4.1 Electronic

7/30/2019 4.1 Electronic

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PROGRAM DIDIK CEMERLANG AKADEMIK

SPM

ORGANISED BY:

JABATAN PELAJARAN NEGERI PULAU PINANG

PHYSICS

MODULE :

FORM 5

4.0 ELECTRONICS


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TOPIC:4.0 ELECTRONIC4.1 UNDERSTANDING THE USES OF THECATHODE RAY OSCILLOSCOPE (C.R.O.)

CONTENTS:1.CHAPTER HIGHLIGHT (30 MINUTES)2.ACTIVITY (50 MINUTES)3.ASSESSMENT (40 MINUTES)4.MARKING SCHEME(ANSWER KEY)


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1.CHAPTER HIGHLIGHTTHERMIONIC EMISSION1. Thermionic emission is a process of emitting of electrons from metal

surface that being heated.2. Thermionic emission rate depends on:

FACTORS THERMIONIC EMISSION RATE

Temperature of the metal

Surface area of the metal

Type of metal Depends on the metal

Surface of the metal coveredwith metal oxide

MALTESE CROSS TUBE

1. A Maltese cross tube is a special cathode ray tube used to investigate theproperties of cathode rays.i) Cathode rays travel in straight lines and they can be stopped by a piece

of metal.ii) Cathode rays transmitted straight lines and produce fluorescent effectiii) Cathode rays have energy and can do work.iv) Cathode rays can be deflected by magnetic field and electric field.


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STRUCTURE OF CATHODE RAYS (C.R.O.)1. A C.R.O. is an instrument that converts electronic and electrical signals to

a visual display.2. A C.R.O uses a cathode-ray tube : an electron gun sends electrons

through the vacuum to a fluorescent screen and a light spot appears on

the screen.3. A C.R.O. consists of three main parts. There are electron gun, deflection

system and the fluorescent screen.4. The diagram below shows the structure of a C.R.O.

Structure of a C.R.O

5. The table shown summaries the function of different parts of a C.R.O.

Main Component Component Function

Filament Heating up cathode

Cathode Emitting electrons bythermionic emission

Electron gun Control grid Brightness control by

controlling amount ofelectrons passingthrough it

Focusing anode

Accelerating anode

i) focusing of electronbeam

ii)Accelerating electronbeam

Deflection system Y- Plates Deflect the electron beamvertically ( upward anddownward)

X- plates Deflect the electron beam

horizontally ( left andright)

Fluorescent screen i) Screen coated with afluorescent material

Change the kineticenergy of the electron tolight energy

ii) Graphite is coated overfluorescent screen andearthed

Electrons that hit thescreen are sent to theearth


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THE WORKING PRINCIPLE OF THE CATHODE RAYOSCILLOSCOPE (C.R.O.)1. The operation of a C.R.O. depends on the thermionic emission.

2. The tube is applied with a potential difference.3. The negative electrode is called a cathode, and the positive is an anode.4. If the cathode is heated, it will become hot and glows.5. The negative charge (negative) leaves the cathode and flows to the anode.6. The electron flows is called cathode rays.

THE USE OF C.R.O.C.R.O. can be used to :

a) Measure potential differencei) Since the vertical deflection of the light spot (trace) is proportional to the

voltage applied across the Y-plates, the C.R.O. can be used as avoltmeter.

ii) It can measure both a.c. and d.c. voltages. It measure the peak voltage ofa.c. input signal.

Example 1.A C.R.O. shows a trace of a d.c. voltage as in the following diagram. Thetime base setting is 20 ms/cm and the Y-gain setting is 5V/cm.

Amplitude of the waveform = 1 cmThe voltage is given by

V = (5)(1) = 5 V

The wave corresponds to a + 5V d.c. input voltage.


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Example 2

When an a.c. voltage is connected to the Y-gain of the C.R.O., a waveformas shown below is displayed on the screen. The time-base setting is 5ms/cm and the Y-gain setting is 1V/cm.

Amplitude of the waveform = 2 cmThe peak voltage is given by

V = (1)(2) = 2 VThe waveform corresponds to an a.c. input with a peak voltage of 2V.

b) Measure short time intervals

Examples : period of signal input the C.R.O., time interval between atransmitted pulse and its reflected pulse.

i) The diagram below shows the waveform of an a.c. signal input to aC.R.O.. The time interval between point P and Q is the period.


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ii) If the time base is set to 5ms/cm, the periodT = 5 ms/cm x 4 cm

= 20 ms ( or 0.02 s)

iii) The frequency of the a.c. signal can be determined by

f = 1 = 1 = 50 HzT 0.02s

c) Display waveform.

When a signal is connected to the Y-input and earth terminals, a brightspot on the screen moves vertically. Switching on time-base will move thebright spot horizontally. The combination of the horizontal and verticalmovements of the bright spot will generate a wave forms.

a.c voltage across y-plate Time base voltage resultant waveformmakes spot move up and sweeps spot acrossdown the screen


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ACTIVITIESA) Match the correct statement.

