THERMIONIC EMISSION
Is the process of emission of electrons from a heated
metal’s surface
OR
Is the discharge of electrons from the surfaces of heated materials
Electrons are emitted from the metal surface because the thermal energy given to the carrier overcomes the binding potential.
This binding potential also known as work function of the metal
FACTORS AFFECTING THE RATE OF THERMIONIC EMISSION
- Work function of the metal
The lower the work function the greater the rate of emission of electrons from the surface.
- Temperature of the surface
The higher the temperature the high the rate of emission as the electrons will have more kinetic energy to leave the surface.
- Surface area of the metal
The Larger the surface area of the metal, the high rate of emission as thermionic emission
CATHODE RAYS
Are stream of fast – moving electrons.
OR
Are beam of electrons moving at high speed in vacuum.
The electrons move in a specific direction.
PROPERTIES OF CATHODE RAYS
- They travel in straight lines from the negative pole (cathode).
- The produce fluorescence in the glass walls of the discharge tube.
- They possess heating effect.
- They can cause ionization in gases.
- They consist of negatively charged particles.
- They produce X- rays when fall only certain metal such as copper.
- They can penetrate through thin metallic sheets.
- They are deflected when subjected to a strong magnetic field and electrical fields
CATHODE RAY TUBE
Cathode ray tube (CRT) is a vacuum tube containing an electron gun, with a source of electrons and fluorescent screen
Cathode ray tube nearly as the same as Cathode Ray Oscilloscope (CRO)
STRUCTURE OF CATHODE RAY OSCILLOSCOPE (CRO)
Uses of cathode ray tube
- As picture Tube in TV and computer monitors
- Radar System to detect enemy aircraft.
- To measure voltage or current, the drop across a resistor is measured.
- To study waveforms: A visual display is seen.
- Measurement of frequency and phase angle.
CLASS ACTIVITY
Describe applications of cathode ray tube.
Electron gun
- Filament
When a current passes through the filament, the filament becomes hot and heats up the cathode.
- Cathode
Emit electrons when it is hot
- Control Grid
Control the number of electrons reaching the fluorescent screen
Control the brightness of the spot on the screen
- Focusing Anode
To focus the electrons onto the screen
- Accelerating Anode
To accelerate the electrons to high speed
Deflecting system
- Y – Plates
To deflect the electron beam vertically
- X – Plates
To deflect the electron beam horizontally
Fluorescent screen
The screen is a glass surface coated with a fluorescent material (Zinc sulphide)
The fluorescent material converts the kinetic energy of electrons to heat and light energy when the electron collides with the screen.
Where images are displayed
APPLICATIONS OF CATHODE RAY OSCILLOSCOPE (CRO)
Cathode Ray Oscilloscope, CRO is used: - To display waveform of waves
- To measure the potential difference
- To measure short time interval
- To measure the frequency of oscillating signals
X – RAYS
X – Rays are electromagnetic waves of high energy and very short wavelength produced when fast moving electrons strikes a metal target.
X – Rays are produced when fast moving electrons strike a metal target releasing their energy which is converted to X – rays.
THE X – RAY TUBE
It consists of emitter (cathode) for emitting electrons, Anode for accelerating the electron through the tube, metal target for producing X- rays when electrons strokes to it, the battery that supplies high current for heating cathode filament made of tungsten.
External high voltage source for maintaining cathode and anode
PRODUCTION OF X – RAYS
When the cathode is heat at high temperature produces electrons.
Electrons accelerated by anode strike tungsten metal target. When electrons strike target produces X- rays.
Energy converted to X – rays are of very small percentage of their energy, with the rest grit being converted to heat.
Oil or any other cooling system can be used for cooling anode and reduce energy loss into heat to the anode.
TYPES OF X – RAYS
There two categories of X – rays depending on their wavelengths which give rise to different properties
- Hard X – rays
- Soft X – rays
HARD X – RAYS
Are X- rays of shorter wavelengths
PROPERTIES OF HARD X – RAYS
They have high energy.
They have more penetrating power.
They are produced by a high accelerating potential
They are produced by very high velocity electrons that strike the target.
SOFT X - RAYS
Are X- rays of longer wavelengths
PROPERTIES OF SOFT X – RAYS
They have weak penetrating power.
They have lower energy.
They are produced by lower accelerating potential.
They are produced by low velocity electrons that strike the target.
APPLICATIONS OF X – RAYS
- IN THE MEDICAL FIELD
X- Rays are used in hospitals to detect broken or fractured bones or some diseases in soft tissues.
They are used in the treatment of cancer by the destruction of diseased tissues.
NOTE
Soft X – rays are used in medical cases
- CRYSTALLOGRAPHY
This is the experimental study of arrangement of atoms in solids.
OR
Is the study of the arrangement of crystals.
- ASTRONOMY
X – Rays emitted from celestial objects are used in observational astronomy.
- SECURITY INSTALLATIONS
X – Rays are used for non – invasive security searches at Air pot and Seaports.
- INDUSTRIES
X – Rays are used in industries to inspect metal casting and welded joints for hidden faults.
CLASS ACTIVITY
- What are cathode rays?
- Sketch a diagram of cathode – ray – tube and label its parts and describe functions of each part.
- Describe how cathode rays are produced in a cathode ray- tube.
- Give five properties of cathode rays.
- Give three (3) uses of CRO
- With the aid of diagram explain how X – Rays are produced.
- Distinguish between hard and soft X – Rays.
- List five properties of X – Rays.
