From ideas to implementation

Transcription

1 Physics HSC Course Stage 6 From ideas to implementation Part 2: The amazing cathode ray tube

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3 Contents Introduction... 2 Cathode ray tubes (CRT)... 3 Types of cathode ray tube...3 What makes up a CRT?...4 Cathode ray oscilloscope (CRO)...5 Using the CRO...6 Cathode ray tubes in television sets Safety requirements...11 Inside the picture tube...12 The picture...14 Controlling the cathode ray beam...19 Electron microscopes Types of electron microscopes...22 Lightning conductors Photocopiers Summary Suggested answers Exercises Part Part 2: The amazing cathode ray tube 1

4 Introduction In the last part of this module you learned about the history of the development of the cathode ray tube and its role as an instrument of scientific research into the nature of subatomic particles, namely the electron. In this part you will learn about some of the applications for the cathode ray tube. Many of these applications directly affect daily life and have improved our understanding of the world around us. In Part 2 you will be given opportunities to learn to: outline the role in a cathode ray tube of: electrodes in the electron gun the electric field the fluorescent screen outline applications of cathode rays in oscilloscopes, electron microscopes and television sets discuss the impact of increased understanding of cathode rays and the development of the oscilloscope on experimental physics. In Part 2 you will be given opportunities to: gather, analyse and process information on the use of electrically charged plates and point charges in photocopying machines and lightning conductors gather secondary information to identify the use of magnetic fields in television sets. Extracts from Physics Stage 6 Syllabus Board of Studies NSW, originally issued The most up-to-date version can be found on the Board's website at 2 From ideas to implementation

5 Cathode ray tubes (CRT) The German scientist Karl Braun invented the cathode ray tube with a fluorescent screen making up one end of the tube in It rapidly became a research tool for scientist such as Thomson. Braun discovered that a stream of electrons would make a screen coated with fluorescent material glow with light. Braun was the developer of the cathode ray oscilloscope. He demonstrated the first oscilloscope tube in 1897, after research work on high frequency alternating currents. Cathode ray tubes previous to Braun's work had produced uncontrolled cathode ray streams. Braun succeeded in producing a narrow stream of electrons directed by means of alternating voltages that could trace patterns on a fluorescent screen. Types of cathode ray tube CRTs are divided into two major groups based on how the electron beam (or beams) are directed to the desired location on the tube's screen. CRTs where the electron beam is deflected electrostatically or by electric fields. This system of electron beam deflection is used primarily for oscilloscopes where great speed is required to position the electron beam to a desired location on the screen in order to follow rapidly changing waveforms. CRTs where the electron beam is electromagnetically deflected. This type of CRT is used almost universally in television, computer displays and radar. The figure following shows a computer monitor CRT. Notice the copper electromagnetic deflection coils. Part 2: The amazing cathode ray tube 3

6 Computer monitor cathode ray picture tube. Photo: Ric Morante. Color CRTs are similar to other electromagnetically deflected CRTs except that they contain the equivalent of three conventional electron guns in one envelope along with a more complex screen configuration in terms of the distribution of the phosphors into discrete areas in the fluorescing screen. What makes up a CRT? The basic components of all modern CRTs are outlined below. An electron beam source and beam intensity control mechanism. One or more accelerating electrodes. These increase the electron s velocity so that when the electrons hit the screen they have enough energy to ensure appropriate light output from the screen. The faster the electrons are travelling, the brighter the fluorescence produced by the screen. A focusing section consisting of electric fields that bring the electron beam to a sharp focus precisely at the screen. Most CRTs have a curved front screen so that the focusing mechanism required by the television produces a focused beam at the same distance from the filament, no matter where on the screen the electron beam hits. 4 From ideas to implementation

7 A deflection system consisting of magnetic coils that positions the beam to a desired location on the screen or is used to scan the beam across and down the screen in a repetitive pattern. A phosphor screen that converts the kinetic energy of the electrons in the invisible electron or cathode ray beam into visible light. A mechanical structure known as the envelope or tube outer that allows a vacuum state (eg. the glass tube). This must also provide a location for electrical connections to the various electrodes, and must insulate those connections and components from each other. Why does the CRT screen glow? The screen glows because of phosphorescence. Cathode ray tubes and television picture tubes have a layer of the phosphor coated on the inside of the glass screen. The beam of electrons projected by the cathode excites the fluorescent phosphor layer on the screen. That is, the phosphors glow. Most cathode ray tubes use zinc sulfide that glows with a characteristic blue-green trace. In a colour TV tube, the screen is coated with three phosphors that fluoresce one red, another green and the third blue. With phosphorescence, the emission of light from a phosphorescent substance can continue for a brief time after the exciting radiation of the electron beam is cut off. That is why when the TV is finally turned off at night the screen appears to have a soft glow in the darkened room for a short time. This is because the phosphor material absorbs energy from earlier exposure to the beam of electrons which is stored and gradually re-emitted later as light. Phosphorescent materials include zinc, calcium, barium and strontium sulfides. Cathode ray oscilloscope (CRO) A cathode ray tube that is configured as a cathode ray oscilloscope is shown in the figure following. Part 2: The amazing cathode ray tube 5

