Chapter 9 Electronic_teacher's

7/31/2019 Chapter 9 Electronic_teacher's

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JPN Pahang Physics Module Form 5

Teachers Guide Chapter 9: Electronics

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 rateof thermionic emission increases.

Surface area of the cathode A larger surface area of the cathode increases the rate ofthermionic emission.

Potential differencebetween the anode and

cathode.

The rate of thermionic emission is unchanged, when thepotential 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

When the temperature of the cathode increases, the

rate of thermionic emission increases.

A larger surface area of the cathode increases the

rate of thermionic emission.

The rate of thermionic emission is unchanged, when

the potential difference increases, but the emitted

electrons accelerate faster towards the anode.

Thermionic Emission is the release of electrons from a heated metal cathode.

towards

potential difference

cathode ray

They are negatively charged particles.

They travel in straight lines.

They possess momentum and kinetic energy.

They are deflected by magnetic and electric field.

electrons

Figure 9.1


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2

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, Vbetween the cathode and the accelerating anode.

[e = 1.6 x 10 -19 C]Solution:

V

101.32

energyKinetic

14-

4

19

2

1025.8

106.1

2

1

=

=

==

V

V

eVmv

2. In a vacuum tube, a cathode ray is produced and accelerated through a potentialdifference 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) J104105.2106.1energypotentialElectric 16319 === eV

(b)162 104

2

1 == eVmv 2109

10431

162

=

v -1714 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 -1715 ms1053.31024.1 ==v

By using the principle of conservation of energy,

eVmv =22

1,

Maximum velocity of electron,m

eVv

2=

v = velocity of electron

V = Potential difference between Anode andCathode

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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3

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

Filament Y-plat

X-platCathode Acceleration

Anode

Focusing Anode

Fluorescent screen

Filament When a current passes through the filament, the

filament becomes hot and heats up the cathode.

Cathode Emits electrons when it is hot.

Control Grid Control the number of electrons hitting the

fluorescent screen.

Control the brightness of the spot on the screen.

Focusing Anode To focus the electrons onto the screen.

Accelerating

Anode

To accelerate the electrons to high speed.

Y-Plates To deflect the electron beam vertically.

X-Plates To deflect the electron beam horizontally.

Glass surface

coated with a

fluorescent

material.

To convert the kinetic energy of the electrons to

heat and light energy when the electrons hit the

screen.

Electron gun

Deflecting

system

Fluorescent

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 ofVpp.Solution:

0.30.25.1 ==V V

Y-shift

Y-Gains Time-base

X-shift

Brightness

Focus

X-inputY-input Earth

To measure a D.C or A.C voltage

To measure a short time intervals

To display the waveform

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

Figure 9.5


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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)

divsV/divVP 42

15 =

VVP 10=

4. Figure shows a trace on an oscilloscope for an a.c source.If the Y-gain is set to 1.5 Vcm

-1and the time-base is 2 ms

cm-1

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

cmVcmV1-

P 42

15.1 =

VVP 0.3=

(b) Calculate the frequency,fof the a.c source.Solution:

24 = cmT ms cm-1

T = 8 ms f = 1251=

THz

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

-1

.

5.

The diagram shows the trace on the screen of a CRO when ana.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.(c)Root-mean-square voltage, Vr.m.s

Figure 9.6

Figure 9.7

Figure 9.8


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6

5 divs

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 = pV

21 = 24.46

21 = V

6. When two claps are made close to a microphone which isconnected to the Y-input and earth terminals, both pulses

will be displayed on the screen at a short interval apart asshown in figure below. Measure the time lapse between the

two claps.

Solution:

7. Figure shows the trace displayed on the screen of aCRO with the time-base is set to 10 ms/div. What is the

frequency,fof the wave?

