Why cathode rays are produced if pressure of the gas discharge tube is lowered to about $10^{-4}$ atm? How bombardment of glass of gas discharge tube from cathode ray result in a faint glow?
$\begingroup$
$\endgroup$
Add a comment
|
7 Answers
$\begingroup$
$\endgroup$
"Cathode rays" are simply beams of electrons. The name exists because cathode ray devices predate the discovery of the electron. The sharp shadows produced by Crookes tubes meant that something was travelling in a straight line from the cathode down the tube, so they were named cathode rays.
The vacuum doesn't have much to do with the production of electrons, per se, but without high vacuum, the electrons collide with gas particles before they travel very far. If there's enough gas, you get a plasma rather than a beam of electrons. The electron beam is produced by thermionic emission in hot cathodes—electrons are liberated from a hot filament when the thermal energy is enough for electrons to exceed the filament's work function. In cold cathodes, electrons are released by field emission, where a large applied electric field allows electrons to tunnel away from a metal electrode.
The glow of the glass envelope is luminescence—either fluorescence or phosphorescence (or both). Basically, the electrons striking the glass excite components of the glass to an elevated electronic state which releases light upon returning to the ground state. Normal glass isn't particularly luminescent so tubes designed to produce light (like a CRT) are coated with phosphors.
answered Oct 28, 2014 at 6:09
$\begingroup$
$\endgroup$
When a high voltage is applied to the tube the electric field accelerates the small number of electrically charged ions always present in the gas created by natural phenomena called photo-ionization and radioactivity.
These collide with other gas molecules knocking electrons off them and creating more positive ions in a chain reaction called Townsend discharge. All the positive ions are attracted to the cathode. When they strike it they knock large numbers of electrons out of the surface of the metal which in turn are repelled by the cathode and attracted to the anode. These are cathode rays.
$\begingroup$
$\endgroup$
When electric current passes through the gas inside the tube which is at low pressure, then the electric current manages to overcome the force between the electron and the nucleus and the the free electrons forms a ray which travels towards the anode in a straight line.
$\begingroup$
$\endgroup$
Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube, also called as a cathode ray tube.
Cathode rays are just a beam of electron observed in vaccum tube. when the dicharge tube or evacuated glass tube is facilitated with 2 electrode and with high voltage, due to emmission of electron from and travelling in perpendicular to cathode,glass opposite of negative electrode(ie cathode) is observed to glow.
High electrical potential is applied between anode and cathode to ionise the residual gas in the tube. the ions are accelerated by electric field and collide with cathode which released electrons. As electrons are -vely charged they are repeled by cathode and are attracted bty anode.
Crookes tube is a rarefied tube evacuated to a pressure below 10−6 atm. It was used in discovery of cathode rays.
Source Wiki
$\begingroup$
$\endgroup$
At low pressure ($10^{-2}$ atm) and higher voltage (10000 V) gases are partially ionised in the discharge tube. Positive ions of gases strike on the cathode. Due to the thermal effect, a beam of electrons releases from the surface of the cathode. This is called a cathode ray.
$\begingroup$
$\endgroup$
1
Cathode rays are originating from cathode as cathode is negatively charged. These rays then hit the gas sample inside the tube and ionize it. The electrons ejected from the ionisation of the gas travel towards anode. These rays are actually electrons produced from the ionisation of the gas inside the tube.
-
1$\begingroup$ This addresses the second question, but not the first. $\endgroup$ Jan 4, 2019 at 15:55
$\begingroup$
$\endgroup$
Cathode rays are the streams of fast moving negatively charged particles originated from the cathode.
At the pressure of 1 mm Hg in the tube, the positive column breaks into alternate and dark dics, then the cathode leaves the glow, this is cathode rays.