8 1 kv potential difference filament horizontal deflection plates vertical deflection plates vacuum cathode grid electron electrons in a beam fluorescent screen a bright spot is formed at the point where the electron beam meets this luminescent screen The internal 3-D view of a modified cathode ray tube that makes up a CRO. The deflection plates are set with positive or negative potentials with an AC current. The changing electric fields across the plates can cause rapid deflection of the electrons in the electron beam. The stream of electrons from the cathode can be focussed to a point at the end of the tube on a fluorescent screen by use of a device called an electron gun. The electron gun consists of: a filament that is the source of electrons a negatively charged metal cylinder that focuses the electron beam in much the same way as a condensing (convex) lens focuses light a series of metal anode cylinders that have progressively higher positive potentials (voltages) relative to the cathode. The beam is focussed to a dot by the use of electric fields from the anode cylinders. The effect of the electric fields from a two anode electron gun are illustrated in the figure following. The invention of the cathode ray oscilloscope was the forerunner of the television picture tube and radarscope. It also became an important laboratory research instrument. Braun's career in science came to an end when he travelled to New York City in 1915 to testify in a radio related patent case. He was detained there because of his German citizenship when the U.S. entered World War I in He died in 1918 before the war ended. He apparently did not contribute to the allied war effort while in detention and so was loyal to his native Germany. 6 From ideas to implementation

9 electrons are emitted at the filament anode 1 anode 2 equipotential lines electron beam F positive negatively charged cylinder acts as a condensing lens to narrow the beam more positive screen is at the beam focus An electron gun. The anodes shown in the figure are each part of a cut away cylinder. These cylinders enable the electron beam to be focussed in cross section to a cylindrical beam. 1 Outline the role of the electron gun in the cathode ray tube. 2 What does the cylindrical electric plates in the anodes do to the electron beam? 3 Why is the fluorescing screen so important in the modern cathode ray tube used in applications such as the CRO or television? Check your answers. Part 2: The amazing cathode ray tube 7

10 Using the CRO You are already familiar with the use of the CRO to study sound waves in the module The world communicates. This use of the CRO is not the typical one made in research. Rather the CRO is used as a research tool wherever low, fluctuating voltages occur. In the electronics industry that application has guided development of the communication age in which you currently live. This means the CRO has been used in the development of just about every communication and computing device known. Locate a local electronics enthusiast or someone who professionally repairs electronic appliances such as televisions or computers. Ask them how they use the CRO in their work. Write down their responses in the space below. How does the CRO work? The voltage to the horizontal deflection plates is adjusted so as to move the electron beam across the screen at a constant rate. The vertical deflection plates are subject to varying or constantly varying AC voltages. This moves the electron beam irregularly in the vertical plane. The combined motion produces a trace on the phosphor screen. This is illustrated in the following where a sine wave trace is produced on the screen of the CRO by a constant AC voltage input. 8 From ideas to implementation

11 dot in the centre of the screen dot moves up (or down) the screen zero voltage across the vertical deflection plates a constant DC voltage across the vertical deflection plates spot oscillates vertically a constant AC current across the vertical deflection plates produces a line spot moves horizontally at steady speed A combination of a constant horizontal deflection and a AC deflection on the vertical plates produces a sine curve The trace from a CRO produced by an AC current. Part 2: The amazing cathode ray tube 9

12 The trace shown above is that produced by a regular AC voltage. The pale text at the bottom indicates that each division on the grid in the y-direction represents 0.25 V. The time base is shown as ms per division of the x-direction grid. 1 Look at the figure on the previous page. What does this indicate about the rate of movement of the electron beam across the screen in the x-direction? 2 What movement of the deflection plates would produce that movement you described above? Which deflection plates in the CRO tube would be involved? 3 What movement of the electron beam is necessary in the y-direction to produce the pattern shown? 4 How can this y-direction movement of the electron beam be produced by the deflection plates in the CRO tube? Check your answers. Do Exercises 2.1 to 2.2 now. 10 From ideas to implementation

