CATHODE RAYS.

Specific objectives.

1. Describe production of cathode rays.
2. State the properties of cathode rays.
3. Explain the functioning of cathode ray oscilloscope and of a TV tube.
4. Explain the uses of cathode ray oscilloscope.
5. Solve problems involving cathode rays oscilloscope.

Production of electrons.

• Electrons may be removed from a surface by;

1. Heating the metal or thermionic emission.
2. Exposing the metal to a certain radiation called photoelectric effect.

• Apply an electric field or high p.d.

Thermionic emission.

• Is the release of electrons from a metal or a metal coated with a certain oxide when heated. The electrons emitted are called thermal electrons.

                                    Production of cathode rays.

Cathode rays are streams of electrons moving from the cathode to the anode.

Are produced by thermionic emission when a metal tungsten is heated strongly in a cathode ray tube. The tube is evacuated to avoid electrons from interacting with any particle before they reach the screen, that is, they prevent electrons from losing kinetic energy. The electrons emitted by the cathode are accelerated towards the screen by an anode voltage which is at a very high positive voltage, that is, the anode is connected to extra high tension supply.

                                    Properties of cathode rays.

1. They travel in a straight line.
2. They cause fluorescence in several substances
3. They are charged negatively hence can be affected by magnetic field towards north pole.
4. They are deflected by electric field towards the positive plate.
5. They possess kinetic energy and therefore have mass (9.1 X 10^-31 kg).
6. They produce x- rays when they strike matter.
7. They have high penetrating power and penetrate a thin piece of metal e.g. aluminum.
8. They cause heating effect.

Cathode ray oscilloscope.

• Is a qualification of cathode ray tube.
• It consists of.

1. Electron gun.
2. Deflecting plates.
3. The screen.
4. The tube.

1. Electron gun.

• The electron gun consists of;

1. Tungsten filament which is the cathode and coated with metal oxide. The tungsten produces a lot of electrons by thermionic emission. It is made up with respect to anode.
2. The grid. The grid controls the number of electrons in the beam and hence the brightness of the screen. When it is more negative, it repels some of the electrons back to the cathode and reduces the brightness of the spot. It is connected to the negative side of the source. To have a brighter spot, the grid is made less negative.
3. The anode. It is a metal with a hole in it. It is connected to the positive side of the source and has a higher voltage (3- 5 KV) above the cathode. This high voltage attracts and accelerates the electrons to a high speed and pass through the hole and the beam of electrons is obtained. It is used to focus the theme on the screen.

Deflecting plates.

• There are two sets of plate;

1. Horizontal plates. They deflect the beam of electrons up and down the vertical axis.
2. Vertical plates. They deflect the beam of electrons sideways.
3. The deflection of both plates causes the beam to reach every part of the screen.

The screen.

1. It is coated with phosphor which is a fluorescent material. When the electron beam strikes the screen at a spot. The phosphor coating converts the kinetic energy of the high speed electrons into light energy. This enables us to see a bright spot on the screen.

The tube.

• The inside of the tube is coated with graphite. The graphite has three functions.

1. Conduct stray electrons to the earth.
2. Shield the beam of electrons from external electric field.
3. Accelerates the electrons towards the screen as it is at the same p.d as the earth.

Uses of cathode rays.

1. Timing device in the radar system. A pulse is first displayed on the screen at the distance it is emitted by a radar system. It is then sent into the space. It is again displayed after a reflection by an object in the earth’s atmosphere or in the outer space. The distance between the two pulses on the screen gives the time taken for the return journey of the pulse.
2. Television tube.
3. To display wave forms.
4. As a voltmeter to measure A.C current and voltage.
5. To compare frequencies.

C.R.O as a voltmeter.

Advantages.

1. Has infinite resistance and does not therefore take in current hence no interference with the circuit in which it is connected.
2. Can measure both d.c and a.c voltages.
3. Can measure large voltages without getting damaged.
4. It responds instantaneously unlike ordinary voltmeter whose pointers swing momentarily about the correct reading due to inertia.

C.R.O as a TV tube.

• The TV converts the video signals into pictures and also radio signals into sound.
• The signals are received by the TV aerials demodulated and amplified. These signals control the electron gun of the TV tube.
• The TV tube has two biases;

1. The vertical bias that pull the spot down the spring and return it to the top.
2. Thee horizontal bias that sweep the spot across the screen.

• This causes the electron beam to draw lines on the screen. 625 lines are made per second. This process is called scanning.
• The beam repeats the scanning on the screen about 25 times in a second. The beam is made to vary according to the video signal. It produces light of varying intensity on the TV screen. This produces an image similar to the image on the studio or video camera.
• The incoming signals are applied to the grid of the tube which controls the brightness of the spot. The different light and dark spot combine to form a picture. 25 pictures are formed in a second giving the effect of a movie.
• The deflection of a beam in a TV is done by the magnetic field. This field gives a wider deflection of the beam and make it possible to use a wide screen with a relatively short tube.

                                                Color TV.

Has 3 electron gun besides the main electron gun. The screen is coated with a white pigment plus about 1 million tiny light emitting dots of red green and blue fluorescent pigments. Each electron gun emits electron which will strike specific dots. When electrons strike these dots they each emit one particular color (red, green and blue) which when mixed produce a secondary color. The process is so quick that we see one continuous picture.