SPM PHYSICS FORM 5 electronics

JPN Pahang Physics Module Form 5
Student’s Copy 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?

………………………………………………………………………………………………

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

Figure 9.1

(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 The rate of thermionic emission is unchanged, when the
between the anode and potential difference increases, but the emitted electrons
cathode. 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) ……………………………………………………………………

1


Energy Change in A Cathode Ray

Figure 9.2

By using the principle of conservation of energy,
1 2 v = velocity of electron
mv = eV , V = Potential difference between Anode and
2 Cathode
2eV e = Charge on 1 electron = 1.6 x 10 -19 C
Maximum velocity of electron, v =
m m = mass of 1 electron = 9 x 10 -31 kg

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:
1
Kinetic energy = mv 2 = eV
2
1.32 × 10 = 1.6 × 10 −19 V
-14

V = 8.25 × 10 3 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 −19 × 2.5 × 10 3 = 4 × 10 −16 J
1 4 × 10 −14
(b) mv 2 = eV = 4 × 10 −14 v =
2
×2 v = 8.89 × 1016 = 2.98 × 10 8 ms -1
2 9 × 10 −31

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
mv = eV = 1.6 × 10 −19 × 3.5 × 10 3 = 5.6 × 10 −16
2
5.6 × 10 −16
v2 = × 2 = 1.24 × 1015 v = 1.24 × 1015 = 3.53 × 10 6 ms -1
9 × 10 −31

2


9.1.3 Structure of the Cathode Ray Oscilloscope

1. Label all parts of Cathode Ray Oscilloscope below.

Figure 9.3

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

Electron gun

Deflecting
system

Fluorescen
t screen

3


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

1. Fill in the blank the structure of CRO.

Figure 9.4

9.1.5 Uses of the CRO.
1. The uses of cathode-ray oscilloscope are:
(a) ………………………………………..
(b) ……………………………………….
(c) ……………………………………….

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
1
Peak voltage, Vp = (Y-gains) × (h)
2
1
Effective voltage or root-mean-square voltage, Vr.m.s = Vp
2
Short time intervals, t = no. of divisions between two pulses × time-base value.

2. If the CRO in figure uses Y-gains of 1.5 Vcm-1, calculate the value of Vpp.
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, Vp = (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.
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:
1
V P = 1.5 Vcm -1 × × 4 cm
2 Figure 9.7
V P = 3 .0 V

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

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

Figure 9.8
5


Solution:
(a) Peak-to-peak voltage, Vpp = (Y-gains) × h
= 2V/div × 6 divs
= 12 V
(b) Peak voltage, Vp = 6 V
1 1
(c) Vr.m.s = Vp = × 6 = 4.24 V
2 2
5 divs
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 Figure 9.9
shown in figure below. Measure the time lapse between the
two claps.
Solution:
Length between two pulses = 5 divs
Time taken, t = 5 divs × 10 ms/div
= 50 ms
∴Time interval = 0.05 s

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 two complete wave = 2 divs
∴ Time taken = 2 divs ×10ms/div
= 20 ms
1 1
∴frequency, f = = = 50 Hz
T 20 ms Figure 9.10

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:

Time taken for ultrasonic waves to travel through a distance of
2 d = time between P and Q
= 5 cm × 50 ms sm -1 = 250 ms = 0.25 s
2d
Speed of ultrasonic waves, V =
t
1200 × 0.25
Hance, d = = 150 m Figure 9.11
2

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 of adding a certain amount of other substances called dopants
………………………………………………………………………………………………
such as Antimony and Boron to a semiconductor, to increase its conductivity.
………………………………………………………………………………………………
10. Base on the figure, complete the statement below.
(a) n-type semiconductors

Figure 9.12
pentavalent
Silicon like Silicon doped with ………………atoms such as …………… or
antimony
increases
phosphorus …………. the number of free electron. The phosphorus atoms have
five four
…….. valence electrons, with …… 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
Semiconductor like Silicon doped with ……………….. atoms such as ………… Boron
three
or indium has more positive holes. The Boron atoms have only ………….
one
valence electrons; hence ………. of the covalent bonds has a missing electron.
This missing electron is called a ‘positive hole’. The majority charge-carriers in
positive holes
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?
The function of semiconductor diode is to allow current to flow through it in one direction
………………………………………………………………………………………………
only.
………………………………………………………………………………………………
2. Label the p-n junction below and draw a symbol of the diode.

