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Modern Physics - Part III |
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2.
Cathode
rays Discharge
tube is a hard glass tube which is long (about 30 cm or more) and has two
electrodes attached at its two ends. The electrodes are made of any metal
which is a good conductor, such as copper, aluminum, platinum, etc. The
discharge tube has a facility to connect it to a vacuum pump. The electrodes
can be connected externally to a high voltage source. The discharge tube is
named so because of the gaseous discharge that takes place between the two
electrodes. This we shall see in the ensuing discussions. Let
the electrodes have a potential difference applied to them (about 10 kV).
The electrode connected to the negative terminal is known as the cathode and
the terminal connected to the positive terminal is known as the anode. As
the discharge tube is evacuated, measure the pressure inside in terms of
millimeters of Hg. You will notice various changes in it. The
air inside the discharge tube is a non-conductor of electricity. So
initially the tube looks intact. As the air pressure inside reduces, the gas
starts ionizing. Since a potential difference is maintained inside the tube,
when one gas atom is ionized, the electron escaping from it, ionizes other
gas atoms. There a stream of positive ions and negative electrons gets
created.
These start moving towards cathode and anode respectively. This generates a
current. When
the pressure is not very low, the gas movement looks like bluish streaks. As
the pressure reduces further, the gas inside looks pink. When the discharge
tube is evacuated to a high degree, the inside will start looking black, as
there is no gas inside to conduct any current. This dark space is called
Faradayís dark space. A small glow can be observed at the cathode and the
anode. This is due to residual gases. As
the vacuum is reduced further, there will be a greenish glow behind the
anode. The reason for this can be inferred from the direction. The rays or
particles are coming from the cathode towards the anode. Some of them
overshoot the anode and reach the inner surface of the tube. This causes the
glow. These rays are called cathode rays. Since
the cathode rays are coming towards anode, they must be negatively charged.
It has been proved that cathode rays are nothing but electrons. The
cause for the production of cathode rays can be easily explained. As the
discharge tube is evacuated, the electrons at the cathode get attracted to
the anode due to the high potential difference. Cathode rays are not seen
when the potential difference is low or if the gas pressure is high. Properties
of cathode rays 2.
Keep a strip of paper designed in the shape of a cross in the path of
the cathode rays. You will see a shadow of the design on the fluorescent
screen. This indicates that the cathode rays travel in a straight line. 3.
Make a special arrangement to keep a lightweight paddle wheel in the
path of the cathode rays. Let the paddle wheel be placed in such a manner
that only its upper portion is exposed to the cathode rays. You will notice
that the wheel will start rolling in the direction of the cathode rays, away
from the cathode itself. This shows that the cathode rays exert mechanical
force on the object in their path. This also proves that cathode rays are particulate in nature. They
are nothing but negatively charged electrons. 4.
Notice first the position of the spot on the fluorescent screen where
the cathode rays are striking. Now keep a horseshoe magnet near the
discharge tube. You will notice that the spot of the cathode rays is getting
deflected from its initial position. Change the direction of the horseshoe
magnet. You will see the spot on the fluorescent screen is getting deflected
in the opposite direction.
This clearly shows that the cathode rays are affected by the presence
of a magnetic field. 5.
An application of an electric field will show that the cathode rays
get deflected towards the positive plates. This is because the negatively
charged electrons or the cathode rays are attracted towards the positive
plate and is repelled by the negative plates. 6.
As seen before, cathode rays are capable of ionization of gas atoms
if the potential difference is large and the gas pressure is not high. 7.
Cathode rays also produce fluorescence in some materials. As they are
energetic electrons, when they strike a certain substance, the substance
starts to glow. 8.
Depending on the energy of the cathode rays, they can penetrate thin
sheets of paper or metal foils. 9.
When cathode rays are stopped they produce X-rays (see the next
section). X-rays are very small wavelength rays which find many practical
applications. 10.
Cathode rays also affect photographic plates when they strike them. Uses
of cathode rays 2.
CRT is the basic component in all television and computer screens.
The signals are sent to the vertical and horizontal deflecting plates, which
produce a pattern on the fluorescent screen. High energy cathode rays when stopped suddenly produce X-rays. The X-rays have many medical and research applications.
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