Chapter 9 Electronic

JPN Pahang Teacher’s Guide

Physics Module Form 5 Chapter 9: Electronics

CHAPTER 9: ELECTRONICS 9. 1: USES OF THE CATHODE RAY OSCILLOSCOPE (C.R.O) 9.1.1: Thermionic Emission

1. What is Thermionic Emission? Thermionic Emission is the release of electrons from a heated metal cathode. ………………………………………………………………………………………………

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

Figure 9.1

towards the anode by the high electrons emitted are accelerated ………….. (b) The figure shows ………… potential difference between the cathode and anode. difference …………………… cathode ray (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 Effect on the rate of thermionic emission Factor the temperature the cathode increases, the Temperature of the cathode WhenWhen the temperature of theofcathode increases, the rate rate of thermionic emission increases. of thermionic emission increases. A larger surface surface arthe ea of the cathode cathode increases increases Surface area of the cathode A larger surface area ofarea cathode increases the ratethe of rate ofemission. thermionic emission. thermionic Theofrate of thermionic emission is unchanged, The rate thermionic emission is unchanged, when when the Potential difference the potential but theelectrons emitted between the anode and potential differencedifference increases,increases, but the emitted electrons faster towards the anode. cathode. accelerate fasteraccelerate towards the anode. 9.1.2 Properties of Cathode Rays

1. List the four characteristics of the cathode rays. They are negatively charged particles. (i) …………………………………………………………………….. They travel in straight lines. (ii) ……………………………………………………………………. possess momentum and kinetic energy. (iii) They …………………………………………………………………… They are deflected by magnetic and electric field. (iv) ……………………………………………………………………

1



Energy Change in A Cathode Ray

Figure 9.2

By using the principle of conservation of energy, 1 mv 2 = eV , 2 2eV

Maximum velocity of electron, v =

m

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

-14

1. In a cathode ray tube, an electron with kinetic energy of 1.32 × 10 J is accelerated. Calculate the potential difference, V between the cathode and the accelerating anode. [ e = 1.6 x 10 -19 C] Solution: 1 Kinetic energy = mv 2 = eV 2

1.32 × 10-14

= 1.6 ×10−19V

V = 8.25 × 104 V

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) Electric potential energy = eV = 1.6 × 10 (b)

1 2

mv

2

= eV = 4 × 10

−16

v

2

=

4 × 10

−16

9 × 10

−31

−19

× 2.5 × 10 3 = 4 × 10 −16 J

×2

v = 8.89 × 10

14

= 2.98 × 10 7 ms -1

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:

1 2 v

mv

2

=

2

= eV = 1.6 × 10 −19 × 3.5 × 103 = 5.6 × 10 −16

5.6 × 10 9 × 10

−16

−31

× 2 = 1.24 × 1015

v = 1.24 × 10

15

2

= 3.53 × 10 7 ms-1



9.1.3 Structure of the Cathode Ray Oscilloscope

1. Label all parts of Cathode Ray Oscilloscope below.

Focusing Anode Cathode

X-plat Acceleration Anode

Filament

Fluorescent screen

Y-plat

Figure 9.3

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

Electron gun gun

Component

Function

Filament

When a current passes through the filament, the filament becomes becomes hot and heats up the cathode. cathode.

Cathode

Emits electrons when it is hot.

Control Grid

à Control the number of electrons hitting the

fluorescent screen. fluorescent screen. à Control the brightness of the spot on the screen.

Deflecting Deflecting system

Fluorescent Fluorescent screen

Focusing Anode

To focus the electrons onto the screen.

Accelerating Accelerating Anode

To accelerate the electrons to high speed.

Y-Plates

To deflect the electron beam vertically.

X-Plates

To deflect deflect the electron electron beam horizontally. horizontally.

Glass surface coated with a fluorescent fluoresc ent material.

To convert the kinetic energy of the electrons to heat and light energy when the electrons hit the screen.

3



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

1. Fill in the blank the structure of CRO.

Brightnesss Brightnes Focus Y-shift

X -shift

Y-Gains

Figure 9.4

Time-base

Y-input

Earth

X-input

9.1.5 Uses of the CRO.

1. The uses of cathode-ray oscilloscope are: measure a D.C or A.C voltage (a) To ……………………………………….. measure a short time intervals (b) To ………………………………………. display the waveform (c) To ……………………………………….

