Discharge tube? NEED high voltage and low pressure why? Im a student of class 11th and right now my schools teaching atomic structure , can anybody tell me that why do we need high voltage and low pressure in discharge tube in cathode ray experiment?
 A: To achieve electric discharge in a gas, you need electrons to accelerate in electric field and achieve kinetic energy high enough to ionize atoms. To this end, you need high electric field and long time between collisions, therefore, high voltage and low density.
A: If the cathode ray experiment has a beam  of electrons hitting a fluorescent screen which glows then:  


*

*You need the low pressure in the tube so that the passage of the beam
of electrons is not impeded very much by collision with air molecules.

*You need a reasonably high accelerating voltage to give the electrons
enough kinetic energy to make the fluorescent screen glow when the electrons hit the screen.


If the cathode ray experiment uses a discharge tube which emits light then the electrons must be accelerated enough (obtain enough kinetic energy) between collision with the gas molecules in the tube to excite/ionise the gas molecules on collision with them.
So a low pressure ensures that the distance between collision is sufficiently large to enable the electron to cause excitation/ionisation of the gas molecules.
On the other hand if there are too few gas molecules the intensity of the emitted would be too small to observe the emitted light.
This means that the gas pressure must not to be too low thus decreasing the distance the electrons travel between collisions with the gas molecules.
Having a high voltage across the tube means that an electron can gain more kinetic energy between collisions with the air molecules and thus a smaller distance between gas molecules (not quite so low a pressure) can be used.
So there is a compromise between the values of voltage and pressure.  
Modern cathode ray tubes use a hot cathode source whereas the older types use a cold cathode which generally require higher voltages to operate.
However, such tubes must not be used with too high a voltage because if they are the electrons on impact with the glass / fluorescent screen would produce a potential health hazard - X-rays.
A: Why high voltage?
To break the molecules of the gas into atoms and to remove the electrons from the outermost orbitals.
Why low pressure?
To allow the rays to move freely from one electrode to another and the possibility of collisions between rays and moleculess are minized.
A: In a closed surface having gas (molecules are easily moves in gas) if we increase the pressure then at some point the gas turns to liquid (molecules come together) and if increase the pressure much more the gas turns into solid ( the molecules come together much more and cant move, they just vibrate ).
So if the pressure of gas is less then the molecules in the discharge tube are easy to move i.e electrons move very easily when we apply electric field.
A: Be decreasing pressure mean we decrease number of molecule and discharge tube and then we applay a high voltage that the electron of the molecules get kinetic energy and get removed from the molecule
A: A very low pressure of about 0.01-0.001mmHg and a very high voltage of 10000-15000V is used to overcome high resistance of air and this allows electric discharges to go through air inside the Crookes tube