Solution:

8. An ultrasound signal is transmitted vertically down to the sea bed. Transmitted andreflected 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 msTime interval = 0.05 s

m4502

0.751200dHance,

t

2dVwaves,ultrasonicofSpeed

s0.75ms750smms150cm5

QandPbetweentimed2ofdistanceathroughtraveltowavesultrasonicfortakenTime

1-

=

=

=

===

=

Distance for a complete wave = 2 divs

Time taken = 2 divs 10ms/div

= 20 ms

frequency, f =T

1=

ms20

1= 50 Hz

Figure 9.9

Figure 9.10

Figure 9.11


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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 offree 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 calleddopants

such asAntimony andBoron to a semiconductor,to increase its conductivity.

pentavalent antimony

increases

negative electrons

five four

Figure 9.12


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8

(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 inone direction

only.

Figure 9.13

p-type n-type

Positive hole Negative electron Symbol

p-n junction

Figure 9.13


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9

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


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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 acapacitor 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 thefirst 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 CROa

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 acapacitor 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 the 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 CROa

A transistor is a silicon chi 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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12

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

V

RR

RVx

yx

x

+

= V

RR

RV

yx

y

Y

+

=

BE : .CE : .

Ib : .

Ic : .

R1 : ...

R2 : ...

E1 : ...

E2 : ...

Rx : ...

Ry : ...

Base circuitCollector 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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13

(a)Figure shows a transistor circuit. The bulb can be lighted up when potential difference, Vacross resistor P is 2V and resistance P is 10 k. Calculate the maximum resistance, S sothat 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,

ICflows to light up the bulb.

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

Bulb

Solution:

VRR

RV

ps

p

p

+=

( )V6

1010

1010V2

3

3

+

=

sR

300001010 3 =+SR == kRS 2020000

IC

IE

IB

RX

RYBase 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 modi ied b interchan in the ositions o the LDR and resistor R.

IC

IE

1k

R

LDR

10 k

6 V

Figure 9.24


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15

(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, ICdrops 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, A whenthe 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

A

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 circui

When a transistor is in the OFF 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

YA

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 01 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 00 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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19

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

P0 0 1 1

0 1 0 1

1 0 0 0

1 1 0 0

1 0 1 0Q

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 S

1 0 1 1 1

0 1 0 0 0

1 1 0 0 0

0 0 1 1 1

Y

A

B

P

Q

0011

0101

1110

01110110

Q

S

RP

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

Input Output

J K L A

0 0 0 0

0 1 0 1

1 0 0 0

1 1 1 1

L

Light

detector

Heat

detector

Input J

Input K

Air-conditioner

- On a hot day or daytime On a hot night

Figure 9.32


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

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 circuitconsisting of two diodes and a bulb.

When the switch is on, the bulb does notlight 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 lightup continuously when the switch is on?

A. P and Q onlyB. P, Q and R onlyC.

R and S onlyD. P, Q and S only

11.Which of the following circuits showsthe connect directions of the base current

IB, emitter current, IE and collectorcurrent, IC?

Figure 9.36

Figure 9.37

Figure 9.38

P

Q


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12.Which of the following statements abouta transistor is not true?A. A transistor can act as an amplifierB. 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 twotypes of semiconductors.

13.What is the function of the transistorcircuit shown in figure 9.39?A. As an amplifierB. As a rectifierC. As a switch deviceD. As a modulator

14.The figure 9.40 shows a transistor beingused as a current amplifier.

Which of the following is correct?A. IB > ICB. IB = ICC. IB < IC

15.Figure 9.41 shows a circuit consisting ofa transistor which acts as an automaticswitch. When the potential differenceacross the thermistor is 3 V and the

resistance of the thermistor is 1000 ,the resistance value of resistor, R is ..

A. 3 kB. 4 kC. 5 kD. 6 kE. 7 k

16.The figure 9. 42 shows a transistorcircuit being used to amplify sound.

Which of the following is not correctabout the circuit?

A. T is an npn transistorB. 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 gatecircuit with input signals, X and Y.

Which of the following is the output

signal?

Figure 9.41

IBIC

Figure 9.40

M-microphoneC- Capacitor

S- speaker

Figure 9.42

Figure 9.43

Figure 9.39


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18.The figure 9.43 shows a logic gatecircuit.

Which of the following is the outputsignal Z?

A. 0110B. 1010C. 1110D.

0101

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

The truth table for the combination of

tree logic gates is as follows.

What is gate X?