To measure a D.C voltage: The unknown voltage, V = (Y-gain) × h To measure a A.C voltage: Peak-to-peak voltage, V pp = (Y-gains) × h 1 Peak voltage, V p = (Y-gains) × (h) 2 Effective voltage or root-mean-square voltage, V r.m.s r.m.s =

1

V p 2 Short time intervals, t = no. of divisions between two pulses × time-base value.

-1

2. If the CRO in figure uses Y-gains of 1.5 Vcm , calculate the value of V pp. Solution: V = 1.5 × 2.0 = 3.0 V

Figure 9.5 4



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: 1 Peak voltage, V p = (Y-gains) × (h) 2 1 V P = 5 V/div × × 4 divs 2 Figure 9.6 V P = 10 V 4. Figure shows a trace on an oscilloscope for an a.c source. -1 If the Y-gain is set to 1.5 Vcm and the time-base is 2 ms -1 cm . (a) Calculate the peak voltage, V p of the a.c source. Solution: 1 -1 V P = 1.5 Vcm × × 4 cm 2 V P = 3.0 V

Figure 9.7

(b) Calculate the frequency, f of the a.c source. Solution: -1 T = 4cm × 2 ms cm 1 ∴ f = = 125 Hz T = 8 ms T (c)

Sketch the trace displayed on the screen if the settings are changed to 1 Vcm -1 ms cm .

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, V pp (b)Peak voltage, V p. (c)Root-mean-square voltage, V r.m.s r.m.s

Figure 9.8 5

-1

and 1



Solution: (a) Peak-to-peak voltage, V pp = (Y-gains) × h = 2V/div × 6 divs = 12 V (b) Peak voltage, V p = 6 V 1 1 (c) V r.m.s × 6 = 4.24 V V p = r.m.s = 2 2

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: Length between between two pulses pulses = 5 divs divs Time taken, t = 5 divs × 10 ms/div = 50 ms = 0.05 s ∴Time interval

5 divs

Figure 9.9

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: Distance for a complete wave = 2 divs ∴ Time taken = 2 divs ×10ms/div = 20 ms 1 1 ∴ frequen frequency, cy, f = = = 50 Hz T 20 ms

Figure 9.10

8. An ultrasound signal is transmitted vertically down to the sea bed. Transmitted and -1 reflected signals are input into an oscilloscope with a time base setting of 150 ms cm . The diagram shows the trace of the two signals on the screen of the oscilloscope. The -1 speed of sound in water is 1200 ms . What is the depth of the sea? Solution: Time taken for ultrasonic waves to travel through a distance of 2 d = time between P and Q

= 5 cm × 150 ms sm -1 = 750 ms = 0.75 s Speed of ultrasonic waves, V = Hance, d =

1200 × 0.75 2

2d t

= 450 m

Figure 9.11

6



9.2 SEMICONDUCTOR DIODES 9.2.1 Properties of Semiconductors

1. Semiconductor is a group of materials that can conduct better than insulators but not as …………………………………………………………………………….. good as metal conductors. ……………………………. 2. Give the examples of pure semiconductor: Silicon (a) …………………………… Germanium (b) …………………………... Selenium (c) …………………………… 9. What is the “doping” process? Doping is a process process of of adding a certain certain amount amount of other substance substancess called dopants ………………………………………………………………………………………………

Boron to a semiconductor, to increase increase its conductivity conductivity. such as Antimony and Boron ……………………………………………………………………………………………… 10. Base on the figure, complete the statement below. (a) n-type semiconductors

Figure 9.12 pentavalent pentava lent Silicon like Silicon doped with ………………atoms such as …………… antimony or increases the number of free electron. The phosphorus atoms have phosphorus …………. five valence electrons, with four …….. …… being used in the formation of covalent bonds.

The fifth electron is free to move through the silicon. The silicon has negative electrons ….………………….. as majority charge-carriers and it thus known as an n-type semiconductor.

