Chapter 9-Electronics(Teacher's Guide)

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

1. What is Thermionic Emission?

………

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

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

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

3. Factors that influence the rate of thermionic emission

Factor Effect on the rate of thermionic emission

Temperature of the cathode When the temperature of the cathode increases, the rate of thermionic emission increases.

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

Potential difference

between the anode and cathode.

The rate of thermionic emission is unchanged, when the potential difference increases, but the emitted electrons accelerate faster towards the anode.

9.1.2 Properties of Cathode Rays

1. List the four characteristics of the cathode rays.

(i) ……….. (ii) ………. (iii) ………

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. electrons

Energy Change in A Cathode Ray

1. In a cathode ray tube, an electron with kinetic energy of 1.32 × 10-14 J is accelerated. Calculate the potential difference, V between the cathode and the accelerating anode. [ e = 1.6 x 10 -19 _C] Solution: V 10 25 . 8 10 6 . 1 10 1.32 2 1 energy Kinetic 3 19 14 -2         V V eV mv

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) Electricpotentialenergy eV 1.610192.5103 41016J

(b) 2 4 10 14 2 1 _{  }  eV mv 2 10 9 10 4 31 14 2     _ v v 8.891016 2.98108ms-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: 16 3 19 2 10 6 . 5 10 5 . 3 10 6 . 1 2 1 _{  }  _{   }  eV mv 15 16 2 10 24 . 1 2 10 6 . 5       v v 1.241015 3.53106ms-1 By using the principle of conservation of energy,

eV mv2  2 1

,

Maximum velocity of electron,

m eV v 2

v = velocity of electron

V = Potential difference between Anode and Cathode

e = Charge on 1 electron = 1.6 x 10 -19 C m = mass of 1 electron = 9 x 10 -31 kg

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-plat Cathode 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

To convert the kinetic energy of the electrons to heat and light energy when the electrons hit the screen. Electron gun Deflecting system Fluorescen t screen Figure 9.3

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

1. Fill in the blank the structure of CRO.

9.1.5 Uses of the CRO.

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

2. If the CRO in figure uses Y-gains of 1.5 Vcm-1, calculate the value of Vpp.

Solution: 0 . 3 0 . 2 5 . 1    V V Y-shift Y-Gains _Time-base X-shift Brightness Focus X-input Y-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 = Vp

2 1

Short time intervals, t = no. of divisions between two pulses × time-base value. Figure 9.4

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:

Peak voltage, Vp = (Y-gains)

2 1  (h) divs V/div V_P 4 2 1 5    V V_P 10

4. Figure shows a trace on an oscilloscope for an a.c source. If the Y-gain is set to 1.5 Vcm-1 and the time-base is 2 ms cm-1.

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

Solution: cm Vcm VP -1 4 2 1 5 . 1    V VP 3.0

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

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

5. The diagram shows the trace on the screen of a CRO when an a.c voltage is connected to the Y-input. The Y-gain control is set at 2 V/div and the time base is off.

Calculate the value of : (a) Peak-to-peak voltage, Vpp

(b)Peak voltage, Vp.

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

Figure 9.6

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 = Vp 2 1 = 6 4.24 2 1   V

6. When two claps are made close to a microphone which is connected to the Y-input and earth terminals, both pulses will be displayed on the screen at a short interval apart as shown in figure below. Measure the time lapse between the two claps.

Solution:

7. Figure shows the trace displayed on the screen of a CRO with the time-base is set to 10 ms/div. What is the frequency, f of the wave?

Solution:

8. An ultrasound signal is transmitted vertically down to the sea bed. Transmitted and reflected signals are input into an oscilloscope with a time base setting of 150 ms cm-1. The diagram shows the trace of the two signals on the screen of the oscilloscope. The speed of sound in water is 1200 ms-1. What is the depth of the sea?

Solution: 10ms/div

Length between two pulses = 5 divs Time taken, t = 5 divs × 10 ms/div

= 50 ms Time interval = 0.05 s 0.25 1200 t 2d V waves, ultrasonic of Speed s 0.25 ms 250 sm ms 50 cm 5 Q and P between time d 2 of distance a through travel to waves ultrasonic for taken Time 1 -      

Distance for two complete wave = 2 divs

 Time taken = 2 divs ×10ms/div = 20 ms frequency, f = T 1 = ms 20 1 = 50 Hz Figure 9.9 Figure 9.10

9.2 SEMICONDUCTOR DIODES 9.2.1 Properties of Semiconductors

1. Semiconductor is

……….. ……….

2. Give the examples of pure semiconductor: (a) ………

(b) ………... (c) ……… 9. What is the “doping” process?

