen when the time-

base is on.

(ii) Sketch waveform of the voltages observed on the CRO screen when a

capacitor is connected in parallel across a resistor, R.

7. The figure shows a full-wave rectifier circuit that is connected to CRO.

(i) Draw arrows to show the current flow in the first half cycle and

to show the current flow in second half cycle in the diagram.

(ii) Sketch the waveform of the voltages observed on the CRO screen when the

time-base is on.

To CRO a

Figure 9.16

Figure 9.17

Figure 9.18



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(iii) Sketch waveform of the voltages observed on the CRO screen when a

capacitor is connected in parallel across a resistor, R.

8. What is the function of the capacitor?

……………………………………………………………………………………………....

9.3 TRANSISTOR

9. 3.1 Terminals of a Transistor.

1. What is a transistor?

………………………………………………………………………………………………

2. Draw and label symbol of n-p-n transistor and p-n-p transistor.

3. State the function for each terminal in a transistor.

(a) The emitter, E :

……………………………………………………………………………….

(b) The base, B :

………………… ……………………………………………………………

(c) The collector, C:

………………………………………………………………………………...

Acts as a current regulator or smoother.

To CRO

a

A transistor is a silicon chip which has three terminals labeled as base, collector and emitter.

n-p-n transistor

Base, B

Collector, C

Emitter, E

p-n-p transistor

Base, B

Collector, C

Emitter, E

Acts as a source of charge carriers, providing electrons to the collector.

Controls the movement of charge carriers (electrons) from the emitter (E) to the collector (C).

Receives the charge carriers from the emitter (E)

Figure 9.19



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9.3.2 Transistor circuit

1. (a) Transistor circuit with 2 batteries.

(b) Transistor circuit with 1 battery.

2. The working circuit of a transistor used as a potential divider can be connected as shown

in figure. The voltage across Rx and Ry can be calculated as follows.

VRR

RVx

yx

x

+= V

RR

RV

yx

y

Y

+=

BE : ………………………………….

CE : ………………………………….

Ib : ………………………………….

Ic : ………………………………….

R1 : …………………………………...

R2 : …………………………………...

E1 : …………………………………...

E2 : …………………………………...

Rx : …………………………………...

Ry : …………………………………...

Base circuit

Collector circuit

Base current

Collector current

Limit the base current

Limit the collector current

Supply energy to the base circuit

Supply energy to circuit.

Potential divider

Potential divider

Ie

Remember:

Ie = Ib + Ic

Ie > Ic > Ib

∆Ic >>>>∆Ib

No Ib, No Ic

Ie

Figure 9.20

Figure 9.21



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(a) Figure shows a transistor circuit. The bulb can be lighted up when potential difference, V

across resistor P is 2V and resistance P is 10 kΩ. Calculate the maximum resistance, S so

that the bulb is lighted up.

9.2.3 Transistor as an Automatic Switch.

1. Complete the statement below.

The switching action is produced by using a potential divider. In a working circuit

shown in figure, a resistor, RX and a …………………………. are being used to form a

potential divider. If the variable resistor is set to zero, the base voltage is ………. and

the transistor switches ………. However, if the resistance of the variable resistor is

increased, the base voltage will……………. When the base voltage reached a certain

minimum value, the base current, IB switches on the transistor. A large collector current,

IC flows to light up the bulb.

2. What type of transistor is used in an automatic switch circuit?

………………………………………………………………………………………………

Bulb

Solution:

VRR

RV

ps

p

p

+=

( )V 6

1010

1010V 2

3

3

×+

×=

sR

300001010 3 =×+SR

Ω=Ω= kRS 20 20000

IC

IE

IB

RX

RY Base voltage

Battery voltage

increases

variable resistor

zero

off

Transistor n-p-n

Figure 9.22

Figure 9.23



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3. (a) Light Controlled Switch

(i) Complete the statement below.

Figure shows a transistor-based circuit that functions as a light controlled switch.