7



(b) p-type semiconductors

Figure 9.13

trivalent atoms such as ………… Boron Semiconductor like Silicon doped with ……………….. three or indium has more positive holes. The Boron atoms have only …………. one of the covalent bonds has a missing electron. valence electrons; hence ……….

This missing electron is called a ‘ positive hole’. The majority charge-carriers in positive holes holes and this semiconductor is thus known this semiconductor are the ……………….

as a p-type semiconductor.

9.2.2 The p-n junction (Semiconductor diode)

1. What is the function of semiconductor diode? The function of semiconductor diode is to allow current to flow through it in one direction direction ……………………………………………………………………………………………… only. ………………………………………………………………………………………………

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

n-type

Positive hole

Negative electron

p-n junction junction

Figure 9.13

8

Symbol



3. (a) Forward-biased positive terminal terminal (i) In forward-bias, the p-type of the diode is connected to …………………. and the nnegative terminal of the battery. type is connected to the ……………………

(ii)

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

-

+

The bulb is light up

ü

The bulb does not light up Figure 9.14 (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 …………………. negative terminal, and the n positive terminal terminal of the battery. type is connected to the ……………………

(ii)

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

+

-

The bulb is light up The bulb does not light up ü Figure 9.15 4.

Draw arrows

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

5. What the meaning of rectification? Rectification Rectifica tion is a process process to convert convert an alternating alternating current current into into a direct current current by using using a diode. diode. ……………………………………………………………………………………………… ………………………………………………………………………………………………

9



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 timebase is on.

Figure 9.16 (ii)

Sketch waveform of the voltages observed on the CRO screen when a capacitor is connected in parallel across a resistor, R.

Figure 9.17 7. The figure shows a full-wave rectifier circuit that is connected to CRO. cycle and (i) Draw arrows to show the current flow in the first half cycle 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.

a

To CRO

Figure 9.18

10


(iii)


Sketch waveform of the voltages observed on the CRO screen when a capacitor is connected in parallel across a resistor, R.

a

To CRO

Figure 9.19 8. What is the function of the capacitor? Acts as a current current regulator regulator……………………………………………………… or smoother. smoother. …………………………… ……………………………………………………… …………………………………….... ……….... 9.3 TRANSISTOR 9. 3.1 Terminals of a Transistor.

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

2. Draw and label the symbol of n-p-n transistor and p-n-p transistor. Collector, C Collector, C

Base, B

Base, B

Emitter, E

Emitter, E

n-p-n transistor

p-n-p transistor

3. State the function for each terminal in a transistor. (a) The emitter, E : Acts as a source source of charge charge carriers, carriers, providing providing electrons electrons to the collector. collector. ………………………………………………………………………………. (b) The base, B : Controls the movement of charge carriers (electrons) from the emitter (E) to the collector (C). ………………… …………………………………………………………… (c) The collector, C: Receives the charge carriers from from the emitter emitter (E) ………………………………………………………………………………...

11



9.3.2 Transistor circuit

1. (a) Transistor circuit with 2 batteries.

I e

BE

circuit circuit : Base …………………………………. ………………………… ……….

CE

Collector circuit : ………………………… …………………………………. ……….

I b

current current : Base …………………………………. ………………………… ……….

I c

Collector current : ………………………… …………………………………. ……….

R1

the base base current current ………... : Limit …………………………………... ………………………… Limit the collector collector current current : ………………………… …………………………………... ………... Supply energy to the base circuit : ………………………… …………………………………... ………... Supply energy to circuit. : ………………………… …………………………………... ………... Potential divider : ………………………… …………………………………... ………... Potential divider : ………………………… …………………………………... ………...

R2

Figure 9.20

E 1 E 2

(b) Transistor circuit with 1 battery.

Rx Ry

Remember: Ie = Ib + Ic

Ie > Ic > Ib

∆Ic >>>>∆Ib

I e

No Ib, No Ic

Figure 9.21

2. The working circuit of a transistor used as a potential divider can be connected as shown in figure. The voltage across R x and Ry can be calculated as follows.

 R x   Vx =   R x + R y  V  

V Y

12

 R  =  y  V  + R R  x y 



(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 k Ω. Calculate the maximum resistance, S so that the bulb is lighted up. Solution:

 R  =  p  V  + R R  s p   10 × 10 3   6 V 2 V =  3  ( ) + × R 10 10  s  RS + 10 × 10 3 = 30000 RS = 20000 Ω = 20 k Ω V p

Bulb

Figure 9.22 9.2.3 Transistor as an Automatic Switch.