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

(a) n-type semiconductors

Silicon like Silicon doped with ………atoms such as ……… or phosphorus …………. the number of free electron. The phosphorus atoms have …….. valence electrons, with …… being used in the formation of covalent bonds. The fifth electron is free to move through the silicon. The silicon has

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

semiconductor.

a group of materials that can conduct better than insulators but not as good as metal conductors.

Silicon Germanium Selenium

Doping is a process of adding a certain amount of other substances called dopants such as Antimony and Boron to a semiconductor, to increase its conductivity.

pentavalent _antimony

increases

negative electrons

five four

(b) p-type semiconductors

Semiconductor like Silicon doped with ……….. atoms such as ………… or indium has more positive holes. The Boron atoms have only ………….

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

This missing electron is called a „positive hole‟. The majority charge-carriers in this semiconductor are the ………. and this semiconductor is thus known as a p-type semiconductor.

9.2.2 The p-n junction (Semiconductor diode)

1. What is the function of semiconductor diode?

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

trivalent Boron

positive holes

three one

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

Figure 9.13

p-type n-type

Positive hole Negative electron Symbol

p-n junction

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 is light up





Rectification is a process to convert an alternating current into a direct current by using a diode. Figure 9.14

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

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

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

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

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

(ii) Sketch the waveform of the voltages observed on the CRO screen when the time-base is on. To CRO a . Figure 9.16 Figure 9.17 Figure 9.18

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

8. What is the function of the capacitor?

………....

9.3 TRANSISTOR

9. 3.1 Terminals of a Transistor.

1. What is a transistor?

………

2. Draw and label 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 CRO

a

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

n-p-n transistor Base, B Collector, C Emitter, E p-n-p transistor Base, B Collector, C Emitter, E

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

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

Receives the charge carriers from the emitter (E) Figure 9.19

9.3.2 Transistor circuit

1. (a) Transistor circuit with 2 batteries.

(b) Transistor circuit with 1 battery.

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

V R R R Vx y x x           V R R R V y x y Y          BE : ………. CE : ………. Ib : ………. Ic : ………. R1 : ………... R2 : ………... E1 : ………... E2 : ………... Rx : ………... Ry : ………... Base circuit Collector circuit Base current Collector current Limit the base current Limit the collector current Supply energy to the base circuit Supply energy to circuit.

Potential divider Potential divider

I

I

(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. Calculate the maximum resistance, S so that the bulb is lighted up.

9.2.3 Transistor as an Automatic Switch.

1. Complete the statement below.

The switching action is produced by using a potential divider. In a working circuit shown in figure, a resistor, RX and a ………. are being used to form a potential divider. If the variable resistor is set to zero, the base voltage is ………. and the transistor switches ………. However, if the resistance of the variable resistor is increased, the base voltage will………. When the base voltage reached a certain minimum value of the base current, IB switches ON the transistor. A large collector

current, IC flows to light up the bulb.

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

……… Bulb Solution: V R R R V p s p p         





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

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

In bright light, the LDR has a very ………. resistance compared to R. Therefore, the base voltage of the transistor is too …….. to switch on the transistor.

In darkness, the resistance of the LDR is very ……… and the voltage across the LDR is ……… enough to switch ON the transistor and thus lights up the bulb. This circuit can be used to automatically switch …… the bulb at night.

(ii) Complete the table below.

Condition RLDR VLDR R VR Transistor (ON or OFF) Daylight

Darkness

Remember _{Ic >>>>}_Ib

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

………..

light-dependent resistor dark

bright light resistor

low low large high on low low high high high high low low ON OFF

The circuit can be modified by interchanging the positions of the LDR and resistor R. IC IE 1k R LDR 10 k 6 V Figure 9.24

(b) A Heat-Controlled Switch

(i) Complete the statement below.

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

A ………..is a special type of resistor. Its resistance becomes very ……… when it is cold. When the thermistor is heated, its resistance ………… rapidly. At room

temperature, the thermistor has a ………. resistance compared to R. Therefore, the base voltage of the transistor is too low to switch on the transistor.

When the thermistor is heated, its ………. drops considerablely compared to R. Therefore, the ………., VB is high enough to switch ……. the transistor. When the

transistor is switch on, the relay switch is activated and the relay is switched ………. The circuit can also be used in a fire alarm system.

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

……….. ……….. (iii) Complete the table below.

Temperature RThermistor VThermistor R VR Transistor (ON or OFF) High Low Remember _{Ic >>>>}_Ib thermistor high drops high resistance base voltage on

To protect the transistor from being damaged by the large induced e.m.f in the relay coil when the collector current, IC drops to zero.

low low high high high high low low ON OFF on Relay Alarm RB R Thermistor Diode Figure 9.25

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 when

the resistance of R is reduced?

……… 4. A transistor is said to have two states, the „ON‟ state and „OFF‟ state.

(a) Explain the meaning of the „ON‟ state of a transistor.