13 Cathode ray tubes in television sets Traditional television sets contain a modified cathode ray tube as a picture tube. The picture tube is a funnel shaped cathode ray tube made of special glass. The glass tubes must be able to seal to the metal electrodes that carry high voltages (the anode buttons) and to the conductors supplying the heating power and control voltages to the cathode. Together these form part of the electron beam gun. electron gun magnetic deflection coils silvered tube fluorescent screen A television cathode ray picture tube. Safety requirements The glass used to manufacture the picture tube has specific requirements for safety reasons. The television picture tube is essentially a particle accelerator. Electrons are accelerated from the cathode toward the anode and eventually the phosphor coated screen. Accelerating an electron causes it to give out electromagnetic radiation. That happens in the cathode ray picture tube of a television or computer monitor. The result is often that low levels of high energy electromagnetic radiation such as X-rays are produced. Part 2: The amazing cathode ray tube 11

14 The electrons accelerated toward the screen in the picture tube are undergoing an electrical energy (E = qv) loss and a kinetic energy gain Ê 1 2 mv ˆ Ë 2. People are exposed to the radiation from the television picture tubes for extended periods of time. Because of this the glass used in the tube must contain appropriate amounts of heavy oxides such as barium oxide, lead oxide and strontium oxide in order to ensure potentially harmful X-rays produced by the tube in its operation are absorbed. Survey the number of hours the television set in your home is on per week. Do this by simply recording the time(s) that the television is turned on and off each day for a week. Many dwellings have the television operating for up to 120 hours per week. Sun Mon Tues Wed Thurs Fri Sat AM PM Total hours Consider that low levels of X-rays can be produced by television picture tubes. The dosage of X-rays that can lead to health problems is cumulative. If the body is exposed to excess levels of X-ray radiation it can cause problems such as cancer. Can you see why it is important to prepare the glass tubes that make up the television picture tube from glass that absorbs X-rays? Inside the picture tube In a cathode ray picture tube, the cathode is a heated filament similar to the filament in a normal light bulb. The cathode filament is heated in a vacuum created inside a glass funnel shaped tube. As a result, electrons naturally escape from the heated cathode into the vacuum of the tube. The free electrons are negative. The anode is positive, so it attracts the electrons given off from the cathode and accelerates them towards it. The accelerated electrons actually pass through the hole in the anode, overshoot and continue on a path toward the other end of the tube that makes up the television screen. 12 From ideas to implementation

15 In a television s cathode ray picture tube, the accelerating stream of electrons (or cathode ray) is focused into a narrow beam by a set of focusing anodes. This arrangement is again called an electron gun. The electron gun is located at the rear of the glass discharge tube called the picture tube. The electron gun makes the cathode ray beam that is eventually responsible for painting the picture on the fluorescing screen. (Photo: Ric Morante) The cathode ray beam flies through the vacuum in the tube until it hits the screen at the other end of the tube. This screen is really just the end of a cathode ray tube coated with phosphor. This phosphor coating is concentrated in small dots or rectangles on the screen. The phosphor glows when struck by the beam. Rapid movement of the electron beam across and down the screen at the end of the tube produces the images you see on the television. If you look at the labelled photograph of the cathode ray picture tube above you will see there is a conductive silver coating inside the tube. This conductive coating is there to soak up the electrons that pile up at the screen-end of the tube. Hence if you look at the inside a broken picture tube you will see it is silver coated. Part 2: The amazing cathode ray tube 13

16 The picture How is a single beam of electrons able to produce a picture on the television screen? The viewing of a picture on a television screen relies on some quirks of how your brain works. The first of these is that if you divide a still image into a collection of small dots, your brain will reassemble the dots into an image. Look closely at a photograph in a newspaper. You should be able to distinguish the picture as being made up of a series of dots. If you have difficulty doing this, use a magnifying glass or convex lens. This can be made with a water drop on a sheet of clear plastic or cellophane wrapping. 1 What colour are all the dots that make up the image? Now look closely at a colour image from a magazine either with a magnifying glass or a convex water lens on a sheet of clear plastic. 2 How many colours are all the dots that make up the image? As you can see making a picture from dots is easy. Now check a television for evidence of a similar technology for making images. Carry out the following activity. Turn your television on. Flick a few drops of water onto your television screen. Look closely at the small convex lenses formed by the water droplets. 3 Describe what you see. Check your answers. Picture resolution If you have access to a computer with a television style monitor rather than a plasma or LCD screen and can put a multicoloured image on the screen repeat the experiment with the water droplets described above. You should see identical results although you will probably see the dots or rectangles are closer together than on a television screen. 14 From ideas to implementation