1. Complete the statement below.

RX Battery voltage Base voltage

I C C

I B

RY

I E

Figure 9.23 The switching action is produced by using a potential divider. In a working circuit variable resistor shown in figure, a resistor, R X and a …………………………. are being used to form a zero and potential divider. If the variable resistor is set to zero, the base voltage is ………. off the transistor switches ………. However, if the resistance of the variable resistor is increases When the base voltage reached a certain increased, the base voltage will…………….

minimum value, the base current, I B switches on the transistor. A large collector current, I C C flows to light up the bulb.

2. What type of transistor is used in an automatic switch circuit? Transistor n-p-n ………………………………………………………………………………………………

13



3. (a) Light Controlled Switch (i) Complete the statement below.

10 k Ω

R I C C

1k Ω

6V I E

LDR Figure 9.24 Figure shows a transistor-based circuit that functions as a light controlled switch. light-dependent resistor (LDR) has a very high resistance in the …….… dark and a low The ……………………….. bright light R is a fixed ……………. resistor The LDR and R form a potential resistor in ………………...

divider in the circuit. low resistance compared to R. Therefore, the base In bright light, the LDR has a very ………. low to switch on the transistor. voltage of the transistor is too …….. large and the voltage across the LDR is In darkness, the resistance of the LDR is very ……… high enough to switch on the transistor and thus lights up the bulb. This circuit can be ……… on the bulb at night. used to automatically switch ……

(ii) Complete the table below. Condition

R LDR

VLDR

R

VR

Daylight

low

Darkness

high

Transistor (ON or OFF)

low

high

high

OFF

high

low

low

ON

Remember ∆Ic >>>>∆Ib (iii) How can the circuit in figure be modified to switch on the light at daytime? The cir circu cuit it ca can n be mod modii ied b int inter erch chan an in the os ositi itions ons o the LDR an and d res resist istor or R. …………………………………………………………………………………………..

14



(b) A Heat-Controlled Switch

(i) Complete the statement below.

Thermistor

Diode

Relay

RB Alarm

R Figure 9.25

Figure shows a transistor-based circuit that function as a heat controlled switch. thermistor high when it is A ……………..is a special type of resistor. Its resistance becomes very ……… drops rapidly. At room cold. When the thermistor is heated, its resistance ………… high resistance compared to R. Therefore, the base temperature, the thermistor has a ……….

voltage of the transistor is too low to switch on the transistor. resistance drops considerablely compared to R. When the thermistor is heated, its ……………. on the transistor. When the voltage V B is high enough to switch ……. Therefore, the base ……………., on 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? To protect the transistor from being damaged by the large induced e.m.f in the relay ………………………………………………………………………………………….. coil when the collector current, I C C drops to zero. …………………………………………………………………………………………..

(iii) Complete the table below. Temperature R Thermistor VThermistor

R

VR

Transistor (ON or OFF)

High

low

low

high

high

ON

Low

high

high

low

low

OFF

Remember ∆Ic >>>>∆Ib

15



9.2.4 Transistor as a Current Amplfier

1. Complete the statement below.

mA

I C C

R2 R1

µA IB

R

I E

Figure 9.26 A transistor functions as a current amplifier by allowing a small current to control a collector current I C is primarily determined by the larger current. The magnitude of the …………………., base current big small change in the base current, I will cause a …….. ………………….., I . A ……….. B

B

change in the collector current, I C. The current amplification can be calculated as follows: Current Amplification

=

∆ I C ∆ I B

2. Name the type of the transistor used. n-p-n transistor ………………………………………………………………………………………………

3. What will happened to the readings of the miliammeter, mA and microammeter , µ A when the resistance of R is reduced? The readings on miliammeter and microammeter increase. ……………………………………………………………………………………………… 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. When a transistor is in the ‘ON’ state, currents flow in the base and in the collector circuit. ………………………………………………………………………………………

(b)

