CATHODE RAYS
AND CATHODE RAY TUBE
Introduction
Ø
When
a metal is heated to very high temperatures, electrons may be emitted from its
surface.
Ø
The
electrons are strongly held by the forces of the nuclei.
Attraction of nucleus and the outer electrons
Position
of the outer electrons
Nucleus
Ø
When
strongly heated, the force of attraction weakens and they are likely to break
loose. Using the kinetic theory of matter, the energy provided by the heat
makes the electrons to become excited and they may eventually break away from
the surface. The electrons are therefore produced through thermionic emission.
Thermionic emission
Ø
Thermionic emission -is the
production of electrons from the surface of a metal due to heat (thermal) energy.
Demonstrating thermionic emission
Ø The diagram
below can be used to demonstrate thermionic emission.
Ø The evacuated
glass bulb has a cathode made of a mixture of metal oxides (barium and
strontium oxides).
Ø Barium and
strontium oxides have low work function. Work function is the minimum energy
required to dislodge/remove or emit electrons from a metal surface.
Ø There is an
anode on the opposite side of the cathode.
Ø A low voltage
supply i.e. 6 V drives a current that in turn helps to heat the cathode.
Ø Before switching
on the heater current, no current is recorded on the milliammeter (mA).
Ø When the current
is switched on, the current is gradually increased and it is registered in the
milliammeter.
Ø The hot cathode
emits electrons. The electrons are then
attracted by the anode. This completes
the gap between both electrodes (the cathode and the anode).
Production of
cathode rays
Ø
Cathode
rays are just streams of electrons moving from the cathode to the anode.
Ø
They
are produced in a cathode ray tube.
Ø
Electrons
are produced at the cathode by thermionic emission.
Ø
They
are accelerated towards the fluorescent screen by the anode. The anode is normally connected to the
positive terminal of the extra-high tension source.
NB: the tube is evacuated to prevent the
electrons from colliding with any particles before reaching the screen. The
collision can cause loss of energy in the moving electrons.
Ø
When
the cathode rays hit the screen, it glows (becomes bright).
Properties of cathode rays
1.
They travel in straight lines----- when an
object e.g. a Maltese cross is placed on the path of the electrons/ cathode
rays between the cathode and the screen, a shadow is formed on the screen.
2.
They cause certain substances to fluoresce/ glow. ---- For
example, when an electron beam hits phosphor, e.g. zinc sulphide, it glows.
3.
They
are charged---
they are deflected by both the magnetic and electric fields. In the electric field, they are deflected
towards the positive side. On the magnetic field the direction of deflection is
determined by the Flemings left hand rule.
4.
They posses kinetic energy-----
5.
They can produce x-rays when they
are suddenly stopped by a metal target---
NB: Cathode rays are
just a stream of moving electrons and this is confirmed by the above two
properties.
CATHODE RAY OSCILLOSCOPE
Ø The cathode ray
oscilloscope is an improvement of the cathode ray tube.
Ø It consists of
the following parts
(i)
The
electron gun
(ii)
A
system of plates for deflecting the electron beam
(iii)
An
evacuated strong glass tube
(iv)
A
fluorescent screen at one end of the glass envelope.
(a)
The electron gun
Ø The purpose of
the gun is to supply electrons, accelerate them towards the screen and focus
the beam on to a point on the screen
Ø The components
of an electron gun include the heated cathode, a grid that controls the rate of
flow of electrons, and anodes to accelerate and focus the electron beam.
Ø The cathode is
coated with oxides of thorium or strontium (which have low work function).
Ø The anode
consists of cylinders and disks maintained at high positive potential relative
to the cathode. Their work is to attract
the emitted electrons and direct them on to point in the screen.
v Focusing
the beam
Ø The anodes A1
and A2 are kept at different potentials. The potential of A1
is higher than that of A2.
Ø An electric
field therefore develops between them.
Ø The direction of
the field acts such as to converge the diverging beam from the cathode as it
leaves the aperture of anode A1 as shown above.
Ø When the
potential difference between anode A1 and A2 is great,
then the electric field intensity also is high.
This means that the system will achieve high/desired degree of focusing.
v The
Grid
Ø This is a hollow
cylinder surrounding the cathode and has a small hole at the end.
Ø The cylinder is
at a small negative potential relative to the cathode.
Ø It is used t
control the intensity of the electron beam. i.e. when it is made less negative,
more electrons cross over. When it is made more negative, fewer electrons cross
over.
Ø Controlling the
intensity of the beam ultimately controls the brightness of the spot on the
screen.
v Deflection
system
Vertical deflection
Ø
When
the beam passes through uncharged horizontal plates, it strikes the screen at
A. the beam is not deflected.
Ø
When
the switch is closed, the plates become charged and the beam is attracted
towards the positive plate Y2. It is deflected to hit the screen at B.
Ø
When
the polarity of the plates is changed, the spot will shift to C through A.