17 This produces a higher resolution on the computer screen. The still image allows you to examine various colours of the dots that make up the screen colour in a particular part of the screen. You should see the dot colour combinations vary in relative intensities. Mixing the intensities of the different coloured dots enables the eye to construct all the colours that can be seen on a screen. You may be familiar with the term, dpi in printing. This refers to the number of dots per inch. The more dots per inch the higher the resolution of the image. A similar thing happens with computer and television screens. The more dots per inch, the higher the resolution. Each of the different coloured dots you see in a rectangle, stripe or dot is made to fluoresce by an electron beam specifically aimed at that portion of the dot. In other words there is an electron beam that is aimed only at the red dots, another only aimed at the blue dots and a third aimed only at the green dots. This means whereas a black and white television has a single electron beam a colour television picture tube needs three beams. How does the image form? You should now know that the image formed on a television screen is made up of a series of dots. The question you should now ask yourself is how does a single electron beam on a white phosphor screen produce an image in black and white, or how does three electron beams produce a coloured image? The answer lies in another quirk of the eye. This quirk is one you are already familiar with and probably experience everyday. When you look at a bright light such as a light globe filament the image of that bright object is clearly impressed on the eye for some time even after you stop looking at the bright object. This is even more apparent if you close your eyes after looking at the bright object. You will still see a residual image of the bright object apparently impressed on your eyelids. Another example of this happening is if you spin a torch or bright light around in a circular motion on a dark night. The passage of the torch forms a circular ring pattern on the retina of the eye. This creates the appearance of a ring of light even though the torch can only be in one place at any one time. Part 2: The amazing cathode ray tube 15

18 Moving pictures The final quirk of the eye that allows you to see moving picture is as follows. If you photograph a moving object a number of times in close succession, a sequence of still pictures is produced. If you show the still pictures in rapid succession, your brain reassembles the sequence of still pictures back into a moving scene. You don t have to be special to do this. In fact, cartoons and all animations are based on this concept. You may even have tried to produce your own animation using something as simple as stick figures on the corners of the pages of a book that you can flick through rapidly to give the stick figures an appearance of motion. If you have never tried this you should have a go now with the corners of the page of the learning materials. Watching motion If you have access to a video cassette recorder (VCR) and a tape try the following activity. 1 Put a recorded video in your VCR. 2 Press play. 3 Press pause and advance the video by the frame advance if the VCR has that option. Can you see the individual frames that are assembled and played one after the other to produce the effect of a moving image from still frames taken close together? If the frames are flicking through and changing fast enough your brain and eyes do not seen any of these individual frames. You see a moving picture scene. A clear picture The bright spot left by the moving electron beam in the cathode ray tube moves across and down the screen many times per second (50 times for most televisions but can be higher for example 100 times or 100 Hz for flicker free televisions). Everywhere it touches the phosphor screen the screen fluoresces. The result is a picture produced on the retina of your eye of the pattern arced out by the bright spot on the screen. This picture appears to be on the screen of the cathode ray picture tube even though in reality each electron beam is only causing illumination at one point on the screen at any one instant of time. 16 From ideas to implementation

19 To assist this process there is also a slight lag time where the fluorescence of the screen is maintained even though the electron beam moves on. However, the length of this lag time glow must be short to produce a clear picture and avoid interference. Instead of deflection plates, steering coils produce the movement of the electron beam within the cathode ray tube. Steering coils are copper windings around the outside of the tube. These current carrying coils create magnetic fields inside the tube when an electric current passes through them. The electron beam is deflected by the magnetic fields. One set of steering coils creates a magnetic field that pushes the electron beam down the screen (the vertical deflection plates of the electron gun). Another set of steering coils creates the magnetic fields that push the beam horizontally (the horizontal deflection plates of the electron gun). A computer monitor showing deflection coils. (Photo: Ric Morante.) By controlling the size and direction of the currents flowing in the coils you can position the electron beam to any point on the screen. In a television picture tube, to paint the entire screen and trick the eye into seeing this illumination as a constantly repainted moving picture, the magnetic field generated by the current carrying coils to need to move the electron beam in a raster scan pattern across and down the screen. In other words, the beam paints one line across the screen from left to right. Part 2: The amazing cathode ray tube 17