The ……………………….. (LDR) has a very high resistance in the …….… and a low

resistor in ………………... R is a fixed ……………. The LDR and R form a potential

divider in the circuit.

In bright light, the LDR has a very ………. resistance compared to R. Therefore, the base

voltage of the transistor is too …….. to switch on the transistor.

In darkness, the resistance of the LDR is very ……… and the voltage across the LDR is

……… enough to switch on the transistor and thus lights up the bulb. This circuit can be

used to automatically switch …… the bulb at night.

(ii) Complete the table below.

Condition RLDR VLDR R VR Transistor (ON or OFF)

Daylight

Darkness

Remember ∆Ic >>>>∆Ib

(iii) How can the circuit in figure be modified to switch on the light at daytime?

…………………………………………………………………………………………..

light-dependent resistor dark

bright light resistor

low

low

large

high

on

low low

high high

high high

low low ON

OFF

The circuit can be modified by interchanging the positions of the LDR and resistor R.

IC

IE

1kΩ

R

LDR

10 kΩ

6 V

Figure 9.24



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(b) A Heat-Controlled Switch

(i) Complete the statement below.

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

A ……………..is a special type of resistor. Its resistance becomes very ……… when it is

cold. When the thermistor is heated, its resistance ………… rapidly. 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.

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

Therefore, the ……………., VB is high enough to switch ……. the transistor. When the

transistor is switch on, the relay switch is activated and the relay is switched ………. The

circuit can also be used in a fire alarm system.

(ii) What is the function of a diode is used in the heat-controlled circuit?

…………………………………………………………………………………………..

…………………………………………………………………………………………..

(iii) Complete the table below.

Temperature RThermistor VThermistor R VR Transistor (ON or OFF)

High

Low

Remember ∆Ic >>>>∆Ib

thermistor high

drops

high

resistance

base voltage on

To protect the transistor from being damaged by the large induced e.m.f in the relay

coil when the collector current, IC drops to zero.

low low

high high

high high

low low

ON

OFF

on

Relay

Alarm

RB

R

Thermistor Diode

Figure 9.25



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9.2.4 Transistor as a Current Amplfier

1. Complete the statement below.

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

larger current. The magnitude of the …………………., IC is primarily determined by the

………………….., IB. A ……….. change in the base current, IB will cause a ……..

change in the collector current, IC. The current amplification can be calculated as follows:

2. Name the type of the transistor used.

………………………………………………………………………………………………

3. What will happened to the readings of the miliammeter, mA and microammeter, µA when

the resistance of R is reduced?

………………………………………………………………………………………………

4. A transistor is said to have two states, the ‘ON’ state and ‘OFF’ state.

(a) Explain the meaning of the ‘ON’ state of a transistor.

………………………………………………………………………………………

(b) Explain the meaning of the ‘OFF’ state of a transistor.

………………………………………………………………………………………

………………………………………………………………………………………

(c) What is the function of the rheostat, R?

………………………………………………………………………………………

(d) What is the function of the resistor, S?

IC

IE

R1

R2

R IB

mA

µA

Current AmplificationB

C

I

I

∆

∆=

collector current

base current small big

When a transistor is in the ‘ON’ state, currents flow in the base and in the collector circuit.

When a transistor is in the ‘ON’ state, there is no current in the base and in the collector

circuit.

To change the base current.

To control and limit the base current.

n-p-n transistor

The readings on miliammeter and microammeter increase.

Figure 9.26



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………………………………………………………………………………………

9.4 Logic Gates

9.4.1 Analysing Logic gates

1. What is a logic gate?

………………………………………………………………………………………………

2. Complete the table below.

Gates Symbol Truth table

AND gate

Input Output

A B Y

0 0 0

0 1 0

1 0 0

1 1 1

OR gate

Input Output

A B Y

0 0 0

0 1 1

1 0 1

1 1 1

NOT gate

Input Output

A Y

0 1

1 0

A switching circuit that is applied in computer in computer and other electronic devices.