Ø
With
an alternating voltage, the spot moves up and down in accordance with the instantaneous
voltage at the frequency of a.c.
Ø
If
the frequency is high enough, a vertical line is observed rather than a moving
spot. This is due to persistence of
vision.
Ø
Since
the plates cause deflection in a vertical direction, then they are referred to
as the Y-plates.
v Horizontal
deflection
Ø When the plates
are arranged as shown below, the beam of electrons is deflected horizontally.
Ø Since the
deflection is horizontal the plates are referred to as the X-plates.
Screen
Ø The screen is
coated with a fluorescent substance, (e.g. zinc sulphide). Called phosphor.
Ø Phosphor glows
on impact with electrons.
Ø The inside of
the tube is coated with graphite which has the following functions
(i)
Conduction
of electrons to earth
(ii)
Shielding
the beam from external electric field.
(iii)
Accelerating
the electron beam towards the screen because it is at the same potential
(ground) as the anode.
USES
OF C.R.O
(a) C.R.O
as a voltmeter- the time base is switched off. The x-plates
are earthed and the voltage to be measured connected across the y-plates. The vertical displacement of the spot on the
screen is measured. The voltage is then
determined using the formula
Voltage=displacement
x sensitivity (volts/ division).
-
As
a voltmeter the C.R.O is advantageous as
compared to the normal voltmeter/ other meters for the following reasons;
(i)
It
has infinite resistance and does not therefore take any current from the
circuit in which it is connected.
(ii)
It
can measure both the direct and the alternating voltages.
(iii)
It
responds instantaneously- this is in contrast to the ordinary meters whose
pointers swing momentarily about the correct reading due to inertia.
(iv)
Can
measure large voltages without getting damaged.
v
The television tube
-
The
television tube is a cathode ray tube but with the following modifications;
·
The deflecting of the spot is by magnetic coils- they are
normally positioned in pairs to effect both the vertical and horizontal
deflection of the beam.
Magnetic fields
are preferred to electric fields as they give a wider deflection of the
beam. This makes it possible to work with a wide screen with a relatively
short tube.
-
The incoming signal from the aerial is fed into the
grid.
This effects variation on the
intensity of the beam as it sweeps across the screen.
Dots of varying
brightness in successive lines build up the image on the screen.
Since this
occurs at very high frequency, there is persistence of vision which gives an
impression of a steady picture.
NB: a color
television has three electron guns. Each of the electron guns carries details
of the three primary colors (red, blue, green).
the electron gun
Television Picture
Tube
A color television
picture tube contains three electron guns, one corresponding to each of the
three primary colors of light—red, green, and blue. Electromagnets direct the
beams of electrons emerging from these guns to continuously scan the screen. As
the electrons strike red, green, and blue phosphor dots on the screen, they
make the dots glow. A screen with holes in it, called a shadow mask, ensures
that each electron beam only strikes phosphor dots of its corresponding color.
The glow of all the dots together forms the television picture.
© Microsoft
Corporation. All Rights Reserved.
The television picture tube receives video signals
from the tuner and translates the signals back into images. The images are
created by an electron gun in the back of the picture tube, which shoots a beam
of electrons toward the back of the television screen. A black-and-white
picture tube contains just one electron gun, while a color picture tube
contains three electron guns, one for each of the primary colors of light (red,
green, and blue). Part of the video signal goes to a magnetic coil that directs
the beam and makes it scan the screen in the same manner as the camera
originally scanned the scene. The rest of the signal directs the strength of
the electron beam as it strikes the screen. The screen is coated with phosphor,
a substance that glows when it is struck by electrons (see Luminescence).
The stronger the electron beam, the stronger the glow and the brighter that
section of the scene appear.
In color television, a portion of the video signal is
used to separate out the three color signals, which are then sent to their
corresponding electron beams. The screen is coated by tiny phosphor strips or
dots that are arranged in groups of three: one strip or dot that emits blue,
one that emits green, and one that emits red. Before light from each beam hits
the screen, it passes through a shadow mask located just behind the screen. The
shadow mask is a layer of opaque material that is covered with slots or holes.
It partially blocks the beam corresponding to one color and prevents it from
hitting dots of another color. As a result, the electron beam directed by
signals for the color blue can strike and light up only blue dots. The result
is similar for the beams corresponding to red and green. Images in the three
different colors are produced on the television screen. The eye automatically
combines these images to produce a single image having the entire spectrum of
colors formed by mixing the primary colors in various proportions.
Color Television
Camera
Light entering a
television camera is first separated into three primary colors—red, green, and
blue. Each color is then directed to a camera tube, where it strikes a surface
in the camera tube that is extremely sensitive to light. Using this sensitive
surface, the camera tube transforms variations in light intensity into
variations in electric charge, or current. The current from each tube is then
combined into one electric video signal, which is transmitted to home
televisions via radio or cable.