20 These lines are rows of dots of different brightness varying with the intensity of the electron beam. At the end of a line the beam is switched off then quickly turned back on when aimed at the left hand side of the screen again but has been pushed down screen slightly. It then paints another horizontal line and the whole process is repeated. This continues down the screen until the entire screen has been painted. The electron beam is then switched off until it is aimed at the top left hand corner again ready to begin repainting the screen once more. The whole screen is painted 50 times per second to produce the effect of a moving picture scene. If the television is a 100 Hz model the process described above is repeated 100 times per second. The more times per second the screen is painted, the greater the flicker free effect of the picture. Manufacturers of televisions claim this flicker free television is less demanding on the eyes and promotes more comfortable viewing. These 100 Hz televisions are not as common as 50 Hz televisions because of their higher cost. Computer monitors that work in almost the identical manner are often designed so that the screen is painted in excess of 50 times per second to produce higher resolution flicker free images. Some computer monitors allow you to change the refresh or repaint rate of the monitor to suit your own requirements up to around 100 Hz. Interestingly most video cassette machines send the signal to the television for repainting the picture only 25 times per second yet the picture is relatively flawless and appears continuous. 1 Seek out a 50 Hz television, a 100 Hz television, a television playing a video tape and a high resolution computer monitor. This may be most easily accomplished by visiting an electrical goods shop. Compare the image in terms of viewing quality. Write down your impressions as to whether you can tell the difference or see the flickering of the image. 2 Watch television until you see a shot of a working television on the screen. This often happens during the news. Observe the image of the screen of the television on your screen. Do you notice the definite flicker? Alternatively if you have access to a video camera take a video of the screen of an operating television. Replay the video though your television. You should see the image of the screen flicker. 18 From ideas to implementation

21 Why is this flickering of the television screen image visible to you much more prominently in the video footage even though if you were watching the television you wouldn't notice the flicker? Check your answer. Controlling the cathode ray beam In the module The world communicates you learned that a signal is necessary to produce a response in the electronic communication devices. When a television aerial receives a broadcast radio signal, or when you use a video cassette recorder (VCR) or digital video disc player (DVD) to play a program on your television, the signal has to contain the information to control the electron beams in the cathode ray picture tube. That is the signal needs to contain the information that tells the electronics controlling the beams of electrons hitting the phosphor screen how to behave. This is so that the image on the phosphor screen is accurate to the one the TV station, DVD or VCR sends. The TV station or VCR therefore sends a signal to the TV that contains four different parts. Intensity signals for the electron beam as it paints each line. This controls the brightness of the glow on the phosphor screen to enable shading on the image. Horizontal retrace signals the TV when to move the electron beam very quickly back at the end of each line. Vertical retrace signals to move the beam from bottom right to top left in conjunction with the horizontal. A signal indicates which of the electron beams aimed at the coloured phosphor dots on the screen should be turned on or off to illuminate phosphors necessary to make the colour required in that part of the screen. A signal that contains all of these components is called a composite video signal. Of course that signal will only give you the picture. The sound is produced by the decoding of a simultaneously transmitted FM audio signal! Note that both signals are radio frequencies. Part 2: The amazing cathode ray tube 19

22 Magnets and the screen To do the following activity you will need access to a television monitor and a magnet. Any magnet will do, even a strong fridge magnet. Take care when doing this activity. It is possible with some older televisions to permanently damage the screen. Never bring the magnet close enough to touch the screen. Do not leave the magnet in place near the screen for more than a second or so. Procedure Turn on the television. Bring the magnet close to the television screen and move it around near the screen but do not allow the magnet to come into contact with the screen. If you do it could damage your screen permanently. 1 Describe what you see happening to the image on the screen. 2 How does this suggest that the image produced on the television screen is produced as a result of a beam of electrons? Check your answers. In times past, television screens could be dramatically affected by magnetic fields from common devices such as the magnetic field associated with a vacuum cleaner motor. To eliminate this screen image problem it was often necessary to get a TV repairman to degauss the screen to restore a damaged image. Now it is common for TV screens to have a slightly greenish tinge in parts. This is often found to be due to having some source of a magnetic field too close to the screen. Possible sources of magnetic fields include external speakers or poorly designed additional electrical appliances. 20 From ideas to implementation

23 It is now common for computer monitors to have a built in adjustment that allows you to degauss the screen should it be affected by a stray magnetic field by accident. Other devices such as televisions are degaussed when you switch the television of and on. Locate a TV repairman and ask about the role of magnetic fields in producing a clear picture on the television screen. Part 2: The amazing cathode ray tube 21

