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Thermionic emission: is the discharge of electrons from the surfaces of  
heated materials.  
OR  
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Thermionic emission is the process by which free electrons are emitted from  
the surface of a metal when external heat energy is applied  
N.B The rate of escaping (discharging) of electrons from a metal surface  
increases with the increase in temperature  
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How Thermionic Emissions Occur?  
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Thermionic emission occurs when kinetic energy of electrons from the surface  
of the heated metal overcome the work function of the metal  
Work Function  
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Is the minimum kinetic energy needed by an electron for it to escape  
completely from the surface of the metal  
OR  
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Is the minimum energy needed to remove an electron from a solid to a point in  
the vacuum immediately outside the solid surface.  
Types of Thermionic Emission  
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Cathode rays  
•
X- rays  
Cathode Rays  
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Are fast moving electrons emitted from cathode moving to the anode in a  
cathode ray tube  
Properties of Cathode Rays  
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They travel in straight lines.  
They carry negative charges.  
They cause fluorescence (glow) when they strike materials.  
They have energy and momentum.  
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They are deflected by electrical and magnetic fields (toward South Pole).  
Cathode rays can ionize gas atoms if the potential difference is large and the  
gas pressure is not high.  
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They can penetrate thin sheets of paper or metal foils depending on their energy.  
They affect photographic plates.  
They produce X-rays when stopped suddenly  
The Cathode-Ray Tube (CRT)  
•
Is an evacuated vacuum tube containing electron gun used to accelerate and  
deflect the electron beam(s) onto the phosphorescent screen to create the image  
OR  
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Is an electronic tube designed to display electrical data (see the fig. below)  
Why Cathode-Ray Tube is evacuated?  
•
Cathode-Ray Tube is evacuated so as to minimize air or electric resistance in  
order to make electrons travel without colliding with other particles  
OR  
The tube is evacuated so as to prevent the electrons from losing their energy  
as a result of interacting with air particles before reaching the screen  
What would happen if CRT not vacuums?  
•
If gas is maintained in the tube (at atmospheric or high pressure), the tube will  
behave like an open circuit (insulator), when p.d across it is strong enough it will  
cause an electric spark which will ionize the air and make it conduct electricity  
•
Therefore if the gas is maintained in the tube there is no production of cathode  
rays thus the fluoroscope screen will not form image  
NB:  
•
Conduction in gases and response of the tube, depend on pressure of gas  
atmospheric pressure of a gas will behave simply as an insulator  
.
At  
Components of the Cathode-Ray Tube  
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Electron Gun  
Deflection system  
Fluorescent Screen  
1. Electron Gun  
•
•
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It is used to produce electrons at a high fixed velocity.  
This is done through the process of thermionic emission  
It consists of Heater, cathode control grid, accelerating and focusing anode.  
,
(a) Heater  
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Its heating element is used to heat cathode to high temperatures from 800°C to  
several thousand degrees Celsius either directly by an electric current or indirectly  
(b) Cathode  
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This is a metal filament such as tungsten heated by electron gun resulting  
metal electrons attains enough kinetic energy than a Work Function of a metal  
and escape the cathode by thermionic emission  
(c) Control grid  
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This controls the brightness (intensity) of the beam by controlling the rate of  
flow of electrons  
(d) Accelerating anode  
•
This is a metal disk maintained at a high positive voltage of 5 000 V to 50 000  
V used to pull electrons from cathode to focusing anode  
(e) Focusing anode  
•
This is a metal disk maintained at a high positive voltage of 5 000 V to 50 000  
V used to pull electrons received from accelerating anode to deflection system  
up to fluoresce screen  
2. Deflection system  
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It is used to deflect the electron beam either vertically or horizontally  
It is used to control the image produced by controlling the position that the  
electrons hit the screen  
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It consists of horizontal (x) deflection plates and vertical (y) deflection plates  
(a) Horizontal (X) Deflection Plates  
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They are used to deflect the electron beam horizontally (left or right).  
(b) Vertical (Y) Deflection Plates  
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They are used to deflect the beam vertically (up or down)  
3. Fluorescent Screen  
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This is the display component of the CRT where image displayed.  
