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When applying DC to a neon lamp, only one electrode glows:

http://commons.wikimedia.org/wiki/File:Neonlamp3.JPG The voltages across the lamps are left: DC (left lead positive), middle: DC (right lead positive), and right: AC.

But... why? The electrodes are the same shape, so the electric field around them should be same shape, and the gas should break down at the same electric field strength. It seems like the fields would be symmetrical. Is there a difference in threshold between positive and negative coronas? If so, do both sides light up at high enough voltage? Or maybe only one type of corona is possible in neon since it's a noble gas? If it contained air would it glow at both electrodes?

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Quick guess: Light emission occurs once electrons are fast enough that their energy upon a collision is in the range of visible light. With DC, the electric field points in exactly one direction and hence acceleration occurs in one direction. –  Lagerbaer Aug 15 '11 at 3:42
    
is the third picture made with AC source? –  troyaner Aug 15 '11 at 13:15
    
@troyaner: yes. –  endolith Aug 15 '11 at 13:41
    
I'm with the 'charge carrier' explanation, ie its to do with the movement/KE of electrons, this matches the pictures above. –  Nic Aug 15 '11 at 14:44
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The ions are, however, way heavier than the electrons and therefore don't get accelerated that much –  Lagerbaer Aug 15 '11 at 15:35
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4 Answers

The asymmetry comes from the different masses of electrons and neon ions (neon ions are about 36000 times heavier).

This mass asymmetry results in different cross sections for the excitation of neon atoms by electrons and ions. There are some plots of this here http://webhost.ua.ac.be/plasma/pdf_papers/sab97comparison.pdf (figure 1a for electrons, figure 1b for ions). The interesting processes of excitation occur above around 10eV for electrons and 100eV for ions.

There is a very cool Java simulation of discharges: http://phet.colorado.edu/en/simulation/discharge-lamps . It covers electron excitation, the atomic structure of neon, acceleration and excitation cross-sections very well, and demonstrates why the glow can be localized (it glows at the point that electrons have been accelerated to the necessary energy), and why it is asymmetric (all electrons start from the cathode and can gain energy on the way to the anode).

I hope this answers why only one side can glow. I wish I could explain exactly why the glow is next to the cathode in the picture, but I would just be guessing.

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Yeah, the applet is backwards from reality. The closer you get to the negatively-charged cathode, the less light there is because the electrons haven't been accelerated yet. Reality: "When driven from a DC source, only the negatively charged electrode (cathode) will glow." –  endolith Aug 18 '11 at 2:39
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In the Manitoba Grade 9 curriculum handbook, the following explanation is presented: "The neon bulb emits electrons from the negative electrode which crash into the neon atoms, emitting a reddish-orange glow at the negative cathode." I am presently taking my teacher certification and it seems I am expected to let my Grade 9 students figure this out for themselves by applying the Particle Model of electricity. Does anyone find this explanation convincing?

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This is right, but what urges the electrons to leave the cathode? –  Georg Oct 29 '11 at 10:08
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There is an article at Physics of Plasmas,Phys. Plasmas 19, 072113 (2012); http://dx.doi.org/10.1063/1.4737189, which explains the phenomenon discussed here.

The physical mechanism behind neon glow is induced-dipole interaction. Unfortunately, it is too difficult to explain physics without any mathematics; and, one should refer to the article for details.The figure 3 in the article gives pictorial idea behind charged-particle oscillation.

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Answers should contain an explanation, not just a link. meta.stackoverflow.com/a/8259/130885 –  endolith Sep 4 '12 at 13:59
    
It's unfortunate that even the link is behind a paywall. –  adavid Oct 6 '12 at 23:16
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Well, I guess, in the case of AC too, the lamp is glowing in only one electrode, but since its alternating at 50Hz( depends on your geography/country) the persistence of vision is making you see both of them glow at the same time.

In DC case, the lamp is glowing because of Ne ions hitting the cathode(negative electrode). Its simply a gas discharge tube. wiki

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The cathode itself isn't glowing. The gas around it is. –  endolith Aug 17 '11 at 14:20
    
Also the Wikipedia talk page says that the ions sputtering the cathode has nothing to do with the glow. –  endolith Aug 17 '11 at 14:28
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