Explain the meaning of the ‘OFF’ state of a transistor. When a transistor is in the ‘OFF’ state, there is no current in the base and in the collector ……………………………………………………………………………………… circuit. ………………………………………………………………………………………

(c)

What is the function of the rheostat, R? To change the base current. ………………………………………………………………………………………

(d)

What is the function of the resistor, S? To control and limit the base current. ………………………………………………………………………………………

16



9.4 Logic Gates 9.4.1 Analysing Logic gates

1. What is a logic gate? A switching circuit that that is applied applied in computer computer in computer computer and other other electronic devices. devices. ……………………………………………………………………………………………… 2. Complete the table below. Gates

Symbol

Truth table

Input A Y

AND gate B

Output

A

B

Y

0

0

0

0

1

0

1

0

0

1

1

1

Input A Y

OR gate B

NOT gate

A

Y

17

Output

A

B

Y

0

0

0

0

1

1

1

0

1

1

1

1

Input

Output

A

Y

0

1

1

0



Input NAND

A

gate

Y B

Output

A

B

Y

0

0

1

0

1

1

1

0

1

1

1

0

Input

NOR gate

A Y B

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. A

0011

P

1100 Y

B

0101

Figure 9.27

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

18

0100



2. P

0011

A

1100 Y

B

1010

Q

0101

1000

Figure 9.28


3.

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

0011 A

0001

X

B 0101

Y

0100

B 1010

Figure 9.29


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

19



4. 0011

A

P 1110

Y

0110

0111 B

Q

0101

Figure 9.30


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

5. R

P

S

Q Q

The truth table:

Figure 9.31

Input

Output

A

B

P

Q

S

1

0

1

1

1

0

1

0

0

0

1

1

0

0

0

0

0

1

1

1

20



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

Light detector

Input J L Air-conditioner Air-conditioner

Heat detector

Input K

Figure 9.32

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: Input

Output

J

K

L

A

0

0

0

0

0

1

0

1

1

0

0

0

1

1

1

1

(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. - On a hot day or daytime – On a hot night ………………………………………………………………………………………………

21



Reinforcement Chapter 9

A. B. C. D. E.

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.

2.5 V 5.5 V 7.5 V 12.5 V 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.

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

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.

3. A beam of electrons is being deflected due to a potential difference between plates P and Q. P Figure 9.33 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.

7. The figure 9. 35 shows the arrangement of silicon atoms after an atom P is doped to form an extrinsic semiconductor. Figure 9.35

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?

Which of the following is not true? A. The conductivity of the semiconductor increases. B. The semiconductor becomes an ntype. C. The majority charge carrier is electron. D. Atom P is a trivalent atom.

Figure 9.34

22



8. The figure 9.36 shows a rectifier circuit. Which of the following statements is true?

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

P Q

11. Which of the following circuits shows the connect directions of the base current IB, emitter current, I E and collector current, IC?

Figure 9.36 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? Figure 9.37

A. B. C. D.

Replace the diode with a new one. Reverse the connection of the diode. Increase the number of bulbs. Connect a resistor in series with the bulb.

10.

Figure 9.38 Figure 9.38 shows four identical bulbs, P, Q, R and S, and four electronic

23



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.

Figure 9.41 A. B. C. D. E.

Figure 9.39

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

3 k Ω 4 k Ω 5 k Ω 6 k Ω 7 k Ω

16. The figure 9. 42 shows a transistor circuit being used to amplify sound. M-microphone C- Capacitor S- speaker

14. The figure 9.40 shows a transistor being used as a current amplifier. IB

IC

Figure 9.42 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.

Figure 9.40 Which of the following is correct? A. IB > IC B. IB = IC C. IB < IC

17. The figure 9.43 shows a logic gate circuit with input signals, X and Y.

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 ..

Figure 9.43 Which of the following is the output signal?

24



What is gate X? A. AND B. NOR C. OR D. NAND

18. The figure 9.43 shows a logic gate circuit.

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.

Figure 9.43 J

Which of the following is the output signal Z? A. 0110 B. 1010 C. 1110 D. 0101

Y

K

Figure 9.45 Which of the following logic gates can be used to represent J and K?

19. The figure 9.44 shows the combination of three logic gates. Output 1110

A. B. C. D.

Figure 9.44 The truth table for the combination of tree logic gates is as follows.