It is phosphor coated so that it emits light wherever the electrons strike it  
NB  
:
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The horizontal and vertical deflection plates can direct the beam towards any point  
on the screen. In some devices, the electrically charged plates are replaced by  
poles of electromagnets.  
•
The deflection plates move the electron beam to different points on the screen  
resulting in the formation of an image for a short time about (20-1 sec) or 0.05 Hz  
Operation of the Cathode-Ray Tube  
Cathode is heated indirectly by a heater (electric gun) until thermal electron emission.  
The electrons emitted are controlled by grid towards anode. After travelling through  
the hole in the anode the electrons hit the luminescent screen, causing them to slow  
down and excite the phosphor in the screen to fluorescence  
Applications of the Cathode-Ray Tube  
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computer display (Monitor)  
Televisions (TV)  
As a display device in RADAR  
cathode-ray oscilloscopes (CRO)  
Televisions (TV)  
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It may be black and white television or coloured television  
Operation of black and white television  
The signal is first amplified and then applied to the vertical deflection plates to deflect  
the beam vertically. At the same time, a voltage is applied to the horizontal deflection  
plates thus causing the beam to be deflected horizontally. The image is formed on the  
screen by varying the brightness at thousands of points on the screen. The brightness  
of a point on the screen depends on the number of electrons that strike it  
Operation of colored television  
,
The signal is first amplified and then applied to the vertical deflection plates to deflect  
the beam vertically. At the same time, a voltage is applied to the horizontal deflection  
plates thus causing the beam to be deflected horizontally. The image is formed on the  
screen (phosphors of primary colour  
:
red  
,
green and blue). The image is formed  
by varying the intensity of the electron beam that strikes the different phosphors  
NB:  
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The intensity of the electron beam can be varied by changing the  
voltage on the grid located between the cathode and the anode  
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The grid has a negative charge and so repels the electrons coming from  
the cathode  
By changing the grid’s voltage it allows more or fewer electrons to pass  
on to the anode and ultimately to the screen  
•
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Some color televisions use a single electron gun whereas others use three  
The process where plate voltage (signal) converted to image on  
fluorescent screen is called scan  
•
The horizontal and vertical motion of electrons towards fluorescent  
screen is called scanning  
The Computer Displays  
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Computer displays work in the same way as the television  
The Cathode-Ray Oscilloscope (CRO)  
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The cathode-ray oscilloscope is typically used to display signals in wave forms.  
It operates in a way similar to a television  
Operation of Cathode-Ray Oscilloscope  
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The signal is first amplified and then applied to the vertical deflection plates to deflect  
the beam vertically. At the same time, a voltage is applied to the horizontal deflection  
plates thus causing the beam to be deflected horizontally at a uniform (constant)  
rate. The signal applied to the vertical plates is thus displayed on the screen as a  
function of time. The horizontal axis serves as a uniform time scale. The screen of  
the CRO is covered with a grid to facilitate measurements.  
Uses of Cathode-Ray Oscilloscope (CRO)  
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Measuring frequencies  
Measuring Voltages  
Measuring phase differences  
Measuring small time intervals  
Comparison of frequencies  
Advantages of the C.R.O for use as a Voltmeter  
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It can measure both direct and alternating current  
It can measure very large voltages without getting damage  
It does not take any current due to its high resistance and therefore does not  
interfere with the circuit  
X-Rays  
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Are electromagnetic waves with very high frequencies and short wavelengths  
How are they produced?  
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X-rays are produced whenever fast moving electrons are sopped suddenly by  
metallic targets  
The X-Ray tube  
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Is a vacuum tube that converts electrical input power into X–Rays  
OR  
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Is a device for generating X- Rays by accelerating electrons to high energies  
and causing them to strike a metal target from which the X–Rays are emitted  
OR  
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Is the highly evacuated glass bulb contains cathode and anode made of  
platinum and tungsten or heavy metal of high melting point  
X – Rays tube consists of:-  
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Heater – produce heat.  