24 Electron microscopes Almost everyone has looked at photographs taken by an electron microscope. Usually these photographs show exquisite detail of very small objects. What most people do not realise is that the electron microscope is really behaving as a giant cathode ray tube. The main difference between an electron microscope and a light microscope is the way that the device forms images. The electron microscope uses a stream of electrons instead of a stream of light photons to form the image. So, electron microscopes are really just scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. Electron microscopes were developed to overcome the magnification and resolution limitations of the light microscopes. Light microscopes are limited by the physics of light to 500X or 1000X magnification and have a resolution limit of 0.2 mm. The problem is that resolution is determined by the wavelength of the radiation being employed. By the early 1930s these limits had been achieved. The desire to see finer detail was expanding with biologists seeking to see the interior structures of cells and the structure of the cell membrane. To see more detail required resolution and magnification at least ten times greater than could be supplied by the light microscope. The size of electron vibrations is tiny compared to the wavelength of light. This means a greater resolving power for the electron microscope. The aim was, therefore, to build an electron microscope. Types of electron microscopes There are two basic types of electron microscopes: the transmission electron microscope (TEM) and the scanning electron microscope (SEM). 22 From ideas to implementation

25 The transmission electron microscope (TEM) was the first type of electron microscope to be developed. It was developed by Max Knoll and Ernst Ruska in Germany in Its mode of operation was patterned on the light transmission microscope you are most probably familiar with. The difference is that a focused beam of electrons was used to 'see' through the specimen rather than light. The first experimental scanning electron microscope (SEM) operated in 1942 but the first commercial instruments didn't become available until around A scanning electron microscope works in much the same way as a light reflection microscope except that instead of the image forming from reflected light the image forms from reflected electrons. All electron microscopes work in basically the same way. 1 A stream of electrons is formed by a cathode filament and accelerated toward and through an anode toward the specimen in a vacuum tube. 2 The electron stream or cathode ray is focused using metal apertures and electromagnetic lenses into a thin, focused beam. The metal apertures simply stop any stray electrons from outside the main beam interfering with the production of a clear image. The role of these devices is similar to the electron gun in a television or CRO. 3 The cathode ray beam is focused onto the sample using an electromagnetic lens. In general the beam scans across and down the specimen. 4 Interactions occur between the electrons and the specimen. These directly affect the electron beam transmission or reflection and it is this alteration in the electron beam that is detected and transformed into an image on a phosphor screen. The transmission electron microscope This type of microscope is generally used to examine biological specimens. The microscope is essentially a sealed vacuum tube. Heating a tungsten filament at voltages usually at around 500 V produces the cathode or electron source. Then the electrons are accelerated at voltages generally ranging from to V. This high voltage accelerates the electrons toward the anode with the result that the electrons pass through the specimen. For the electrons to pass through the sample it must be cut very thin. Because electron beams are invisible to the eye, the images they form are revealed on a fluorescent phosphor screen that is essentially a high resolution black and white television screen and can then be photographed. Part 2: The amazing cathode ray tube 23

26 To increase the contrast in the thin sample it is stained with electron absorbing heavy metal salts that are preferentially absorbed in some parts of the specimen. The sample is loaded into a chamber at normal air pressure. The chamber is attached to a vacuum pump and the whole system is evacuated before the electron beam can begin to look at the sample. filament acting as a cathode system under vacuum magnetic field electron beam that pases through the thin sample the person views the light image that forms on the phosphor screen light image from the phosphor screen phosphor screen electromagnets acting as a condenser lens system condenser ring aperture to collect stray electrons sample cut thinly magnetic fields refocus the beam and act as an objective lens projector lens magnetic fields electron beam hits a phosphor screen and forms a magnified image just like an image on a television screen The transmission electron microscope. The first lens largely determines the spot size the general size range of the final spot that strikes the sample. The second lens changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam. A condenser ring is used to prevent any stray electrons that are not part of the tight beam hitting the object and producing a fuzzy halo on the image. This also restricts the electron beam. The tightly focussed beam strikes the specimen. Some of the beam is directly transmitted, while some is absorbed or scattered. The transmitted portion is focused by the objective lens to form an image that is passed down the tube through intermediate and projector lenses. These lenses have the role of enlarging the image before it forms on the phosphor screen. 24 From ideas to implementation

27 At the phosphor screen the energy of the moving electrons is converted into light. This allows the user to see the image or photograph it. Darker areas of the image represent those that fewer electrons were transmitted through because they were thicker or denser. This is aided by the contrast enhancing chemicals mentioned earlier. Lighter areas of the image represent areas of the sample that more electrons were transmitted through because they were thinner or less dense or absorbed less by less of the electron absorbing salts. The scanning electron microscope This type of microscope is also a large modified cathode ray tube under near vacuum conditions. filament source of electrons beam of electrons vacuum chamber condenser lens magnetic fields focus the cathode ray beam an image of the object is produced line by line as on a normal television but faithful to the information collected from the reflected electron beam electromagnet condenser ring aperture to collect stray electrons electromagnet producing a magnetic field to deflect electron beam scans the sample that has been coated with gold information from the detector is relayed to a cathode ray tube monitor detector collects the scattered and reflected electrons reflected electrons from sample the path of the electron beam is from side to side and down the sample the gold coated sample reflects electrons from the beam to the detector Scanning electron microscope. Part 2: The amazing cathode ray tube 25