……… (b) Explain the meaning of the „OFF‟ state of a transistor.

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

……… (d) What is the function of the resistor, R1?

IC IE R1 R2 R IB mA A Current Amplification B C I I    collector current

base current small big

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

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

To change the base current. n-p-n transistor

The readings on miliammeter and microammeter increase. Figure 9.26

9.4 Logic Gates

9.4.1 Analysing Logic gates

1. What is a logic gate?

……… 2. Complete the table below.

Gates Symbol Truth table

AND gate Input Output A B Y 0 0 0 0 1 0 1 0 0 1 1 1 OR gate Input Output A B Y 0 0 0 0 1 1 1 0 1 1 1 1 NOT gate Input Output A Y 0 1 1 0

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

Y A B Y A Y A B

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

9.4.2 Combinations of logic Gates

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

The truth table:

Input Output A B P Y 0 0 1 0 0 1 1 1 1 0 0 0 1 1 0 0 Y A B Y A B Y A B P 0 0 1 1 0 1 0 1 0 1 0 0 1 1 0 0 Figure 9.27

2.

The truth table:

3.

The truth table:

Input Output A B P Q Y 0 0 1 1 1 0 1 1 0 0 1 0 0 1 0 1 1 0 0 0 Input Output A B _B X Y 0 0 1 0 0 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 Y A B P 0 0 1 1 0 1 0 1 1 0 0 0 1 1 0 0 1 0 1 0 Q Y A B B 0011 0101 0 0 0 1 1 0 1 0 0 1 0 0 X Figure 9.28 Figure 9.29

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 P Q R Q S 1 0 1 1 1 0 1 1 0 0 1 1 0 0 0 0 0 1 1 1 Y A B P Q 0011 0101 1110 0111 0110 Q S R P Q Figure 9.30 Figure 9.31

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

Keys:

The light detector (Input J): In the day, logic “1”. At night, logic “0”. The heat detector (Input K): Hot, logic “1”. Cool logic “0”.

(a) Complete the truth table below:

(b) Based on the truth table in (a), state the conditions in which the air-conditioner conditions in which the air-conditioner will operate and function normally.

……… Input Output J K L 0 0 0 0 1 1 1 0 0 1 1 1 Light detector Heat detector Input J Input K Air-conditioner

- On a hot say or daytime – On a hot night Figure 9.32

Reinforcement Chapter 9 Part A: Objective questions

1. Which of the following is not a property of cathode rays?

A. It is positively charged. B. It travels in a straight line.

C. It can be deflected by magnetic field. D. It can be deflected by electric field. 2. Cathode rays consists of

A. Fluorescent particles B. Light rays from a screen C. Beams of fast moving particles D. Light rays from hot filament 3. A beam of electrons is being deflected

due to a potential difference between plates P and Q.

Which of the following statements is not true?

A. The potential at plate P is positive. B. The deflection would be greater if

the potential difference is greater. C. The deflection would be greater if

the electrons are moving faster. D. The electron beam will return to

straight line if a suitable magnetic field is applied between the plates. 4. The figure 9.34 shows the trace

displayed on a CRO with the Y-gain control is turned to 3.75 V/div. What is the maximum value of the potential difference being measured?

A. 2.5 V B. 5.5 V C. 7.5 V D. 12.5 V E. 15.0 V 5. In p-type semiconductor

A. The number of holes are equal to the number of electrons.

B. The number of the holes are more than the number of electrons.

C. The number of the holes are less than the number of electrons.

6. Which of the following is not true about diode?

A. It can be used to rectify alternating current.

B. It can only conduct electricity when it is connected in forward in forward bias in a circuit.

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

D. The majority charge carriers in the diode are electrons.

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

Which of the following is not true? A. The conductivity of the

semiconductor increases.

B. The semiconductor becomes an n-type.

C. The majority charge carrier is electron.

D. Atom P is a trivalent atom. Figure 9.34

P

Q

Figure 9.33

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

A. A rectifier changes d.c to a.c. B. Device P allows current to flow in

any directions.

C. Device Q acts as a rectifier.

D. The rectifier circuit would still work if device P is reversed.

9. The figure 9.37 shows a circuit consisting of two diodes and a bulb. When the switch is on, the bulb does not light up.

What needs to be done to light up the bulb?

A. Replace the diode with a new one. B. Reverse the connection of the diode. C. Increase the number of bulbs. D. Connect a resistor in series with the

bulb. 10.

Figure 9.38 shows four identical bulbs,

components connected in a circuit. Which of the following bulbs will light up continuously when the switch is on? A. P and Q only

B. P, Q and R only C. R and S only D. P, Q and S only

11. Which of the following circuits shows the connect directions of the base current IB, emitter current, IE and collector current, IC? Figure 9.36 Figure 9.37 Figure 9.38 P Q

12. Which of the following statements about a transistor is not true?

A. A transistor can act as an amplifier B. A transistor can act as a relay switch. C. The function of a transistor is the

same as that of two diodes.