Y

A

B

Y A

Y

A

B



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NAND

gate

Input Output

A B Y

0 0 1

0 1 1

1 0 1

1 1 0

NOR gate

Input Output

A B Y

0 0 1

0 1 0

1 0 0

1 1 0

9.4.2 Combinations of logic Gates

1. Find the output Y for each combination of logic gates.

The truth table:

Input Output

A B P Y

0 0 1 0

0 1 1 1

1 0 0 0

1 1 0 0

Y

A

B

Y

A

B

Y

A

B

P 0 0 1 1

0 1 0 1

0 1 0 0

1 1 0 0

Figure 9.27



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2.

The truth table:

3.

The truth table:

Input Output

A B P Q Y

0 0 1 1 1

0 1 1 0 0

1 0 0 1 0

1 1 0 0 0

Input Output

A B B X Y

0 0 1 0 0

0 1 0 0 1

1 0 1 0 0

1 1 0 1 0

Y

A

B

P 0 0 1 1

0 1 0 1

1 0 0 0

1 1 0 0

1 0 1 0 Q

Y

A

B

B

0011

0101

0 0 0 1

1 0 1 0

0 1 0 0

X

Figure 9.28

Figure 9.29



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4.

The truth table:

5.

The truth table:

Input Output

A B P Q Y

0 0 1 0 0

0 1 1 1 1

1 0 1 1 1

1 1 0 1 0

Input Output

A B P Q Y

0 0 1 0 0

0 1 1 1 1

1 0 1 1 1

1 1 0 1 0

Y

A

B

P

Q

0011

0101

1110

0111

0110

Q

S

R P

Q

Figure 9.30

Figure 9.31



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6. Figure shows a logic gate system which switches on an air-conditioner automatically.

Keys:

The light detector (Input J): In the day, logic “1”.

At night, logic “0”.

The heat detector (Input K): Hot, logic “1”.

Cool logic “0”.

(a) Complete the truth table below:

(b) Based on the truth table in (a), state the conditions in which the air-conditioner conditions

in which the air-conditioner will operate and function normally.

………………………………………………………………………………………………

Input Output

J K L

0 0 0

0 1 1

1 0 0

1 1 1

L

Light

detector

Heat

detector

Input J

Input K

Air-conditioner

- On a hot say or daytime – On a hot night

Figure 9.32



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Reinforcement Chapter 9

Part A: Objective questions

1. Which of the following is not a property

of cathode rays?

A. It is positively charged.

B. It travels in a straight line.

C. It can be deflected by magnetic field.

D. It can be deflected by electric field.

2. Cathode rays consists of

A. Fluorescent particles

B. Light rays from a screen

C. Beams of fast moving particles

D. Light rays from hot filament

3. A beam of electrons is being deflected

due to a potential difference between

plates P and Q.

Which of the following statements is not

true?

A. The potential at plate P is positive.

B. The deflection would be greater if

the potential difference is greater.

C. The deflection would be greater if

the electrons are moving faster.

D. The electron beam will return to

straight line if a suitable magnetic

field is applied between the plates.

4. The figure 9.34 shows the trace

displayed on a CRO with the Y-gain

control is turned to 3.75 V/div.

What is the maximum value of the

potential difference being measured?

A. 2.5 V

B. 5.5 V

C. 7.5 V

D. 12.5 V

E. 15.0 V

5. In p-type semiconductor

A. The number of holes are equal to the

number of electrons.

B. The number of the holes are more

than the number of electrons.

C. The number of the holes are less than

the number of electrons.

6. Which of the following is not true about

diode?

A. It can be used to rectify alternating

current.

B. It can only conduct electricity when

it is connected in forward in forward

bias in a circuit.

C. It is formed by joining an n-type and

a p-type semiconductor.

D. The majority charge carriers in the

diode are electrons.

7. The figure 9. 35 shows the arrangement

of silicon atoms after an atom P is doped

to form an extrinsic semiconductor.

Which of the following is not true?

Figure 9.34

P

Q

Figure 9.33

Figure 9.35



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A. The conductivity of the

semiconductor increases.

