CHEM1006 Lecture Notes - Lecture 8: Cathode Ray, Thermionic Emission, Cold Cathode

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1 Aug 2016
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Lecture 8
Cathode rays are streams of electrons observed in vacuum tubes.
If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass
opposite the negative electrode is observed to glow from electrons emitted from the cathode.
Electrons were first discovered as the constituents of cathode rays.
The image in a classic television set is created by focused beam of electrons deflected by
electric or magnetic fields in cathode ray tubes.
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 must first be detached from the atoms of the cathode.
The early cold cathode vacuum tubes, called Crookes tubes, used a high electrical potential
between the anode and the cathode to ionize the residual gas in the tube.
The electric field accelerated the ions and the ions released electrons when they collided with
the cathode.
Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire
filament that 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 and 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 traveling in straight lines from the
cathode.
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