p-type n-type

Positive hole Negative electron Symbol

p-n junction
Figure 9.13

8


3. (a) Forward-biased
(i) positive terminal
In forward-bias, the p-type of the diode is connected to …………………. and the n-
negative terminal
type is connected to the …………………… of the battery.
(ii) Complete the diagram below to show the diode is in forward-bias.

+ -

The bulb is light up

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 …………………. , and the n-
negative terminal
positive terminal
type is connected to the …………………… of the battery.
(ii) Complete the diagram below to show the diode is in reverse-bias.

- +

The bulb is light up

The bulb does not light up

Figure 9.15

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

5. What the meaning of rectification?
Rectification is a process to convert an alternating current into a direct current by using a 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 time-
base 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.
(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.

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 regulator or smoother.
……………………………………………………………………………………………....

9.3 TRANSISTOR
9. 3.1 Terminals of a Transistor.
1. What is a transistor?
A transistor is a silicon chip which has three terminals labeled as base, collector and emitter.
………………………………………………………………………………………………
2. Draw and label 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 of charge carriers, providing electrons to the 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 the emitter (E)
………………………………………………………………………………...

11


9.3.2 Transistor circuit
1. (a) Transistor circuit with 2 batteries.

BE Base circuit
: ………………………………….
CE Collector circuit
: ………………………………….
Ib Base current
: ………………………………….
Ic Collector current
: ………………………………….
R1 Limit the base current
: …………………………………...
Ie
Limit the collector current
R2 : …………………………………...
Figure 9.20 Supply energy to the base circuit
E1 : …………………………………...
Supply energy to circuit.
(b) Transistor circuit with 1 battery. E2 : …………………………………...
Potential divider
Rx : …………………………………...
Potential divider
Ry : …………………………………...

Remember:
Ie = Ib + Ic
Ie > Ic > Ib

Ie ∆Ic >>>>∆Ib
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 Rx and Ry can be calculated as follows.
⎛ Rx ⎞ ⎛ Ry ⎞
Vx = ⎜ ⎟V VY = ⎜ ⎟V
⎜R +R ⎟ ⎜R +R ⎟
⎝ x y ⎠ ⎝ x y ⎠

12


(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.
Solution:
⎛ Rp ⎞
Vp = ⎜ ⎟V
Bulb ⎜R +R ⎟
⎝ s p ⎠

⎛ 10 × 10 3 ⎞
2V =⎜ ⎟
⎜ R + (10 × 10 ) 6 V
3 ⎟
⎝ s ⎠
RS + 10 × 10 = 30000
3

RS = 20000 Ω = 20 kΩ
Figure 9.22

9.2.3 Transistor as an Automatic Switch.
1. Complete the statement below.

RX
IC
Battery voltage IB

Base voltage RY
IE

Figure 9.23

The switching action is produced by using a potential divider. In a working circuit
variable resistor
shown in figure, a resistor, RX and a …………………………. are being used to form a
zero
potential divider. If the variable resistor is set to zero, the base voltage is ………. and
off
the transistor switches ………. However, if the resistance of the variable resistor is
increases
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?
Transistor n-p-n
………………………………………………………………………………………………

13


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

10 kΩ R
IC
1kΩ
6V
IE
LDR

Figure 9.24
Figure shows a transistor-based circuit that functions as a light controlled switch.
light-dependent resistor
The ……………………….. (LDR) has a very high resistance in the …….… and a low dark
bright light resistor
resistor in ………………... R is a fixed ……………. The LDR and R form a potential
divider in the circuit.
low
In bright light, the LDR has a very ………. resistance compared to R. Therefore, the base
low
voltage of the transistor is too …….. to switch on the transistor.
large
In darkness, the resistance of the LDR is very ……… and the voltage across the LDR is
high
……… enough to switch on the transistor and thus lights up the bulb. This circuit can be
on
used to automatically switch …… the bulb at night.

