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Chapter Electronics Physics JPN Pahang

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Chapter Electronics Physics JPN Pahang
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   JPN Pahang Physics Module Form 5 Teacher’s 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 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) …………………………………………………………………… (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   JPN Pahang Physics Module Form 5 Teacher’s Guide Chapter 9: Electronics 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, V   between the cathode and the accelerating anode. [   e  = 1.6 x 10 - 19 C] Solution: V101.32 energyKinetic 14- 4192 1025.8 106.1 21 ×=×=×== − 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) J104105.2106.1energy potentialElectric 16319  −− ×=×××== eV   (b) 162 10421  − ×== eV mv  2109104 31162 ×××= −− 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 106.5105.3106.1 21  −− ×=×××== eV mv   1531162 1024.12 109106.5 ×=×××= −− v   -1715 ms1053.31024.1  ×=×= v     By using the principle of conservation of energy, eV mv  = 2 21, Maximum velocity of electron, meV 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 Figure 9.2   JPN Pahang Physics Module Form 5 Teacher’s Guide Chapter 9: Electronics 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-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 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   JPN Pahang Physics Module Form 5 Teacher’s Guide Chapter 9: Electronics 4 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 V   pp . Solution: 0.30.25.1  =×= 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,  V   pp = (Y-gains) × h  Peak voltage, V   p   = (Y-gains) 21 × ( h ) Effective voltage or root-mean-square voltage, V  r.m.s   =  p V  21 Short time intervals, t   = no. of divisions between two pulses × time-base value. Figure 9.4 Figure 9.5   JPN Pahang Physics Module Form 5 Teacher’s Guide Chapter 9: Electronics 5 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, V   p   = (Y-gains) 21 × ( h ) divsV/divV  P 4215  ××=  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, V   p of the a.c source. Solution: cmVcmV  1-P 4215.1  ××=  V V  P 0.3 =   (b)   Calculate the frequency,  f   of the a.c source. Solution: 24  ×= cmT  ms cm -1   T = 8 ms ∴  f = 1251 = 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,  V   pp (b)Peak voltage, V   p . (c)Root-mean-square voltage, V  r.m.s  Figure 9.6 Figure 9.7 Figure 9.8
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