J AND NAND OR NOR

K NOR NOR AND AND

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.

Scale: 1 division = 1 cm The Y-gain is set at 3 V/cm The time base is set at 5 ms/cm

Figure 9.46

(a) Explain what is meant by thermionic emission. Emission of electrons electrons from from the surface surface of a metal metal by heat. ……………………………………………………………………………………………… (b) Determine the peak voltage of a.c voltage. 2 x 3 = 6V ………………………………………………………………………………………………

(c) Determine the time for one complete oscillation on the screen. 2 x 5 = 10 ms ………………………………………………………………………………………………

25



(d) What is the frequency of the a.c voltage? -3 f =1/T = 1 / 10 x 10 =50 Hz ……………………………………………………………………………………………… (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 9.47

(a) When the polarity of the a.c supply voltage is positive at A, state the two diodes which are forward biased. D1 and D3 ………………………………………………………………………………………….. (b) When the polarity of the a.c supply voltage is negative at A, state the two diodes which are forward biased. D2 and D4 …………………………………………………………………………………………… (c) Using the axes in figure 9.48, sketch the voltage-time graph across the resistor, R.

Voltage/V Time/ms

Figure 9.48

26



(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. Voltage/V Figure 9.49

Time/ms

(e) Explain how the capacitor causes the voltage across the resistor to vary with time in the way that you have drawn. The charging of the capacitor by the power supply and the discharging of the capacitor ……………………………………………………………………………………………… through the resistor will smooth the output. ………………………………………………………………………………………………

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 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. 12 V Logic gate A X

B

Relay switch

0V

Figure 9.50

Keys: Buttons A and B : X Output

:

When pressed, Not pressed, Motor is activated,

27

logic “1” logic ”0” logic “1”

240 V

Motor



(a) The truth table below shows the operations of the logic gates in a lift motor control system. Input Output A

B

X

0

0

0

0

1

1

1

0

1

(i)

Using the keys given, complete the truth table.

(ii)

Name the logic gate in the circuit in the figure 9.50. OR …………………………………………………………………………………

(iii)

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

(b) Why is a relay switch needed in the circuit? Activates large current current in the main main secondary secondary circuit supply// supply// small small current ……………………………………………………………………………………………… at the output cannot activate the motor. ………………………………………………………………………………………………

(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 activated and the lift door will close. Figure 9.51 shows an electronic circuit for the control system of the lift door.

Figure 9.51 240 V

R

28

Motor


(i)


State the relationship between the resistance and the intensity of light received by the light dependent resistor, LDR. The higher the light intensity, the lower the resistance of the resistor. …………………………………………………………………………………

………………………………………………………………………………… (ii)

Complete the circuit in figure 9.51 by drawing the resistor and the light dependent resistor using the symbols given below.

Resistor

(iii)

Light dependent resistor

Explain how the circuit functions. – High light intensity intensity produces produces lower lower resistance resistance and high high base voltage voltage ………………………………………………………………………………… base current current flows and activates activates the the transistor transistor - A bigger base ………………………………………………………………………………… collector current current flows through through the relay relay switch switch and activates activates the - A big collector ………………………………………………………………………………… circuit of the door motor. ………………………………………………………………………………… …………………………………………………………………………………

29



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.

Figure 9.52

Figure 9.53

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

(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.

Circuit W

Circuit Y

Circuit X

Circuit Z

30


(i) (ii) (iii)


What is meant by rectification? [1mark] Explain the working principle of a transformer. [3 marks] 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 I B of a transistor. R1 = 1k Ω A2 I C C

R2 = 56k Ω

A1

I B

6V

T

R3 = 2k Ω

Figure 9.56 Transistor T is connected to fixed resistor R 1 =1k Ω and R2 = 56 k Ω and a rheostat R 3 as shown in figure 9.56. The battery supplies a voltage of 6 V to the transistor circuit. Rheostat R 3 is adjusted until the current IB detected by microammeter A 1 is 10 µA. The collector current, I C recorded by miliammeter A 2 is shown in figure 9.57(a). 3

