1026902-Nota-Padat-Fizik-F5-Electronic - 9.1 CATHODE RAY...

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9.1 CATHODE RAY OSCILLOSCOPE What is thermionic emission? Thermionic emission is the process of emission of electrons from a heated metal’s surface. How does thermionic emission occur? Metal consists of a large number of electrons which are free to move. At room temperature, the electrons are free to move but remain inside the metal. The electrons cannot escape at the surface because they are held back by the attractive forces of the atomic nucleus. If the metal is heated at a high temperature, some of the free electrons may gain sufficient energy to escape from the metal. What is cathode rays? Thermionic emissions can be used to produce a continuous flow of electrons in a cathode ray tube. When the cathode is connected to the anode by an extra high tension (EHT) voltage supply, a narrow beam of fast electrons will move to the anode. The beam of electrons moving from the cathode to the anode is called cathode rays . Describe the properties of cathode rays Cathode rays can be used in picture tube of a television, a cathode ray oscilloscope and the visual display on a radar screen. The properties of cathode rays can be summarized: 1. Negatively charged particles called electrons. 2. Travel in straight lines and cast sharp shadows. 3. Travel at very high speed and have kinetic energy. 97
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kinetic energy of the electrons is converted into light energy) 5. Deflected by electric and magnetic fields. To Investigate the properties of electron streams in a Maltese cross cathode ray tube. Procedure Observation on the fluorescent screen Connect only the 6.3 V power supply to the filament A dark shadow of the Maltese Cross is formed on the screen. Connect the 6.3 V and EHT to the electrodes A darker shadow of the Maltese Cross is seen on the screen. The shadow is surrounded by green light Bring a pole of a bar magnet near to the neck of the tube. Two shadow are seen on the screen. The light shadow remains at the centre of screen while the dark one is shifted. Reverse the pole of the bar magnet The light shadow remains at the centre of screen while the dark one is shifted to the opposite direction. Discussion 1. When the 6.3 V power supply is switched on, the filament is heated. The Maltese cross shadow is formed on the screen due to the light from the filament. 2. When the EHT power supply is switched on, a high voltage is applied between the cathode and anode causing electrons to accelerate at high speeds from cathode to anode. These electrons are the cathode rays . 3. The cathode rays blocked by the Maltese Cross causing a shadow to form on the screen. The cathode rays travel in straight lines. 98
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1026902-Nota-Padat-Fizik-F5-Electronic - 9.1 CATHODE RAY...

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