THERMIONIC EMISSION
Thermionic emission:- Is the discharge of electrons from the surfaces of heated materials.
OR
Is the emission of electrons from the metal surface by application of heat.
Work function:- Is the minimum amount of energy required to remove electrons from a metal surface.
CATHODE RAYS:- These are streams of fast-moving electrons.
❖ The electrons move in a specific direction.
THE CATHODE-RAY TUBE (C.R.T)
• This is a vacuum tube producing cathode rays.
❖ It contains an electric gun, with the source of electrons and a fluorescent screen. These have internal or external means to accelerate and deflect electron beam that is used to form image in the form of light emitted from the fluorescent screen.
❖ The single electron beam can be processed in such a way as to display moving pictures in natural
colours.
PARTS OF CATHODE-RAY TUBE AND THEIR FUNCTIONS
1. Electron gun: Used to accelerate and focuses electrons to the screen.
2. Cathode: Used as a source of electrons (for electron emission).
3. Anode: Accelerates and focus electron ejected from cathode to the screen.
4. Horizontal deflection plates: Used to deflect electron beams horizontally (left or right)
5. Vertical deflection plates: Used to deflect electron beams vertically (up or down)
6. Fluorescent screen: Is where the image in natural colour is formed.
PRODUCTION OF CATHODE RAY
▪ Cathode rays are produced by the cathode in a cathode-ray tube after being heated.
PROPERTIES OF CATHODE RAYS:
• They travel in straight lines
• They carry negative charges.
• They have energy and momentum.
• They are deflected by electric and magnetic field.
• They produce X-Ray when stopped suddenly.
• They affect photographic plates.
• They can penetrate thin sheets of paper or metal foils depending on their energy.
• They cause fluorescence (glow) when they strike materials.
APPLICATIONS OF THE CATHODE-RAY TUBE
▪ Cathode-ray tube is applied in computer, television and cathode-ray oscilloscopes (C.R.O) to do
the following uses:-
➢ Used as a voltmeter to measure voltage.
➢ Display waveform.
➢ To measure time intervals.
➢ To measure phase relationship.
➢ Comparison of frequencies.
X-RAYS
• These are short wavelength of electromagnetic radiations.
PARTS OF X-RAY TUBE AND THEIR FUNCTIONS
1. Power supply: Used as a source of energy/ heat
2. Concave cathode: Used to focus cathode rays (electrons) to a spot on tangent.
3. Tungsten target: Used to convert some of the electrons into X-rays radiation.
4. Copper anode/rod: Used conduct heat away from the target.
PRODUCTION OF X-RAYS
▪ X-rays are produced when the cathode rays (electrons) strike/hits a solid metal target (tungsten
target) at the anode.
❖ It is only a very small percentage of their energy is converted to X-rays, with the rest of it being
transformed to heat.
TYPES OF X-RAYS
i) Hard X-rays
ii) Soft X-rays
i) HARD X-RAYS
• This is the type of x-ray which has high penetrating power.
PROPERTIES OF HARD X-RAYS
▪ Have shorter wavelength
▪ Have upper range of frequencies
▪ Have high energy
▪ More penetrating power
▪ Produced by high accelerating potential
▪ The velocity of electrons that strike the target is extremely high.
ii) SOFT X-RAYS
• This is the type of x-ray which has low penetrating power.
PROPERTIES OF SOFT X-RAYS
▪ Have longer wavelength
▪ Have lower range of frequencies
▪ Have less energy
▪ Less penetrating power
▪ Produced by lower accelerating potential
▪ The velocity of electrons that strike the target is extremely low