The electron gun with deflection coils. Trinitron Cathode Ray Tube
PAST REVISION
K.C.S. E QUESTIONS
TOPIC
7
CATHODE RAYS AND CATHODE RAY TUBE
PAST KCSE QUESTIONS ON THE TOPIC
1.
State
two differences between the cathode ray tube (CRT) of a T.V and the cathode ray
oscilloscope (CRO)
- The deflection of the beam in a television is done
by magnetic fields from magnetic coils while in a cathode is done by electric
field.
- The incoming signals is fed into the grid by the
aerial.
2. Distinguish between a photon and a quantum.
3. How does the energy of ultra violet light
compare to that of yellow light
given that the
energy E of a wave frequency f, is given by E = hf, where h is plank’s
constant?
4. A photon has energy of 5x10-19J.
Calculate the wavelength associated with this photon.
5. The control grid in a cathode Ray
Oscilloscope (CRO) is used to control Brightness of the beam on the screen. How
is this achieve
6. a) Figure
14 shows the features of a cathode ray tube.
i)
Name
the parts labeled A and B. (2mks)
ii) Explain how
the electrons are produced in tube. (2mks)
iii) State two
functions of the anodes. (2mks)
iv) At what part
of the cathode ray tube would the time base be connected? (1mk)
v) Why is a
vacuum created in the tube? (1mk)
b) The graph in Figure 15 was obtained
on a cathode ray oscilloscope (CRO) screen when the output of an a.c generator
was connected to the input of the CRO. The time-base calibration of the CRP was
set at 20 milliseconds per centimeter and the y- gain at 5 volts per
centimeter.
i) Determine
the pick voltage of the generator. (2mks)
ii) Determine
the frequency of the voltage. (3mks)
iii) On
the same grid, redraw the graph for the same voltage when the time base
calibration is set at 40 milliseconds per centimeter and the 7-gain at 10volts
per centimeter. (Show at least one complete cycle). (2mks)
7. Sketch the picture seen on the screen of a
cathode ray oscilloscope when the oscilloscope is adjusted so that the spot is
in the middle of the screen and the output terminals from a transformer
connected to the mains are connected across the Y-plates.
8. The diagram shows the screen of a cathode
ray tube, and behind it the position of the X and y plates which deflect the
electron beam. The beam forms a spot on the screen.
a) Draw
a labeled diagram showing a side view of the cathode ray tube.
b) How
is the brightness of the spot controlled?
c) The “X-shift” control on the front of the
cathode ray oscilloscope moves the spot sideways on the screen. What kind of
voltage direct, alternating or zero) does it apply to:
i) The X plates
ii) The Y plates
The
‘time–base’ voltage normally applied to the X-plates in a RCO varies with
time
as shown.
i) Describe
the motion of the spot when the time-base is on.
ii) Illustrate on the diagram above what is
seen on the screen if an alternating voltage is applied to the Y-plates with
the time-base on.
State two uses of the CRO.
9. The control grid in a cathode ray
oscilloscope (CRO) is used to control the brightness of the beam on the screen.
Explain how this is achieved.
10. State and explain three uses of main parts of
a CRT in an oscilloscope.
Answers
a)
CATHODE RAYS AND
CATHODE RAY TUBE
1.
1990: Photon – particle of light energy.
Quantum – Packet
of energy.
2.
Ultra
violet has a higher energy than yellow light.
3.
E
= hf = hc
λ
λ = 6.63 x 10-34
x 3 x 108
5 x 10-19
Λ = 3.978 x 10-7
4.
Low
negative voltage is applied on control grid, which controls the number of
electrons reaching the screen.
5.
1998: (i) In
TV (CRT) deflection is by magnetic field while in CRO
deflection
is by electric field
(ii) CRO forms a spot on screen, CRT forms
an image.
(iii) CRO displays waves while CRT displays
pictures.
(a) (i) A – Grid
B – Filament
(ii) Filament heats cathode electron boil
off cathode (thermionic emission)
(iii) Accelerating Focusing.
(iv) Across X-plates.
(v) To reduce collision with air molecules that could lead to
ionization.
(b) (i) Height
= 4cm
Peak
value = 4 x 5 = 20V
(ii) 2 wavelength = 16cm
T
= 8 x 20 x 10-3
=
0.16s
F
= 1 =
1 = 6.25Hz
T
0.16
(iii)
No answers for question 1& 2
6.
Low
negative voltage is applied on control grid, which controls the number of
electrons reaching the screen.
7.
(i) Electron gun – produces direct
electrons.
(ii) Deflecting system – deflecting the
beam to necessitate the study of external circuit.
(iii) Fluorescent screen – to display the
pattern being studied.
FURTHER QUESTIONS
1. Describe
how a modern color television works.
2.
How is the modern television system different from a normal CRT?
3.
What property of cathode rays shows that they are particles?
4.
Where is the time-base connected in a CRO?
5. What
is the difference between the cathode rays and the x-rays?
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