Why do cathode ray tubes have to contain a gas at LOW PRESSURE?

[mahela007]

Why should the gas inside a cathode ray tube be a low pressure in order for the current to flow? (i.e the cathode rays)

Why don't the rays appear when there is a gas at normal pressure inside the tube?

[insane_alien]

because if the gas was at normal pressure there would be too much gas in the way and it would stop being a beam.

 

the electrons in the beam can be deflected if they hit molecules of gas. if too many of them are deflected either none will rech the target or the spot on the target will be very diffuse.

 

lowering the pressure means that there are less molecules to collide with so everything works nicely.

 

an analogy would be if you had some flour in your hand and you blew on it. due to the atmosphere it forms a billowy cloud that doesn't really go very far. but if you did in a vacuum (well, used a can of air rather than blew on it yourself due to the low pressure) the flour would probably go clear across the room.

[mahela007]

Thanks for your reply.. great analogy by the way.

[anish114]

can u tell me xactly in simple words dat why do we always require low pressure in the crt??

also

plz ans my some questions more

 

what is known as discharge in discharge tube ?

 

why do we require high voltage in the crt ?

[mahela007]

well, I just leaned the answer on this thread so I'll try to explain the answer to your first question.

If there were many, many air molecules (normal or high pressure) inside the tube, then the electrons would simple smash into them and the electrons would not be able to travel easily to the other end of the tube.

Imagine that you are an electron and you are driving in your car on a road... if there was a traffic jam (the cars jamed together would be like air molecules at high pressure) you wouldn't be able to find a way through to keep travelling.You would constantly be bumping into other cars. (OOPS!) If the other cars were magically sucked out into space (Just like the air molecules are sucked out to create low pressure) then you'd have a nice, fast, pleasant drive..

Hope that helps

 

(To [insane_alien] ...)

(Was my analogy correct?)

[insane_alien]

yeah, thats a good analogy. although the ar molecules aren't as ordered as a traffic jam.

 

perhaps if you relplace the traffic jam with a crowd you're trying to run through it would be a bit closer.

 

then again, its an analogy isn't really meant to be an accurate representation.

[swansont]

yeah, thats a good analogy. although the ar molecules aren't as ordered as a traffic jam.

 

perhaps if you relplace the traffic jam with a crowd you're trying to run through it would be a bit closer.

 

then again, its an analogy isn't really meant to be an accurate representation.

 

Or running through a forest, blindfolded. The concept here is called the mean free path — how far can you go before you hit something. Target density (which depends on pressure) is one of the factors.

[parasar6942]

what actually happens is that when the pressure is normal very weak current which can just be measured through quadrant electrometer is there...

 

when applied pressure is lowered then no. of ions remains same but as pressure is decresed the electrons present in anions leave the ion and start moving freely and thus then electricity is conducted with the help of electron and as the mass of electron is less than any ion so it will more efficiently and thus current will be more......

 

rest concept depends on current as u ppl knows.......

Edited June 13, 2017 by parasar6942

[Carrock]

plz ans my some questions more

 

what is known as discharge in discharge tube ?

 

why do we require high voltage in the crt ?

If the pressure is high, most of the (small number) of free electrons are slowed by collisions and never acquire enough energy to ionise other atoms and create more free electrons.

 

The higher the voltage (i.e. volts/meter) the higher the pressure can be for a discharge.

 

At lower pressure, there are fewer collisions and electrons gain enough energy from the electric field to ionise some of the atoms they do hit.

 

If you slowly increase the voltage, eventually there is a chain reaction with a sudden large increase in conductivity, loosely called a discharge.

Edited June 14, 2017 by Carrock

[studiot]

Didn't cathode ray tubes go out of date shortly after 2009?

 

[pzkpfw]

Didn't cathode ray tubes go out of date shortly after 2009?

 

 

 

As I read this, I have an old 12" colour CRT sitting in front of me, on my desk at work (waiting for me to take it home). It's a Taxan Vision EX. (Apparently the Sinclair QL used a re-badged version of it).

 

Bought it off an auction website; from a person who coincidentally turned out to work for the same company as me, different floor in the same building.

 

Admittedly, I got it for running old computers (it has a composite input, and a TTL RGB input).

 

On the same auction site I currently have a bid on a BBC Master 128. At home I have a TRS-80 Model I whose monitor vanished long ago.

Edited June 14, 2017 by pzkpfw

[MigL]

What are you starting, a museum ?

I used a TRS-80 model 1 ( with 16k of memory ! ) as a controller for my undergrad thesis.

And built a ZX-81 kit for my first computer.

 

They almost seem more quaint than vacuum tubes/crts.

Edited June 15, 2017 by MigL

[Mordred]

Lol try building a 3d engine to run a set of distancing lasers to map a room in 3d on a 8085 Intel PC in hexadecimal. (As the interface was strictly hexadecimal).

 

Each set of trig calcs took the poor thing 45 minutes.

Edited June 15, 2017 by Mordred

[studiot]

Lol indeed

 

You youngsters have it so easy these days.

 

When I did my thesis ( A complete hydrographic survey of Plymouth Sound), just after this thread was started,

 

The most common computer commetn was

 

!Runtime Error.in line 35

 

We did our computing by manually puching a set of cards, putting an elastic band arond them an puttinf them into the 'in' wire basket in the computer dept hatch.

 

Some days later (next day if we were very lucky) the cards would beappear in the 'out', folded in the output printout on that famous 'music paper..

 

It took me a smester to design and program a least squares analysis of the survey control net.

Lol try building a 3d engine to run a set of distancing lasers to map a room in 3d on a 8085 Intel PC in hexadecimal. (As the interface was strictly hexadecimal).

Each set of trig calcs took the poor thing 45 minutes.

[Mordred]

Surprisingly enough I still often use hex directly for programming for microchips. Just now I can cheat more such as with the basic microchips. (though their basic compiler leaves a lot to be desired). Their chips are great for hobby projects.

[studiot]

Of course the interesting thing is that before the PC came along (monochrome version) we had the Compucolor II with graphics, inbuilt drive, and an inbuilt cathode ray tube that was a pig to align properly oh and I nearly forgot , virus immunity.

 

That was built on an 8080 processor.

[Bender]

If the pressure is high, most of the (small number) of free electrons are slowed by collisions and never acquire enough energy to ionise other atoms and create more free electrons.

 

The higher the voltage (i.e. volts/meter) the higher the pressure can be for a discharge.

 

At lower pressure, there are fewer collisions and electrons gain enough energy from the electric field to ionise some of the atoms they do hit.

 

If you slowly increase the voltage, eventually there is a chain reaction with a sudden large increase in conductivity, loosely called a discharge.

I don't think there is a discharge at low pressures. The voltage needs to be high to give the electrons enough energy to leave the metal, after that, they become free electrons and fly through the (for practical purposes) near vacuum.

[Maheen]

Why light emitt at low pressur but ceased at further low pressure