B. The semiconductor becomes an n-

type.

C. The majority charge carrier is

electron.

D. Atom P is a trivalent atom.

8. The figure 9.36 shows a rectifier circuit.

Which of the following statements is

true?

A. A rectifier changes d.c to a.c.

B. Device P allows current to flow in

any directions.

C. Device Q acts as a rectifier.

D. The rectifier circuit would still work

if device P is reversed.

9. The figure 9.37 shows a circuit

consisting of two diodes and a bulb.

When the switch is on, the bulb does not

light up.

What needs to be done to light up the

bulb?

A. Replace the diode with a new one.

B. Reverse the connection of the diode.

C. Increase the number of bulbs.

D. Connect a resistor in series with the

bulb.

10.

Figure 9.38 shows four identical bulbs,

P, Q, R and S, and four electronic

components connected in a circuit.

Which of the following bulbs will light

up continuously when the switch is on?

A. P and Q only

B. P, Q and R only

C. R and S only

D. P, Q and S only

11. Which of the following circuits shows

the connect directions of the base current

IB, emitter current, IE and collector

current, IC?

Figure 9.36

Figure 9.37

Figure 9.38

P

Q



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12. Which of the following statements about

a transistor is not true?

A. A transistor can act as an amplifier

B. A transistor can act as a relay switch.

C. The function of a transistor is the

same as that of two diodes.

D. A transistor is a combination of two

types of semiconductors.

13. What is the function of the transistor

circuit shown in figure 9.39?

A. As an amplifier

B. As a rectifier

C. As a switch device

D. As a modulator

14. The figure 9.40 shows a transistor being

used as a current amplifier.

Which of the following is correct?

A. IB > IC

B. IB = IC

C. IB < IC

15. Figure 9.41 shows a circuit consisting of

a transistor which acts as an automatic

switch. When the potential difference

across the thermistor is 3 V and the

resistance of the thermistor is 1000 Ω,

the resistance value of resistor, R is ..

A. 3 kΩ

B. 4 kΩ

Figure 9.41

IB IC

Figure 9.40

Figure 9.39



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C. 5 kΩ

D. 6 kΩ

E. 7 kΩ

16. The figure 9. 42 shows a transistor

circuit being used to amplify sound.

Which of the following is not correct

about the circuit?

A. T is an npn transistor

B. The capasitor prevents d.c current

but allows a.c current to pass through

it.

C. Speaker amplifies the sound.

D. R1 and R2 act as potential divider.

17. The figure 9.43 shows a logic gate

circuit with input signals, X and Y.

Which of the following is the output

signal?

18. The figure 9.44 shows a logic gate

circuit.

Which of the following is the output

signal Z?

A. 0110

B. 1010

C. 1110

D. 0101

19. The figure 9.44 shows the combination

of three logic gates.

The truth table for the combination of

tree logic gates is as follows.

What is gate X?

A. AND

B. NOR

C. OR

D. NAND

20. The figure 9. 45 shows a combination of

three logic gates in a logic circuit. When

inputs P and Q are both 1 output Y is 1.

Which of the following logic gates can

be used to represent J and K?

J K

AND NOR

NAND NOR

OR AND

A.

B.

C.

D. NOR AND

M- Microphone

C- Capacitor

S- Speaker

Figure 9.42

Figure 9.43

Figure 9.43

Figure 9.44

Y

J

K

Figure 9.45



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Part B: Structured Questions.

1. Figure 9.46 shows a trace obtained on an oscilloscope screen when an a.c voltage is

connected to the Y-plates of an oscilloscope.

(a) Explain what is meant by thermionic emission.

………………………………………………………………………………………………

(b) Determine the peak voltage of a.c voltage.

………………………………………………………………………………………………

(c) Determine the time for one complete oscillation on the screen.

………………………………………………………………………………………………

(d) What is the frequency of the a.c voltage?

