# why can there be no arcing in a perfect vacuum?

by iScience
Tags: arcing, perfect, vacuum
 P: 214 i understand why there can be no breakdown, however "arcing" is just the transfer of electrons. a breakdown in air just provides a path for the electrons to take. But if you have two sharp surfaces, like a needle, pointing towards each other, separated by some distance d, let's even say that d is small, and if you apply a high enough voltage between the two needles, what is preventing the electrons on the anode surface from getting sapped off to the positively charged needle? why is air needed for this?
 Mentor P: 9,630 You don't need air to get a current flow. Without air, a few electrons will just flow from one side to the other - they cannot ionize more particles on their way. If the energy is not sufficient to kick out atoms on the other side, you don't get an arc, you just get a current flow.
 P: 1,114 In air you can see the ionized atoms. In a vacuum, there are no atoms or very few to ionize so you do not see the path of the electrons. Vacuum tubes operate with a voltage difference between the anode and cathode and work just fine in a vacuum, demonstrating that there is electron flow. Your CRT, a large vacuum tube, of which there is less and less around, operate also on the principle of electron flow within an evacuated chamber. How else would there be anything to see on the display.
P: 214

## why can there be no arcing in a perfect vacuum?

 If the energy is not sufficient to kick out atoms on the other side, you don't get an arc, you just get a current flow.
you mean the electrons of the material on the anode side?

 How else would there be anything to see on the display.
well i thought it worked on the same principle as a crooke's tube; ie electron guns (anode) ejecting electrons to some phosphor screen or .. something like that. but if the electrons are not visible, ie if there is no light emitting beam as seen in a crooke's tube, how can there be an image from invisible rays? this would imply some excitation mechanism on the recieving cathode end however to my knowledge this is not the case. To my knowledge there is the red electron beam, the green electron beam, and the blue electron beam. and the screen that is being shot at selects the percentages of each color. is this incorrect?

also, i've seen the electron supplier being called the cathode before, and i've seen it being called the anode before, which is it?..
 Mentor P: 27,565 An "anode" is usually a surface that has a higher potential than the "cathode". This means that either the anode is at a high positive potential, while the cathode is grounded, or the cathode is at a high negative potential while the anode (or the rest of the system) is grounded. The latter is a common configuration in accelerator facilities that produce or study photocathodes quantum efficiency. To produce an "arc", which is a flash of light that is a result of vacuum breakdown, there has to be some gas specie in the vessel that are excited and/or ionized. Even in a "perfect vacuum", one can still have this. The field-emission electrons that were emitted from these sharp protrusions actually can be accelerated to rather high energies due to the external field. When they hit the surface of the anode, they can cause an increase in the temperature of the surface, thus liberating atoms/molecules into the vacuum. These are the gas specie that can be ionized by the bombarding electrons. A detailed mechanism for this can be found in an extensive review of this process written by Fred Schwirzke's in "The Physics of Vacuum Breakdown" (IEEE, Plasma Science, 1993). Zz.
P: 214
 An "anode" is usually a surface that has a higher potential than the "cathode". This means that either the anode is at a high positive potential, while the cathode is grounded, or the cathode is at a high negative potential while the anode (or the rest of the system) is grounded. The latter is a common configuration in accelerator facilities that produce or study photocathodes quantum efficiency.
but i don't understand, this could be self contradictory then, if i call the electron source (high negative potential) the anode.. i can call the other side the anode as well because it will be the high positive potential.

 To produce an "arc", which is a flash of light that is a result of vacuum breakdown, there has to be some gas specie in the vessel that are excited and/or ionized. Even in a "perfect vacuum", one can still have this. The field-emission electrons that were emitted from these sharp protrusions actually can be accelerated to rather high energies due to the external field. When they hit the surface of the anode, they can cause an increase in the temperature of the surface, thus liberating atoms/molecules into the vacuum. These are the gas specie that can be ionized by the bombarding electrons.
a perfect vacuum is one that has no matter in it.. and that do you mean by "field-emission electron emitted"? can you please reword?
Mentor
P: 9,630
 Quote by iScience if i call the electron source (high negative potential) the anode
Why would you do that? It is called cathode.
PF Patron
P: 35
 To my knowledge there is the red electron beam, the green electron beam, and the blue electron beam
iScience, I think the How TV Works article in the electronics section of the howstuffworks web page may be worth a look before the thread continues. Maybe it's just me, but the thread is becoming unclear.
PF Patron
P: 10,076
 Quote by iScience To my knowledge there is the red electron beam, the green electron beam, and the blue electron beam. and the screen that is being shot at selects the percentages of each color. is this incorrect? also, i've seen the electron supplier being called the cathode before, and i've seen it being called the anode before, which is it?..
Coloured electrons??? That would be very exotic. It's the phosphors they strike that govern the colour in a crt TV picture
A Cathode, by definition, is always a source of negative charges and an Anode is always a source of positive charges. If you have read anything other than that then you should suspect the source - or perhaps you should read it again more carefully.
PF Patron
P: 10,076
 Quote by ChristinaJ iScience, I think the How TV Works article in the electronics section of the howstuffworks web page may be worth a look before the thread continues. Maybe it's just me, but the thread is becoming unclear.
Ha. Lets drag it back to clarity then.
P: 214
 Quote by sophiecentaur A Cathode, by definition, is always a source of negative charges and an Anode is always a source of positive charges. If you have read anything other than that then you should suspect the source.
are you meaning to say that i should be suspicious of the source? or that the cathode is always the "source" of whatever context? ie, source of holes, source of electrons?
PF Patron
P: 35
 Quote by iScience are you meaning to say that i should be suspicious of the source?
You should be suspicious of the source of information, not the source of electrons.

Also, I don't think you're going to get too far with this thread without some reading. Please take note of ZapperZ's citation and google cathode ray tube explanations, alternately you may watch a video here http://highered.mcgraw-hill.com/olcw...e%20Ray%20Tube and always double check information that is not peer reviewed.
PF Patron
P: 10,076
 Quote by iScience are you meaning to say that i should be suspicious of the source? or that the cathode is always the "source" of whatever context? ie, source of holes, source of electrons?
Did you read my post? Is a Hole a negative ion? Or were you just being saucy?
Mentor
P: 27,565
 Quote by iScience are you meaning to say that i should be suspicious of the source? or that the cathode is always the "source" of whatever context? ie, source of holes, source of electrons?
If there is EVER a poster child to strengthen our constant insistence on citing one's source, this thread is it!

http://www.physicsforums.com/blog.php?b=2703

This could have easily been handled in the first few posts had the source of this mistake been cited (i.e. electrons coming out of anodes??!!!). Instead, we are now dealing with terminology (is the cathode an electron source, or is the anode an electron source) while the physics of vacuum breakdown is buried DEEEP down.

iScience: You could have easily revealed your source and we could have tackled it early on. Instead, you chose to simply counter it simply via unverified contradictions without a single valid citation.

This thread appears to have been answered (i.e. the original question, not the mistaken understanding of what one call an electron source). Thus, it is done.

And take note: I want to hear NO MORE COMPLAINTS the next time I ask someone to provide a valid reference as a source!

Zz.

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