Low Pressure and high voltage in cathode rays experiment

[Misr]

Hello ,people
I don't understand what is the use of Low Pressure and high voltage in cathode rays experiment and why do beams of electrons are produced under these conditions?

Thanks in advance...

 

[sophiecentaur]

You need a low pressure because any significant number of gas molecules in the CRT will take the energy from any electrons that happen to leave the cathode. With a very low vacuum and a very hot filament, electrons which are 'boiled off' the filament will pass across the crt to a nearby Anode with a relatively low voltage. This voltage needs to be enough to overcome the attraction of the electrons back to the (net positive) charge on the cathode (which has lost electrons).
When you use a high enough voltage and a focussing electrode, you can produce an actual beam of electrons.

 

[Misr]

U mean that Low pressure was used to reduce the number of gas molecules to be more focused?hmmm still don't understand why :(

 

[sophiecentaur]

To produce electrons from a cathode you need to separate the surface electrons from the cathode. This can either be done using an extremely high voltage on a distant anode (to overcome the local attraction of the cathode protons) or by heating the cathode up, which gives the surface electrons enough kinetic energy to leave the surface. However, an isolated hot cathode will not lose its electrons because they are attracted back to it by the charge imbalance; there is a 'cloud' of electrons around it (a 'space charge'). The electrons in this cloud are fairly easily attracted away by a nearby positive charge (anode). If there is gas in the tube, the boiled-off, 'thermionic' electrons hit gas molecules and are slowed down before they get far away and fall back to the cathode so the anode voltage needs to be much higher to attract them. Hence the advantage of a deep vacuum.
I don't know what you mean by the word "focused" in this context. Nothing has been focused until you use extra magnetic of electric fields, which I am not discussing at this point.

 

[ZapperZ]

U mean that Low pressure was used to reduce the number of gas molecules to be more focused?hmmm still don't understand why :(

The higher the pressure in the tube, the more likely it is for electrons emitted from the source to collide with the gas molecules in the tube. This will prevent the electrons from being a "beam", i.e. moving without being impeded. If you do this at a high enough voltage, such collision might even cause light emission by those gas molecules. This is, after all, what is going on in your fluorescent light bulb and discharge tubes. That's not what we want in a "cathode ray" where we want a stream of electrons having the right energy.

You want high voltage because you want the electrons to quickly leave where it was created. If the electrons simply float around doing nothing, then you'll end up with a space-charge effect where they now start repelling each other and cause (i) the inhibition of further electrons from being emitted, (ii) the electrons to go off in all different directions, rather than where you want them to go.

Zz.

 

[Misr]

Yeah,got it now..Thanks very much for helping me :)

 

[Misr]

Sorry one more question
We need also low pressure to make contact with vacuum in the circuit (closing the circuit)?

 

[sophiecentaur]

Sorry, I don't understand the question. Could you rephrase it please - and more fully?

 

[Misr]

Well, simply I want to ask how do electrodes work? I discovered this problem through this discussion.
Thanks

 

[ZapperZ]

Sorry one more question
We need also low pressure to make contact with vacuum in the circuit (closing the circuit)?

Presuming that I actually understood what you are trying to say, again, I've described this in my response. If you don't have a vacuum, the electron's motion will be impeded. So your "circuit" isn't completed, especially if only a few electrons manage to go from the cathode to the anode. This is why diode tubes are typically evacuated and under (poor) vacuum.

Zz.

 

[sophiecentaur]

With a high enough voltage (electric field) you can make electrons leave a piece of metal and 'jump across' a gap. That's a high voltage spark, when it happens out in the air. You need a lower voltage if you
1. Heat a negative' electrode. (filament cathode as in valves etc)
2. Put the electrode(s) in a vacuum.
As described above.

What more do you want to know?

 

[Misr]

well u don't really understand my question...probably bcoz my English isn't that cool
anyways my question is simpler than u imagine
I mean Gases in standard conditions can't conduct electricity so right
so we also have to reduce the pressure to conduct electricity(the gasses won't be in the standard state then)
so we can consider this a reason for reducing pressure too?

another point..does electrodes connect air to a circuit?how do electrodes work?

Thats all
Thanks very much!

 

[sophiecentaur]

The electrode is the bit of metal that "connects" the circuit to the gas, if you like; electrons will find their way onto an Anode or off a Cathode- constituting a current. But a gas won't conduct a current on its own. You have to provide some separated charges for that to happen. These can come from a heated cathode or by ionising the gas. To ionise the gas, you need to cause one or two random electrons (of which there will, statistically, always be a few, under any conditions) to get fast enough to ionise further atoms and produce an avalanche effect. This can only happen if there is enough space between atoms for the electrons to gather sufficient speed between collisions. If the gas is too dense, the electrons will not have accelerated enough to cause ionisation - hence the need for a low pressure (i.e. plenty of space between atoms) for a 'spark' to occur. Otherwise, any possible spark will be quenched before it forms.

 

[Misr]

This can only happen if there is enough space between atoms for the electrons to gather sufficient speed between collisions.

Yep, briefly we have to reduce the gas pressure to conduct electricity
Thank you very much Sophie this is very helpful

 

[sophiecentaur]

Yes. Unlike a metal, which obeys Ohm's Law even at almost zero volts, a gas is a very non-linear resistance - needing a high voltage to start (lower for low pressures) and then, once it has 'struck', it conducts very well.