28 At the top of the tube is the electron gun producing a stream of electrons. The stream is condensed into a narrow beam by the condenser lenses that use a series of electromagnetic coils. This lens system works in conjunction with a condenser aperture to eliminate any stray electrons not focussed into the beam. The objective lens focuses the scanning beam onto the specimen. The specimen is gold coated to allow it to conduct any charge build up away rapidly. If this is not done small delicate specimens can be affected by the electrostatic repulsion from the beam. They can be blown away from the beam used to investigate them. If the specimen is valuable or unique this can be a serious problem. The specimen is usually coated with a fine coat of gold in a sputtering machine. The electron beam is set to scan across the specimen in a similar raster pattern to the pattern used in the television. When the beam strikes the sample, interactions between the electrons in the beam and sample occur and are detected. These interactions are usually reflections. Before the beam moves on, the number of interactions detected are counted and display as a pixel or dot on a CRT phosphor screen as light energy. The number and nature of electron interactions determine the intensity of a pixel on the screen. More interactions produce brighter pixels. The scanning process continues until the entire grid representing the sample is scanned. The grid scan is repeated up to 30 times per second. The entire process is outlined on the figure above. Do Exercises 2.3 to 2.6 now. 26 From ideas to implementation

29 Lightning conductors Lightning protection systems are designed to prevent damage to people and property due to a large electrical discharge of static electricity that has built up in clouds. The discharges can be of the order of A. The diagram below shows a typical lightning protection system on a domestic dwelling. lightning rods conducting copper wire grounding plates and ground termination rods metal strap A domestic dwelling with a lightning protection system installed. The main purpose of the system is to conduct the electrical charge of a lightning strike safely away from property and people along a designated path. Hopefully, the system intercepts and guides the electric current harmlessly to ground. A typical domestic lightning protection system is made up of several components: Air terminals or lightning rods: These are usually slender metal rods that are installed on the roof at regular intervals. In the past these rods were always pointed though new research suggests that blunt rods may be better suited as lightning rods. Part 2: The amazing cathode ray tube 27

30 Conductors: These are usually copper cables that connect the air terminals and the other system components. Ground terminations: These are metal rods driven into the earth to guide the lightning current harmlessly to ground where it will dissipate. These rods may be attached to a metal sheet or plate to make the dissipation of the electric current more efficient. In large buildings or installations of a commercial nature, such as communications dishes, a conductive strap of metal may be used to ensure that the current is spread evenly over a number of ground terminations. These ground terminations are often buried up to 3 m below the surface in trenches. This reduces the danger to people and equipment that may be close to the ground termination when there is a lightning strike. 28 From ideas to implementation

31 Photocopiers Most photocopiers receive their information one page at a time and print using electrostatic charges, toner and laser light. The actual way individual copiers work varies a lot. The description that follows is an outline of a possible pathway by which a photocopier may operate. Photocopying involves the application of electrostatic charges and heat to produce copies of all kinds of written, printed and graphic matter. The basis of the photocopying process is photoconductivity. Photoconductivity is the ability of certain substances to allow an electric current to flow through them when struck by light. The chemical element, selenium is a photoconductive material. It is a poor electrical conductor in low light levels but, when light energy is absorbed by some of the selenium s electrons and a voltage is applied, the conductivity of the selenium increases. This occurs because electrons are able to pass more freely from one atom to another after absorbing the light energy. When the light source is removed, the electron mobility falls and the selenium becomes a non-conductor again. A photocopier uses an aluminum drum coated with a layer of selenium. The light from the document to be copied is reflected from its surface to the selenium surface of the copier drum. Particles of toner are sprayed through a nozzle, gain a negative charge and stick to the drum in the dark areas where no reflection from the original occurred. This is the print or picture. In doing so this forms an image of the document on the drum that is reversed as when viewed in a mirror. A sheet of copy paper is passed close to the rotating drum. A positive electrically charged plate under the paper causes the negatively charged toner to move across to the copy paper. The toner is then sealed to the paper by a hot roller that fuses the toner particles to the paper. Voila, a copy! Part 2: The amazing cathode ray tube 29

