Introduction to Cathode Ray Oscilloscope and Thermionic Emission

2. 2 Cathode Ray Oscilloscope Introduction Thermionic Emission Cathode Rays Deflection of an electron beam Cathode Ray Oscilloscope Uses of the CRO

3. Thermionic Emission 3 The negative charges , in the form of electrons , escape from the filament when its hot because they have enough energy to get free from the metal surface. There is a certain minimum threshold energy which the electron must have to escape .

4. Cathode Rays 4 Beams of electrons moving at high speed = Cathode Rays Electrons accelerated toward anode pass through the hole miss the cross the screen to fluoresce cast a shadow of the cross

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6. Deflection of an electron beam 6 The deflection of an electron beam can be caused by 1. A magnetic field 2. An electric field Force on electron

7. Cathode Ray Oscilloscope 7 The CRO has three main parts i.e. 1. Electron Gun 2. Deflecting System 3. Florescent Screen

8. Electrons are produced by thermionic emission . Essentially a cathode(negative electrode) is heated and electrons boil off the surface to be attracted by a series of anodes (positive electrodes). The anodes accelerate the electrons and collimate them into a narrow beam. 8 Electron Gun

9. Consist of two pairs of parallel plates : X-Plates and Y-Plates 9 Deflection System To display a waveform , a repetitive reversing voltage is applied to the X-plates . This causes the electron beam to be slowly repelled from the left-hand plate and attracted towards the right-hand plate. On the CRO screen this translates as an illuminated dot moving from left to right.

10. The voltage is then reversed and increased rapidly. The effect is to move the dot very quickly right to left (fly-back). The applied voltage is called the time-base . The curve has the general shape of a ' saw- tooth ' . The p.d. applied to the Y-plates is the signal to be examined. With the p.d. across the X- plates, a sine waves is displayed 10

11. Display The display screen is coated on the inside with a very thin layer of a phosphor called cadmium sulphide . This fluoresces (gives out green light) when electrons impact its surface. 11

12. Uses of CRO 12 A CRO can be used for 1. Measuring Potential Differences 2. Displaying waveforms 3. Measuring time interval & Frequency

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14. 14 Potential Difference Signal Frequency signal

15. 15 X-Rays X-Rays are produced when high-speed electrons are stopped by matter X-rays In an X-ray tube, electrons from a hot filament are accelerated across a vacuum to the anode The anode is a copper block with a target of a high-melting-point metal such as tungsten on which the electrons are focused by the electric field between anode and concave cathode

16. X-Rays X-Ray Properties 1. Penetrate matter up to 1 mm of lead 2. Not deflected by electric or magnetic field 3. Ionize a gas 4. Affect a photographic film 5. Cause fluorescence 6. Give interference and diffraction effects

17. X-RAYs X-RAYs X-rays can travel through soft substances but not hard, dense ones. They travel through soft tissue, but not hard bone. X-rays can travel through soft substances but not hard, dense ones. They travel through soft tissue, but not hard bone. X-rays are used in hospital to make shadow pictures of parts of the body. X-rays are used in hospital to make shadow pictures of parts of the body. X-rays are used at airport to check what may be hidden in the luggage. X-rays are used at airport to check what may be hidden in the luggage.

18. Photoelectric Effect In the photoelectric effect , electrons are emitted from matter (metals and non-metallic) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength , such as ultraviolet light .