(ii) Complete the table below.
Condition RLDR VLDR R VR Transistor (ON or OFF)
Daylight low low high high OFF
Darkness high high low low ON
Remember ∆Ic >>>>∆Ib
(iii) How can the circuit in figure be modified to switch on the light at daytime?
The circuit can be modified by interchanging the positions of the LDR and resistor 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
A ……………..is a special type of resistor. Its resistance becomes very ……… when it is
drops
cold. When the thermistor is heated, its resistance ………… rapidly. At room
high
temperature, the thermistor has a ………. resistance compared to R. Therefore, the base
voltage of the transistor is too low to switch on the transistor.
resistance
When the thermistor is heated, its ……………. drops considerablely compared to R.
on
Therefore, the base voltage VB is high enough to switch ……. the transistor. When the
…………….,
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, IC drops to zero.
…………………………………………………………………………………………..
(iii) Complete the table below.
Temperature RThermistor 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

R2 IC

R1
µA

R IB
IE

Figure 9.26
A transistor functions as a current amplifier by allowing a small current to control a
collector current
larger current. The magnitude of the …………………., IC is primarily determined by the
base current small big
………………….., 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:

∆I C
Current Amplification =
∆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, µA 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 ‘ON’ 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 is applied in computer in computer and other electronic devices.
………………………………………………………………………………………………
2. Complete the table below.
Gates Symbol Truth table

Input Output
A B Y
A 0 0 0
AND gate Y
0 1 0
B
1 0 0
1 1 1

Input Output
A B Y
A
0 0 0
OR gate Y
0 1 1
B
1 0 1
1 1 1

Input Output
A Y

A 0 1
Y
NOT gate 1 0

17


Input Output
A B Y
NAND A 0 0 1
gate Y 0 1 1
B
1 0 1
1 1 0

Input Output
A B Y
A 0 0 1
NOR gate
Y
0 1 0
B
1 0 0
1 1 0

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

0011
A P 1100

Y
0100
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


2.
0011 P
A 1100

Y
1000

B
Q 1010
0101
Figure 9.28
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

3. 0011
A X 0001
B
0101 Y
0100

B
1010
The truth table: Figure 9.29

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

19


4.

0011
A P
1110
Y
0110
0111

B Q
0101 Figure 9.30
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

5.

A P

B Y

B
The truth table: Figure 9.31
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

20


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

Input J
Light
detector L
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
0 0 0
0 1 1
1 0 0
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 say or daytime – On a hot night
………………………………………………………………………………………………

21


Reinforcement Chapter 9
Part A: Objective questions A. 2.5 V
B. 5.5 V
1. Which of the following is not a property
C. 7.5 V
of cathode rays?
D. 12.5 V
A. It is positively charged.
E. 15.0 V
B. It travels in a straight line.
C. It can be deflected by magnetic field.
5. In p-type semiconductor
D. It can be deflected by electric field.
A. The number of holes are equal to the
number of electrons.
2. Cathode rays consists of
B. The number of the holes are more
A. Fluorescent particles
than the number of electrons.
B. Light rays from a screen
C. The number of the holes are less than
C. Beams of fast moving particles
the number of electrons.
D. Light rays from hot filament
6. Which of the following is not true about
3. A beam of electrons is being deflected
diode?
due to a potential difference between
A. It can be used to rectify alternating
plates P and Q.
current.
P B. It can only conduct electricity when
Figure 9.33
it is connected in forward in forward
bias in a circuit.
Q
C. It is formed by joining an n-type and
a p-type semiconductor.
Which of the following statements is not
D. The majority charge carriers in the
true?
diode are electrons.
A. The potential at plate P is positive.
B. The deflection would be greater if
7. The figure 9. 35 shows the arrangement
the potential difference is greater.
of silicon atoms after an atom P is doped
C. The deflection would be greater if
to form an extrinsic semiconductor.
the electrons are moving faster.
D. The electron beam will return to
straight line if a suitable magnetic Figure 9.35
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. Which of the following is not true?
What is the maximum value of the A. The conductivity of the
potential difference being measured? semiconductor increases.
B. The semiconductor becomes an n-
type.
C. The majority charge carrier is
Figure 9.34 electron.
D. Atom P is a trivalent atom.