32 The process of making a copy in the machine is as follows. The selenium coated drum is cleaned of any excess toner from the surface with a rubber blade as it rotates. The drum is given a negative charge of about 600 V. A light beam passes over the paper copy from one end to the other in a band. The reflection increases the conductivity in the lighted areas of the drum. This causes portions of the drum not illuminated by reflected light (dark areas of the original) to become more positively charged. Toner particles made negative so they act as point charges as they pass out of their cartridge are applied to the rotating drum and are attracted to the areas of positive charge. The negative areas of the drum that have been illuminated repel the negatively charged toner particles. The surface of the drum at this point is acting as a rotating charged plate. As the selenium coated drum rotates out of the illuminated area it loses its conductive and positive charged areas. This means the toner in contact with the drum is held only weakly to the surface because there is no longer a strong electrostatic attraction. The paper is fed by rollers under the rotating drum where a large positively charged plate attracts the negatively charged toner from the drum surface to the paper. The drum turns as the paper runs beneath it so the toner mirror image is transferred from the drum to the paper as a replica of the original. The paper runs through the fusing roller that is heated to about 200 C. This fuses the toner onto the paper. Do Exercise 2.7 now. If you have a access to a photocopier open the front cover and look at the drum, the positive plate where the paper picks up the toner, the fusing roller and the toner cartridge. Do not touch, look only, and have someone who knows about photocopiers show you the different parts. 30 From ideas to implementation

33 Summary Electrodes in the electron gun are used to: Electric fields in cathode ray tubes: The fluorescent screen in a cathode ray: The most common use for cathode ray tubes is in the: Oscilloscopes are sensitive to: Oscilloscopes have applications such as: Part 2: The amazing cathode ray tube 31

34 A photocopier works by: 32 From ideas to implementation

35 Suggested answers Cathode ray oscilloscope (CRO) 1 The electron gun produces a focused cathode ray beam that is accelerated toward the fluorescent screen using a system of electric fields. The electron beam is aimed at different sections of the screen using either electric or magnetic fields. 2 The electric cylindrical plates attract the beam toward the screen giving the beam particles kinetic energy and a rapid response time when the beam must be moved on to new spot. 3 The screen converts the energy of the electron beam into light energy that is easy to detect with the human eye. How does the CRO work? 1 The rate of movement in the x-direction is at a constant rate until it snaps back. 2 Horizontal deflection plates provided a constant electric field. When the end of the display is reached the beam must switch off and repaint the screen. 3 The movement of the electron beam is varying in the up and down direction. 4 The y-direction movement is produced by the vertical deflection plates given a varying voltage. Inside the picture tube 1 The dots are all black. By varying their density in any area an image can be built-up. 2 You will see that what appears to be a solid colour is actually made up of separate dots of colour. The dots are all one of four colours. Those colours are cyan, magenta, yellow, blue and black. Part 2: The amazing cathode ray tube 33

36 3 You should see small rectangles or circles. If the television is a coloured television you should see these dots are red, blue or green. If the television is a black and white television you should see the dots are only black or white. A clear picture The sequencing of the refresh rates on the screen are not synchronous so the image is only captured occasionally (relatively speaking) by the video tape. The result is a longer than expected blank screen. If this is long enough it appears as an annoying flicker on the taped TV monitor. Magnets and the screen 1 The image seems to distort. The screen may get a greenish tinge. The image appears to be deflected or distorted by the effect of the magnetic field. 2 Electrons are deflected by a magnetic field. The distortion of the screen image suggests that whatever is producing the image is deflected by a magnetic field. 34 From ideas to implementation

37 Exercises Part 2 Exercises 2.1. to 2.7 Name: Exercise 2.1 Part 2: The amazing cathode ray tube 35

38 Exercise 2.2 Outline the sequence of events that led to the development of the CRO with particular reference to the development of an increased understanding of cathode rays. Exercise 2.3 The cathode ray tube in an oscilloscope, television and electron microscope have a number of features in common. List the common features. Describe the common role each feature has in the operation of the device. 36 From ideas to implementation

39 Exercise 2.4 Describe how the magnetic fields used in television sets control the production of the image. Exercise 2.5 What is the role of the electron gun in a television? Exercise 2.6 An image can be created on the screen of a black and white television even though the size of the dot produced by the electron beam at any instant of time is at a maximum the size of a pin head. Explain how this is possible by referring to the characteristics of the screen and the human eye. Discuss the role of magnetic fields in making the image possible in your answer. Part 2: The amazing cathode ray tube 37

40 Exercise 2.7 The transfer of an image from the drum of a photocopier to the paper is an example of use of electrostatic charges on extremely fine powder particles of toner and charged plates. Outline the main features of that process that involve the use of electrostatics. 38 From ideas to implementation