………………………………………………………………………………………………

(e) With the same a.c voltage applied to the oscilloscope, the time-base setting is altered to

2.5 ms/cm and the Y-gain setting is altered to 2 V/cm. On the space below, sketch the

new trace would appear on the oscilloscope.

2. Figure 9. 47 shows a full wave bridge rectifier. The a.c supply has a frequency of 50 Hz.

Figure 9.46

Emission of electrons from the surface of a metal by heat.

2 x 3 = 6V

2 x 5 = 10 ms

f =1/T=50 Hz

Scale: 1 division = 1 cm

The Y-gain is set at 3 V/cm

The time base is set at 5 ms/cm



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(a) When the polarity of the a.c supply voltage is positive at A, state the two diodes which

are forward biased.

…………………………………………………………………………………………..

(b) When the polarity of the a.c supply voltage is negative at A, state the two diodes which

are forward biased.

……………………………………………………………………………………………

(c) Using the axes in figure 9.48, sketch the voltage-time graph across the resistor, R.

(d) On the figure 9.49, sketch the voltage-time graph across the resistor if a capacitor is

connected across the resistor if a capacitor is connected across the resistor R parallel with

the resistor.

(e) Explain how the capacitor causes the voltage across the resistor to vary with time in the

way that you have drawn.

………………………………………………………………………………………………

………………………………………………………………………………………………

3. A student wants to build a simple lift motor control system which operates using two buttons,

A and B for a two-storey building.

A: Up button

B: Down button

D1 and D3

D2 and D4

Figure 9.47

The charging of the capacitor by the power supply and the discharging of the capacitor

through the resistor will smooth the output.

Time/ms

Voltage/V

Figure 9.48

Time/ms

Voltage/V

Figure 9.49



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The lift motor only activates when someone presses any one of the buttons. Figure 9.50

shows the circuit that can be used to

activate the motor.

Keys:

Buttons A and B : When pressed, logic “1”

Not pressed, logic ”0”

X Output : Motor is activated, logic “1”

(a) The truth table below shows the operations of the logic gates in a lift motor control

system.

(i) Using the keys given, complete the truth table.

(ii) Name the logic gate in the circuit in the figure 9.50.

…………………………………………………………………………………

(iii) In the space below, draw the logic gate symbol in 3(a)(ii).

(b) Why is a relay switch needed in the circuit?

………………………………………………………………………………………………

………………………………………………………………………………………………

(c) The door of the lift is fitted with a light transmitter and a detector which is a light

dependent resistor, LDR. If the light dependent resistor detects light, the relay switch is

Input Output

A B X

0 0 0

0 1 1

1 0 1

1 1 0

Figure 9.51

12 V

0 V

A

B

Logic gate

X 240 V

Relay switch Motor

Figure 9.50

AND Gate

Activates large current in the main secondary circuit supply// small current

at the output cannot activate the motor.



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activated and the lift door will close. Figure 9.51 shows an electronic circuit for the

control system of the lift door.

(i) State the relationship between the resistance and the intensity of light received

by the light dependent resistor, LDR.

…………………………………………………………………………………

…………………………………………………………………………………

(ii) Complete the circuit in figure 9.51 by drawing the resistor and the light

dependent resistor using the symbols given below.

(iii) Explain how the circuit functions.

…………………………………………………………………………………

…………………………………………………………………………………

…………………………………………………………………………………

…………………………………………………………………………………

…………………………………………………………………………………

R Motor

240 V

Resistor Light dependent resistor

The higher the light intensity, the lower the resistance of the resistor.

– High light intensity produces lower resistance and high base voltage

- A bigger base current flows and activates the transistor

- A big collector current flows through the relay switch and activates the

circuit of the door motor.



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Part C: Essay Questions

1.

(a) The diode, bulb and battery in circuit X and circuit Y of figures 9.52 and 9.53 are

identical.