2 1

4 5

0 mA

(a) IB = 10µA

3131



Rheostat R 3 is then adjusted to lower value so that microammeter A 1 gives IB = 20 µA, 30

µA, 40 µA, 50 µA and 60 µA. The corresponding readings of I C on miliammeter, A 2 are shown in figure 9.57(b), 9.57(c), 9.57(d), 9.57(e) and 9.57(f). 3

2 1

4

0

1 5

mA

4

0

(b) IB = 30µA

3 4

1 5

mA

4

0

mA

(d) IB = 50µA

(c) IB = 40µA 3

2 1

0

3

2

1

0

5

mA

(b) IB = 20µA

2

3

2

4

mA

(e) IB = 60µA

32 32

5

5


(a)


For the experiment described identify… The base current, I B (i) the manipulated variable : ..……………………… ..……………………………… ……… The collector current, I C C (ii) the responding variable : ………………………………. ……………………………….. . (iii)

(b)

the fixed variable

The supply voltage : ………………………………. ……………………………….. .

From the figure in 9.57, record the collector current, I C when IB = 10, 20, 30, 40, 50 and 60µA. Tabulate your results for I B and IC in the space given below. IB / µA 10

IC / mA mA 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 I C against IB.

(d)

Based on your graph, determine the relationship between I C and IB. I c is directly proportional to I B B ………………………………………………………………………………………………

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. Power amplifier

Volume control

Microphone

Figure 9.58

33

loudspeaker



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.

34



Scheme Part : No. 4 (a) Inference (a) Inference : The strength strength of the output output signal of of the amplifier amplifier depends depends on the input input current current of the amplifier. (b) Hypothes (b) Hypothesis: is: The larger larger the input current current in an an amplifier circuit, circuit, the larger larger the output output current. current. (c) (i) Aim: To determine the relationship between base current and collector current of a transistor amplifier circuit. (ii) Manipulated variable: Base current, I B B Responding Respondi ng variable variable : Collector Collector current, current, I C C Fixed variable : Supply voltage (iii) Apparatus and materials: npn transistor, 2 batteries, microammeter, miliammeter, rheostat, connecting wires. (iv) Functional arrangement of apparatus. mA

Battery

µA R Battery

(v)- The rheostat is adjusted until the readings of microammeter for base current, I B B = 25 µ A. - The readings of the miliammeter for collector current, I C is recorded. - The steps are repeated for the values of microammeter, I B B = 50, 75,100,125µ A. (vi) Tabulation of data: I B / µ A 25.0 50.0 75.0 100.0 125.0

I C C /mA

(vii) Plot a graph of I B against I C C I B / µ A

I C C /mA

35



Scheme Part C : No. 1 (a) (i) – In a direct current, the current flows in one direction only. - In a alternating current, the current changes reverses it direction periodically. (ii) - Circuit X : the diode is forward biased, the bulb is lighted. - Circuit Y : the diode is reversed biased, the bulb is not lighted. - Function of a diode: Diode only allows current to flow in one direction only. (b)

– A p-type p-type semiconductor semiconductor is produced produced by adding adding trivalent trivalent impurity material such such as boron or gallium to silicon. - In a p-type semiconductor, majority of the charge carriers are the positive holes. - A n-type semiconductor is produced by adding pentavalent impurity material such as phosphorus or arsenic to silicon. - In a n-type semiconductor, majority of the charge carriers are the free electrons.

Scheme Part C : No. 2 (i) Rectification Rectifica tion is a process process of converting converting alternating alternating current current to direct direct current. current. (ii) -When an alternating current flows in the primary coil, a changing magnetic field is produced. produce d. - The changing magnetic field is linked to the secondary coil through the laminated iron core. - As a result, a secondary coil is in a changing magnetic field, this procedures an induced e.m.f at the two ends of secondary coil. (iii) -It must be a step-down transformer to step down the voltage from 240 V to 12 V. -The ratio: The number of turns in the secondary coil 12 1

The number of turns in the primary coil -

=

240

=

20

A full-wave rectification is better than a half-wave rectification, because a halfwave rectification loses half the input power as heat. A smoothing capacitor is required to change the pulsating d.c to a constant d.c. The most suitable circuit to be used is circuit Z.

36

Chapter 9 Electronic

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