Home PHYSICS TOPIC 4: THERMIONIC EMISSION | PHYSICS FORM 4

TOPIC 4: THERMIONIC EMISSION | PHYSICS FORM 4

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Thermionic Emission

Thermionic emission is the discharge of electrons from heated materials, widely used as a source of electrons in conventional electron tubes (e.g., television picture tubes) in the fields of electronics and communications. The phenomenon was first observed (1883) by Thomas A.
Edison as a passage of electricity from a filament to a plate of metal inside an incandescent lamp. The classical example of thermionic emission is the emission of electrons from a hot cathode into a vacuum (also known as thermal electron emission or the Edison effect) in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals tends to become significant only for temperatures over 1000 K. The science dealing with this phenomenon has been known as “thermionics,” but this name seems to be gradually falling into disuse.
Cathode
rays (also called an electron beam or e-beam) are streams of electrons
observed in vacuum tubes.Electrons were first discovered as the
constituents of cathode rays. In 1897 British physicist J. J. Thomson
showed the rays were composed of a previously unknown negatively charged
particle, which was later named the electron. Cathode ray tubes (CRTs)
use a focused beam of electrons deflected by electric or magnetic fields
to create the image in a classic television set.
The Production of Cathode Rays
Explain the production of cathode rays
Cathode
rays are so named because they are emitted by the negative electrode,
or cathode, in a vacuum tube. To release electrons into the tube, they
first must be detached from the atoms of the cathode.
Modern
vacuum tubes use thermionic emission, in which the cathode is made of a
thin wire filament which is heated by a separate electric current
passing through it. The increased random heat motion of the filament
atoms knocks electrons out of the atoms at the surface of the filament,
into the evacuated space of the tube.
Since
the electrons have a negative charge, they are repelled by the cathode
and attracted to the anode. They travel in straight lines through the
empty tube. The voltage applied between the electrodes accelerates these
low mass particles to high velocities. Cathode rays are invisible, but
their presence was first detected in early vacuum tubes when they struck
the glass wall of the tube, exciting the atoms of the glass and causing
them to emit light, a glow called fluorescence.
Researchers
noticed that objects placed in the tube in front of the cathode could
cast a shadow on the glowing wall, and realized that something must be
travelling in straight lines from the cathode.
After
the electrons reach the anode, they travel through the anode wire to
the power supply and back to the cathode, so cathode rays carry electric
current through the tube. The current in a beam of cathode rays through
a tube can be controlled by passing it through a metal screen of wires
(a grid) to which a small voltage is applied.
The
electric field of the wires deflects some of the electrons, preventing
them from reaching the anode. Thus a small voltage on the grid can be
made to control a much larger voltage on the anode. This is the
principle used in vacuum tubes to amplify electrical signals.
High
speed beams of cathode rays can also be steered and manipulated by
electric fields created by additional metal plates in the tube to which
voltage is applied, or magnetic fields created by coils of wire
(electromagnets). These are used in cathode ray tubes, found in
televisions and computer monitors, and in electron microscopes.
The Properties of Cathode Rays
State the properties of cathode rays
Properties of Cathode Rays Include:
  1. Cathode
    rays travel in straight lines. That is why, cathode rays cast shadow of
    any solid object placed in their path. The path cathode rays travel is
    not affected by the position of the anode.
  2. Cathode rays consist
    of matter particles, and posses energy by the virtue of its mass and
    velocity. Cathode rays set a paddle wheel into motion when it is placed
    in the path of these rays one the bladder of the paddle wheel.
  3. Cathode
    rays consist of negatively charged particles. When cathode rays are
    subjected to an electrical field, these get deflected towards the
    positively charge plate (Anode).We know that a positively charged body
    would attract only a negatively charged body, therefore the particles of
    cathode rays carry negative charge.Cathode rays also get deflected when
    these are subjected to a strong magnetic field.
  4. Cathode rays
    heat the object only which they fall. The cathode ray particles possess
    kinetic energy. When these particles strike an object, a part of the
    kinetic energy is transferred to the object. The causes a rise in the
    temperature of the object.
  5. Cathode rays cause green fluorescence
    on glass surface, i.e., the glass surface only which the cathode rays
    strike show a colored shine.
  6. Cathode rays can penetrate through thin metallic sheets.
  7. Cathode rays ionize the gases through which they travel.
  8. Cathode
    rays when fall only certain metals such as copper, but rays produced.
    The X-rays are not deflected by electrical or magnetic fields. X-rays
    pass through opaque materials such as black paper, but stopped by solid
    objects such as bones.
  9. Cathode rays travel with speed nearly equal to that of light.
The Application of Cathode Ray Tube
State the application of cathode ray tube
Application of cathode ray tube includes:
Televisions
Before
LCD or Plasma television, the CRT was used to create a moving image.It
used the same principle as a CRT, and for Black and White televisions,
that worked fine. B&W TVs were essentially the same thing as a CRT,
as all that’s needed is the control of the brightness of the beam.
A
CRT TV works by having the electron beam “scan” the screen at an rate
faster than our eyes can perceive.This means that it shoots across the
screen like a machine gun, and the images we see are actually made from
many fluorescent dots.
The
fluorescence caused by the beam striking the screen lasts a bit longer
so that the next scan can be made without the previous image
disappearing.It scans twice each time, first filling in the odd “holes”
then the even ones.Each scan is about 1/50 of a second.
Colour
CRT TVs had 3 electron guns rather than a single one, a shadow mask,
and a modified fluorescent screen.The 3 electron guns were needed as
there were three primary colours (Red, Green and Blue) that could be
adjusted in different amounts to create any colour.
The
colours are formed as a result of the shadow mask, which is a layer
with holes in it that controls the angle of the incoming electron beams.
This is because the fluorescent screen is separated into multi-coloured
phosphors that are placed adjacent to each other at small intervals.
Thus it isn’t actually a single coloured pixel, but rather 3 very small pixels that join together to form a larger dot.
Cathode Ray Oscilloscopes
A
Cathode Ray Oscilloscope (CRO) is a diagnostic device that allows one
to “see” voltage.It is essential a Cathode Ray Tube with two
perpendicular sets of deflecting electric plates.The vertical set is
where an input voltage is plugged in for the oscilloscope to display.
However,
the horizontal set is connected to a “sweep generator”.This is what
provides a constant, but adjustable, timebase for the sweeping.It
essentially creates a “sawtooth voltage.”This is what causes the image
to be animated, and measured with a linear scale.

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