A. ANDB. NORC. ORD. NAND

20.The figure 9. 45 shows a combination ofthree logic gates in a logic circuit. Wheninputs P and Q are both 1 output Y is 1.

Which of the following logic gates canbe used to represent J and K?

J K

AND NOR

NAND NOR

OR AND

A.

B.C.

D. NOR 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.

(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.

Figure 9.43

Figure 9.44

YJ

K

Figure 9.45

Figure 9.46

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

2 x 3 = 6V

2 x 5 = 10 ms

Scale: 1 division = 1 cmThe Y-gain is set at 3 V/cm

The time base is set at 5 ms/cm


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(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 to2.5 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.

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

..

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

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

f =1/T = 1 / 10 x 10-3

=50 Hz

D1 and D3

D2 and D4

Figure 9.47

Time/ms

Voltage/V

Figure 9.48


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27

(d)On the figure 9.49, sketch the voltage-time graph across the resistor if a capacitor isconnected across the resistor if a capacitor is connected across the resistor R parallel withthe resistor.

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

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

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 9.49

12 V

0 V

A

B

Logic gate

X 240 V

Relay switch Moto

Figure 9.50


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28

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

(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 lightdependent 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.

Input Output

A B X

0 0 00 1 1

1 0 1

R Motor

240 V

Figure 9.51

OR

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

at the output cannot activate the motor.


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29

(i) State the relationship between the resistance and the intensity of light receivedby the light dependent resistor, LDR.

(ii) Complete the circuit in figure 9.51 by drawing the resistor and the lightdependent resistor using the symbols given below.

(iii) Explain how the circuit functions.

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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30

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 currentand 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 circuitare used for the purpose of rectification.

Figure 9.52 Figure 9.53

Circuit W

Circuit X

Circuit Y

Circuit Z


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31

0

1

2 3

4

5

(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 currentExplain 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 collectorcurrent IC to the base current IB of a transistor.

Transistor T is connected to fixed resistor R1 =1k and R2 = 56 k and a rheostat R3 asshown 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 A. Thecollector current, IC recorded by miliammeter A2 is shown in figure 9.57(a).

(a) IB = 10A

mA

T6V

R1 = 1k

A2

IC

A1IBR2 = 56k

R3 = 2k

Figure 9.56

31


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32

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

A, 40 A, 50 A and 60 A. 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

50

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 = 20A (b) IB = 30A

(c) IB = 40A (d) IB = 50A

(e) IB = 60A

mA mA

mA mA

mA

32


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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 and60A. Tabulate your results for IB and IC in the space given below.

IB/A IC/mA10 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.

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

The base current, IB

The collector current, IC

The supply voltage

Ic is directly proportional to IB

Figure 9.58

loudspeakerPower amplifier

Volume control

Microphone


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34

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.


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Scheme Part : No. 4

(a)Inference : The strength of the output signal of the amplifier depends on the input current of theamplifier.

(b)Hypothesis: The larger the input current in an amplifier circuit, the larger the output current.(c) (i) Aim: To determine the relationship between base current and collector current of a

transistor amplifier circuit.(ii) Manipulated variable: Base current, IB

Responding variable : Collector current, IC

Fixed variable : Supply voltage

(iii) Apparatus and materials: npn transistor, 2 batteries, microammeter, miliammeter,

rheostat, connecting wires.

(iv) Functional arrangement of apparatus.

R

Battery

mA

A Battery

(v)- The rheostat is adjusted until the readings of microammeter for base current, IB = 25 A.

- The readings of the miliammeter for collector current, IC is recorded.

- The steps are repeated for the values of microammeter, IB = 50, 75,100,125A.

(vi) Tabulation of data:

IB/A IC/mA

25.0

50.0

75.0

100.0

125.0

(vii) Plot a graph of IB against IC

IB/A

IC/mA


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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 semiconductor is produced by adding trivalent impurity material suchas 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 is a process of converting alternating current to direct current.(ii) -When an alternating current flows in the primary coil, a changing magnetic field is

produced.

- 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:

20

1

240

12

coilprimaryin theturnsofnumberThe

coilsecondaryin theturnsofnumberThe==

- A full-wave rectification is better than a half-wave rectification, because a half-wave 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.

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