D. A transistor is a combination of two types of semiconductors.

13. What is the function of the transistor circuit shown in figure 9.39?

A. As an amplifier B. As a rectifier C. As a switch device D. As a modulator

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

Which of the following is correct? A. IB > IC

B. IB = IC C. IB < IC

15. Figure 9.41 shows a circuit consisting of a transistor which acts as an automatic switch. When the potential difference across the thermistor is 3 V and the resistance of the thermistor is 1000 , the resistance value of resistor, R is ..

A. 3 k B. 4 k C. 5 k D. 6 k E. 7 k

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

Which of the following is not correct about the circuit?

A. T is an npn transistor

B. The capasitor prevents d.c current but allows a.c current to pass through it.

C. Speaker amplifies the sound. D. R1 and R2 act as potential divider. 17. The figure 9.43 shows a logic gate

circuit with input signals, X and Y.

Which of the following is the output signal? Figure 9.41 IB IC Figure 9.40 M-microphone C- Capacitor S- speaker Figure 9.42 Figure 9.43 Figure 9.39

18. The figure 9.44 shows a logic gate circuit.

Which of the following is the output signal Z?

A. 0110 B. 1010 C. 1110 D. 0101

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

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

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

20. The figure 9. 45 shows a combination of three logic gates in a logic circuit. When inputs P and Q are both 1 output Y is 1.

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

A. B. C. D. J K AND NOR NAND NOR OR AND 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 Y J K Figure 9.45 Figure 9.46

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

2 x 3 = 6V Scale: 1 division = 1 cm The Y-gain is set at 3 V/cm The time base is set at 5 ms/cm

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

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

2.5 ms/cm and the Y-gain setting is altered to 2 V/cm. On the space below, sketch the new trace would appear on the oscilloscope.

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

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

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

are forward biased.

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

f =1/T=50 Hz D1 and D3 D2 and D4 Figure 9.47 Time/ms Voltage/V Figure 9.48

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

(e) Explain how the capacitor causes the voltage across the resistor to vary with time in the way that you have drawn.

……… ………

3. A student wants to build a simple lift motor control system which operates using two buttons, A and B for a two-storey building.

A: Up button B: Down button

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 Motor

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

(i) Using the keys given, complete the truth table. (ii) Name the logic gate in the circuit in the figure 9.50.

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

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

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

dependent resistor, LDR. If the light dependent resistor detects light, the relay switch is 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 0 0 1 1 1 0 1 1 1 0 R Motor 240 V Figure 9.51 AND Gate

Activates large current in the main secondary circuit supply// small current at the output cannot activate the motor.

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

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

dependent resistor using the symbols given below.

(iii) Explain how the circuit functions.

……… ……… ……… ……… ………

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

Part C: Essay Questions

1.

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

(i) What is meant by a direct current and an alternating current? [2 marks] (ii) Using Figures 9.52 and figure 9.53, compare the connection of the diodes and the

conditions of the bulbs. Relating the connection of the diodes and the conditions of the bulbs, deduce the function of a diode. [5 marks]

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

(b) A semiconductor diode is made by joining a p-type semiconductor with a n-type semiconductor. Describe and explain the production and the characteristics of a p-type

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

2. Figure 9.55 shows four circuits W, X, Y and Z, each has an ideal transformer and the circuit are used for the purpose of rectification.

Figure 9.52 Figure 9.53 Circuit W Circuit X Circuit Y Circuit Z

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 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.

Transistor T is connected to fixed resistor R1 =1k and R2 = 56 k 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 A. The collector current, IC recorded by miliammeter A2 is shown in figure 9.57(a).

(a) IB = 10A mA T 6V R1 = 1k A2 IC A1 IB R2 = 56k R2 = 2k Figure 9.56

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 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 (b) IB = 20A (b) IB = 30A (c) IB = 40A (d) IB = 50A (e) IB = 60A mA mA mA mA mA

(a) For the experiment described identify…

(i) the manipulated variable : ..……… (ii) the responding variable : ……….. (iii) the fixed variable : ………..

(b) From the figure in 9.57, record the collector current, IC when IB = 10, 20, 30, 40, 50 and 60A. Tabulate your results for IB and IC in the space given below.

IB/A 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.

………

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.

The base current, IB The collector current, IC The supply voltage

Ic is directly proportional to IB Figure 9.58 loudspeaker Power amplifier Volume control Microphone

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.

Scheme Part : No. 4

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

(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, 75,100,125A. (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

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 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 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 coil primary in the turns of number The coil secondary in the turns of number The _{ }

- A full-wave rectification is better than a 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.