The cathode is concave shaped so that it can focus the electrons onto the target  
The anode is made of a good conductor of heat such as copper. This is used to  
ensure that the heat generated when electron hits the target is quickly  
conducted away from the target  
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The target is made of tungsten because tungsten has a high melting point  
therefore can withstand the heat generated without melting  
Cooling in X –ray tube is enhanced by  
(i) The cooling fins outside the tube  
(ii) The oil circulating through the channels in the copper anode  
(iii) The anode made of a good conductor of heat (copper)  
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The tube is highly evacuated so that electrons don’t collide with air particles on their  
way to the target. The air particles can cause the electrons to lose some of their  
kinetic energy  
X –ray tube is surrounded by lead shield .This is used to absorb and stray X-rays  
Alternative diagram  
How it works to produce X –rays?  
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Current flows through the filament, which then becomes hot and electrons are  
then emitted by the process of thermionic emission  
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The emitted electrons are then accelerated towards the target (anode) by the  
high potential difference between the anode and the cathode  
•
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When the electrons hit the tungsten target, they are stopped and X–rays are produced  
Only 0.5% of the kinetic energy of the electrons is converted to X –Rays. The  
rest is converted to heat  
Energy changes occurring in the X –ray tube during the production of X –rays  
• Electric energy → 푯풆풂풕 풆풏풆풓품풚 → 푲풊풏풆풕풊풄 풆풏풆풓품풚 → 푯풆풂풕 풆풏풆풓품풚 풂풏풅 푿− 풓풂풚풔  
Types of X-Rays  
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Soft X-rays  
Hard X-rays  
Soft X-Rays  
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Are the X-rays that produced by lower accelerating potential with longer  
wavelength and lower range of frequency  
•
They have less energy and less penetrating power  
Hard X-Rays  
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Are the X-ray produced by high accelerating potential with short wavelength  
and higher range of frequency.  
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They have high energy and more penetrating power  
Differences between hard and soft x-rays  
Hard x-rays  
Soft x-rays  
Produced by high accelerating potential  
Have shorter wavelength (high frequency)  
Produced by low accelerating potential  
Have longer wavelength (low frequency)  
They have higher energy  
They have less energy  
Have higher penetrating power  
Have lower penetrating power  
N.B  
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The quality (hardness or softness) of X-rays is controlled by the potential  
difference between the filament and the target. The higher the potential  
difference, the harder the X –rays produced  
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The intensity (amount) of X –rays produced is controlled by the heating current  
.The higher the heating current , the more the electrons emitted and hence the  
higher the intensity of the X-rays  
Properties of X-Rays  
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They travel in straight line at the velocity of light  
They cannot be deflected by electric or magnetic field  
They can produce fluorescence  
They affect photographic film  
They penetrate matter but depend on density of matter  
They ionize gases.  
Application (Uses) of X-Rays  
1. In the medical field soft X-rays are used  
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To detect broken or fractured bones or some disease in soft tissue  
Treatment of cancer  
To detect growth inside the body  
To detect foreign objects in the body  
2. Crystallography: Experimental study of the arrangement of atoms in  
solid (study of arrangement of crystals)  
3. Astronomy: X-rays emitted by celestial objects are used in observational astronomy  
4. X-ray microscopic analysis: involves the use of electromagnetic radiation in the  
soft X-ray band to produce image of very small objects  
5. X-rays fluorescence: technique in which X-rays are generated within a specimen and  
detected. The outgoing energy of the identified composition of the sample  
6. Security installation: They are used for non-invasive security searches at airports  
and seaports  
7. In industries  
:
(i) It is used to inspect metal-casting and welded joints for hidden faults  
(ii) They are used to sterilize surgical equipment before packing  
Effect of X – Rays to Human Beings  
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Destroy body cells and can cause cancer  
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Can cause mutation due to destroy of genitals  
Precautions  
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Exposure to X –rays should be limited to short time intervals  
Concrete walls should be used for rooms that store X- rays tubes  
X –rays tubes should be surrounded by lead shield to protect operators from  
stray X–rays  
Class Activity  
1. (a) State one way in which cathode rays differ from electromagnetic waves and  
describe an experiment which illustrates this difference  
(b) Draw a labeled diagram of a longitudinal section view of the cathode ray  
oscilloscope tube showing its main features  
5. (a) (i) What are the two types of X – rays?  