Control grid

Thermionic emission

Cathode ray oscilloscope

X-plates

Y-plates

A process of emitting

electrons from metal surfacethat being heated

Deflect the electron beamhorizontally

To control the number ofelectron through it and controlthe intensity of the light spoton the screen

Deflect the electron beamvertically

An instrument to detect anddisplay electric signal

B. Choose the best answer for every question.

1. What is emitted by the hot metal filament in a cathode ray tube?A particlesB electronsC protonsD X-rays

2. Thermionic emission is a process whereA electrons are emitted from a surface of a hot metal

B electrons are accelerated toward anodeC electrons are emitted from nucleus atom

3. How are the electrons produced in a cathode ray tube?A by applying an electric field to the X-platesB by heating a metal filamentC by ionization of the airD by radioactive decay

4. Which of the following is not a property of a cathode rays?A They travel in a straight lines

B They are deflected by an electric fieldC They are not affected by gravity.D They have momentum and energy

5. The following trace is shown on the screen of an oscilloscope when it isconnected to a transformer. The scale is set at 5V per centimeter.


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What is the value of the peak voltage?A 4 VB 5 VC 10 VD 20 V

Structure question

1. Figure above shows the structure of the cathode ray oscilloscope.a) Name the process of electron emission from the hot surface of the

cathode.

__________________________________________________________

b) How the acceleration of the electron can be increased?

__________________________________________________________

c) State the energy changes of the electron when it hits the fluorescent

screen.

__________________________________________________________

__________________________________________________________


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2. Figure below illustrates the trace obtain on the screen of an oscilloscopewhen a given signal is applied to the input terminals. The voltage sensitivity is0.20 V/cm and the time-base is set to 2.0 ms/cm.

(a) Determine the maximum positive value of the voltage indicated.

________________________________________________________

(b) Determine the frequency of the signal

________________________________________________________

________________________________________________________

ASSESTMENT

1. Which of the following is not a factor that influences the rate of thermionic

emission?A Type of metal D Surface area of the metalB Temperature of the metal E The conductivity of the metalC Surface properties of the metal

2. The diagram shows a trace on an oscilloscope set at 5 V/cm on the verticalaxis.

What is the maximum value of the potential difference being measured?A 1.5 V C 7.5 VB 3.0 V D 15.0 V


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3. Which of the following is not the function of the cathode ray oscilloscope?A measure the potential difference of a.c and d.cB measure short time intervalsC measure the resistanceD investigate the waveform

4. Name the component in cathode ray oscilloscope that can accelerate theelectron.

A focusing anode C filamentB acceleration anode D control grid

Question 5 and 6 base on the diagram below.

5. The following trace is shown on the screen of a cathode ray oscilloscopefor an a.c The scale is set for 5 V/cm and the time base is set to 10ms/cm. Calculate the value of the peak voltage.

A 5VB 10 VC 15 VD 20 VE 25 V

6. Calculate the frequency for this a.c current.A 100 HzB 25 HzC 10 HzD 0.04 HzE 0.01 Hz

STRUCTURE QUESTION1. (a) Describe with the aid of a diagram or diagrams the structure and action of

a cathode ray oscilloscope. Your answer accounts of how the oscilloscope(i) produces a beam of fast moving electrons(ii) makes the beam of electrons produce a visible trace(iii) deflects the beam of electrons


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(b) In order to produce the trace shown in figure below, the voltage sensitivityof an oscilloscope was set at 0.50 V per division and the time-base wasset at 2.0 ms per division.

For the voltage applied to the Y-terminals, determine

(i) the peak value(ii) the period(iii) the frequency


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ANSWER KEY (ACTIVITIES)Part A

Control grid = To control the number of electron through it and control theintensity of the light spot on the screen

Thermionic emission = A process of emitting electrons from metal surface thatbeing heated

Cathode ray oscilloscope = An instrument to detect and display electric signal

X-plates = Deflect the electron beam horizontally

Y-plates = Deflect the electron beam vertically

Part B

1. B

2. A3. B4. C5. C

Structure Question

1 a) thermionic emissionb) increase the potential difference between accelerating anode and cathodec) from kinetic energy to light energy and heat energy

2. a) Maximum positive value of voltage =0.20 x 4 = 0.80 Vb) Period of signal = 2.0 x 5 = 10 ms

Frequency = 1 / period = 1/ 10x 10-3 = 100 Hz


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ASSESTMENT

1. E2. C4. B5. C

6. B

STRUCTURE QUESTION

1. (a) Main feature of the cathode ray tube

The cathode ray tube consists of the electron gun, the deflection system andthe fluorescent screen. In the electron gun, the heater heats the cathode torelease the electrons and the grid controls the amount of electrons reaching thegrid. The anode which is at a positive potential accelerate the electrons andfocuses it into a fine beam. The Y-plates serve to deflect the beam verticallywhile the X-plates deflect the beam horizontally. Both plates cause a waveform tobe formed on the screen.

The screen is coated with a fluorescent material. When the beam strikes it, itglows.

(b) (i) From the trace, the height of the peak value is 4 division.

The peak value = 0.5 x 4 = 2.25 V

(ii) From the trace, there are 3 cycles in 0.02 seconds.

Hence the period = 0.02 = 6 ms = 6.67 ms

(iii) The frequency = 1 = 1 x 103

= 150 Hzperiod 20/3

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