(i) What is meant by a direct current and an alternating current? [2 marks]

(ii) Using Figures 9.52 and figure 9.53, compare the connection of the diodes and the

conditions of the bulbs. Relating the connection of the diodes and the conditions of

the bulbs, deduce the function of a diode. [5 marks]

(iii) State the use of a diode. [1 mark]

(b) A semiconductor diode is made by joining a p-type semiconductor with a n-type

semiconductor. Describe and explain the production and the characteristics of a p-type

semiconductor and a n-type semiconductor. [4 marks]

2. Figure 9.55 shows four circuits W, X, Y and Z, each has an ideal transformer and the circuit

are used for the purpose of rectification.

Figure 9.52 Figure 9.53



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(i) What is meant by rectification? [1mark]

(ii) Explain the working principle of a transformer. [3 marks]

(iii) You are asked to make a 12 V battery charger. Study the circuits W, X, Y and Z in

figures 9.55 and consider the following aspects:

Type of transformer

The number of turns in the primary coil and in the secondary coil.

Type of rectification

Characteristics of output current

Explain the suitability of the above aspects and hence, determine the most suitable

circuit to make the battery charge. [6 marks]

3. A student carries out an experiment to determine the relationship between the collector

current IC to the base current IB of a transistor.

Circuit W

Circuit X

Circuit Y

Circuit Z

T 6V

R1 = 1kΩ

A2

IC

A1 IB R2 = 56kΩ

R2 = 2kΩ

Figure 9.56



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0

1

2 3

4

5

Transistor T is connected to fixed resistor R1 =1kΩ and R2 = 56 kΩ and a rheostat R3 as

shown in figure 9.56. The battery supplies a voltage of 6 V to the transistor circuit.

Rheostat R3 is adjusted until the current IB detected by microammeter A1 is 10 µA. The

collector current, IC recorded by miliammeter A2 is shown in figure 9.57(a).

Rheostat R3 is then adjusted to lower value so that microammeter A1 gives IB = 20 µA, 30

µA, 40 µA, 50 µA and 60 µA. The corresponding readings of IC on miliammeter, A2 are

shown in figure 9.57(b), 9.57(c), 9.57(d), 9.57(e) and 9.57(f).

0

1

2 3

4

5 0

1

2 3

4

5

0

1

2 3

4

5 0

1

2 3

4

5

(b) IB = 20µA (b) IB = 30µA

(a) IB = 10µA

mA

mA mA

mA mA

31



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(a) For the experiment described identify…

(i) the manipulated variable : ..………………………………

(ii) the responding variable : ………………………………..

(iii) the fixed variable : ………………………………..

(b) From the figure in 9.57, record the collector current, IC when IB = 10, 20, 30, 40, 50 and

60µA. Tabulate your results for IB and IC in the space given below.

IB/µA IC/mA

10 0.8

20 1.6

30 2.4

40 3.1

50 3.9

60 4.8

(c) On a graph paper, draw a graph of IC against IB.

(d) Based on your graph, determine the relationship between IC and IB.

………………………………………………………………………………………………

0

1

2 3

4

5

(c) IB = 40µA (d) IB = 50µA

(e) IB = 60µA

mA

32 The base current, IB

The collector current, IC

The supply voltage

Ic is directly proportional to IB



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4. Figure 9.58 shows a microphone connected to a power amplifier. When the microphone

has detected a sound, an amplified sound is given out through the loudspeaker. The sound

becomes louder if the volume of the amplifier is turned on to increase the power.

Using the information based on the observation of the brightness of the bulbs,

(a) Make one suitable inference.

(b) State one appropriate hypothesis that could be investigated.

(c) Design an experiment to investigate the hypothesis stated in (b). Choose suitable

apparatus such as a diode, rheostat and others.

In your description, state clearly the following:

(i) Aim of the experiment,

(ii) Variables in the experiment,

(iii) List of apparatus and materials,

(iv) Arrangement of the apparatus,

(v) The procedure of the experiment, which includes the method of controlling

the manipulated variable and the method of measuring the responding

variable,

(vi) The way you would tabulate the data,

(vii) That way you would analyse the data.

Figure 9.58

Loudspeaker Power amplifier

Volume control

Microphone



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chapter 9 exercise)

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