(ii) X – Rays are said to have harmful effect to human beings when used for  
a long time. Explain the effect that X – rays cause to human beings  
(b) Describe how X – rays are produced in X – ray tube  
(c) Show the three main parts of cathode ray oscilloscope on a well labeled diagram  
6.  
7.  
(a) Write two properties of  
(i) X rays  
(ii) cathode rays  
(b)(i) Give any four uses of cathode ray oscilloscope (CRO)  
(ii) State two ways in which x – rays differ from gamma rays  
(a) Explain briefly the following  
(i) Thermionic emission  
(ii) The production of a stream of electrons in cathode ray oscilloscope (C.R.T)  
(b) What method in a device using the thermionic emission principle ensures  
that the electrons produced  
(i) Do not accumulate at the source?  
(ii) Reach their range undeviated?  
(iii) Travel without meet other forms of particles on their way to the target?  
8. (a) (i) Explain why cathode ray tube (CRT) are evacuated  
(ii) What happens to the CRT when a gas is maintained?  
(iii) If gas is maintained in a CRT, will the image be formed onto the screen? Explain  
(b) In the production of X – rays what are roles of:  
(i) Low voltage  
(ii) High voltage?  
(iii) Tungsten target?  
(c) How is hard X – rays produced?  
9. (a) (i) Define thermionic emission  
(ii) What is X – rays?  
(iii) Mention two uses of X – rays  
(b) With the aid of a diagram, explain how X – rays are produced  
(c) Draw a well labeled diagram of a cathode ray oscilloscope.  
10. State one property of X –rays which makes it possible to detect fractured bones  
11. The penetrating power of x –rays is normally varied depending on the intended  
use. Explain briefly how this is done  
12. X –rays are passed through the air surrounding a charged electroscope. State  
what is observed.  
13. How can the intensity of X-rays in an X –ray tube be increased  
14. The figure below shows a circuit of a special type of a cathode ray tube  
(a) Name the parts labeled A and B  
(ANS: A = Cathode B = Grid  
)
(b)  
C
is a metal can mounted inside the tube and is connected externally to a  
negatively charged electroscope with its casing earthed. The p.d across  
the metal L and M was then adjusted so that the cathode rays were  
deflected into the can and it was observed that the electroscope leaf rose  
steadily  
(i) Why did the leaf rise steadily? (ANS: The leaf rose steadily due to  
the extra accumulation of negative charges. Like charges repel  
)
(ii) What does the result in (ii) above tell you about the charge on the  
cathode rays (ANS: cathode rays are negatively charged)  
(iii) State one other property of cathode rays not mentioned above  
15. (i) What is thermionic emission?  
(ii) Name two factors on which the rate of emission depends.  
ANS.  
(i) Thermionic emission: The phenomenon due to which free surface of the  
metal emits electrons, on being heated  
(ii) Factors determining the rate of emission of thermion:  
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It is inversely proportional to the work function of a material.  
It is directly proportional to the temperature of the surface emitting thermions.  
It is directly proportional to the surface area of the surface emitting thermions.  
16. The figure below shows an X –rays tube  
(a) Name the parts labeled A,B and C(ANS: A =cooling fins B=metal target C= Filament cathode)  
(b) Explain how X –Rays are produced in a tube  
(c) Why it is necessary to use an evacuated tube? (ANS: In order to minimize  
the chance of electrons colliding with air molecules)  
(d) What are the purposes of high and low voltages.  
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Low voltage heats up the cathode filament to emit electrons  
High voltage accelerates electrons towards the metal target  
(e) With reason, state the most appropriate metal to be used to make part B  
(ANS: tungsten/molybdenum, It has a high melting point)  
17. In the production of X – Rays what are the roles of  
(i)  
Low voltage?  
(ii) High voltage?  
(iii) Tungsten target?  
ANS:  
(i) Used for heating the cathode  
(ii) Used to provide a high p.d between the electrodes, for accelerating the electrons  
(iii) Used for absorbing highly energetic electrons and emit X – Rays.(Converting K.E  
of electrons into electromagnetic waves and heat)