22


8. The figure 9.36 shows a rectifier circuit. components connected in a circuit.
Which of the following statements is Which of the following bulbs will light
true? up continuously when the switch is on?
A. P and Q only
P
B. P, Q and R only
C. R and S only
Q D. P, Q and S only

11. Which of the following circuits shows
Figure 9.36 the connect directions of the base current
A. A rectifier changes d.c to a.c. IB, emitter current, IE and collector
B. Device P allows current to flow in current, IC?
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. 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
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. 3 kΩ
B. 4 kΩ
Figure 9.39 C. 5 kΩ
D. 6 kΩ
E. 7 kΩ

13. What is the function of the transistor 16. The figure 9. 42 shows a transistor
circuit shown in figure 9.39? circuit being used to amplify sound.
A. As an amplifier
B. As a rectifier M- Microphone
C. As a switch device C- Capacitor
D. As a modulator S- Speaker

14. The figure 9.40 shows a transistor being
used as a current amplifier.
IC
IB Figure 9.42
Which of the following is not correct
about the circuit?
A. T is an npn transistor
Figure 9.40 B. The capasitor prevents d.c current
but allows a.c current to pass through
Which of the following is correct? it.
A. IB > IC C. Speaker amplifies the sound.
B. IB = IC D. R1 and R2 act as potential divider.
C. IB < IC
17. The figure 9.43 shows a logic gate
15. Figure 9.41 shows a circuit consisting of circuit with input signals, X and Y.
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, Figure 9.43
the resistance value of resistor, R is ..
Which of the following is the output
signal?

24


What is gate X?
A. AND
B. NOR
C. OR
D. NAND
18. The figure 9.44 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 Y
signal Z? K
A. 0110
B. 1010
C. 1110 Figure 9.45
D. 0101
Which of the following logic gates can
19. The figure 9.44 shows the combination be used to represent J and K?
of three logic gates.
J K
A. AND NOR
B. NAND NOR
Figure 9.44
C. OR AND
D. NOR AND
The truth table for the combination of
tree logic gates is as follows.

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 Figure 9.46
The Y-gain is set at 3 V/cm
The time base is set at 5 ms/cm

(a) Explain what is meant by thermionic emission.
Emission of electrons from the surface of a 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?
f =1/T=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 240 V
B

Relay switch Motor

0V
Figure 9.50

Keys:
Buttons A and B : When pressed, logic “1”
Not pressed, logic ”0”
X Output : Motor is activated, logic “1”

27


(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
1 1 0

(i) Using the keys given, complete the truth table.
(ii) Name the logic gate in the circuit in the figure 9.50.
AND Gate
…………………………………………………………………………………
(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 in the main secondary circuit supply// 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

Motor
R

28


(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 Light dependent resistor

(iii) Explain how the circuit functions.
– 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.
…………………………………………………………………………………
…………………………………………………………………………………

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

(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.

Circuit W
Circuit Y

Circuit X Circuit Z

30


(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.

R1 = 1kΩ

A2
IC
R2 = 56kΩ IB 6V
A1 T
R3 = 2kΩ

Figure 9.56
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).

2 3
1 4

0 5
mA

(a) IB = 10µA

31
31


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).

2 3 2 3
1 4 1 4

0 5 0 5
mA mA

(b) IB = 20µA
(b) IB = 30µA

2 3 2 3
1 4 1 4

0 5 0 5
mA mA

(c) IB = 40µA (d) IB = 50µA
2 3
1 4

0 mA 5

(e) IB = 60µA

32
32


(a) For the experiment described identify…
The base current, IB
(i) the manipulated variable : ..………………………………
(ii) The collector current, IC
the responding variable : ………………………………..
(iii) the fixed variable The supply voltage
: ………………………………..
(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.
Ic is directly proportional to IB
………………………………………………………………………………………………

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
Loudspeaker

Volume control

Microphone

Figure 9.58

33


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

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