Plasma Screen TVs
Do these types of TVs produce real plasma? how do they stop it from blowing up like a fusion bomb?
- It would have to undergo the process of either nuclear fission or nuclear fusion, which would be impossible for a plasma TV, as plasma TVs don't actually use or produce any type of plasma. → JarlaxleArtemis 05:58, Jun 19, 2005 (UTC)
(??should these.......??)
(??should these broad fields be broken down here or in separate links - and if in separate links, ho so because the naming is broad and fairly arbitrary??)
(?? I'd say leave the broad picture here then tree out from here on subpages. e.g. as I did Plasma Sources. RBYII ??) --Anon
units
The equation shows 'cm' and very large powers of 10. Can somebody revise the equations to show 'm'? Bobblewik (talk) 11:23, 28 Apr 2005 (UTC)
- It's not necessary, as 1 cm = 100 m. → JarlaxleArtemis 06:00, Jun 19, 2005 (UTC)
Table
I think Iantresman's contribution (Characteristics table, plasma vs gas) is a good idea because it is not always clear why a plasma is special enough to be called a distinct phase of matter, but I am not entirely happy with the content. To begin with, it should be emphasized that all the differences mentioned (except "complex properties") are the result of a single difference, the electrical conductivity. Most of the "complex properties" arise from the presence of multiple species that are distinguished by their electrical charge (and in some cases mass). Finally, the electrical charge provides a means to influence the particle velocities directly, not just through collisions, leading to non-Maxwellian distributions and associated phenomena.
Some specific comments:
| Electrical Conductivity |
Very low
Acts as a dielectric or insulator |
Near perfect
Supports electric currents that can spark or arc; in space plasmas, they are often called Birkeland currents. Such electric currents enable energy to be transferred 'invisibly' from one plasma region to another. |
What is a Birkeland current? Currents do not necessarily transfer energy and energy transfer does not require currents.
| Duality |
None |
Yes: passive and active plasmas
As soon as an electric current passes through a passive plasma, it becomes active and exhibits more complicated features. In magnetohydrodynamics (MHD), it is practical to ignore the current (representing it as curl B); but as Nobel prize-winner for his work in MHD, Hannes Alfven, points out: "this method is unacceptable in the treatment of a number of [plasma] phenomena .. which require that the electric current is taken account of explicitly" (his emphasis). |
Is this standard terminology (in astrophysics)? "Duality" is misleading. What, exactly, was Alfvén talking about?
| Complex properties |
None |
Active plasmas may also exhibit:
- Noise (i.e., oscillations within a large frequency band)
- Non-Maxwellian energy distribution (i.e., not smooth)
- An electron temperature orders of magnitude greater than the ion temperature, which may be much higher than the neutral gas temperature
- At large current densities, may contract into filaments, sometimes called magnetic ropes or plasma cables, (as seen in lightning, aurora and nebulae)
- Can separate component gas mixtures in partially ionized gases
- Can produce double layers, the boundary between adjacent plasmas regions with different physical characteristics, that can explode, facilitate charge separation, and accelerate ions and produce synchotron radiation (such as x-rays and gamma rays)
- Can produce instabilities, which are identified by their distinct morphology (shape)
|
</table>
A smooth velocity distribution can also be non-Maxwellian. What does "separate component gas mixtures" mean? How can a double layer explode? A double layer does not facilitate charge separation, it is charge separation. What is meant by the "shape" of an instability? The Rayleigh-Taylor instability also exists in neutral gases.
There are a number of minor problems with the content and formulation as well. This table is a good start, but I think it needs a lot of work. Before I start it would help to have some answers to the questions above. Art Carlson 07:59, 2005 Jun 9 (UTC)
Ian Tresman writes:
I was a little surprised to find that someone had removed the table in its entirety, rather than simply moving it to another location.
You have some good questions, and thanks for pointing out my lack of understanding of non-Maxwellian, etc. I wonder whether some of these definitions should be answered with their own separate entries? I would certainly like to expand on Double Layers, for example. I'll add some clarification too. Ian Tresman 14:40, 2005 Jun 10 (GMT)
- I didn't remove the table, I moved it to its own section (while editing it substantially).
- Duality: In the wave-particle duality there are two equal aspects of quantum behavior that must both be taken into account to understand some phenomena. In contrast, if you have a model that describes a plasma with current, then you can use also use it to describe a plasma without current simply by setting j = 0. Besides, current is not the real issue. There are many phenomena involving currents that can be described perfectly well within MHD.
- Non-Maxwellian: "Uneven" isn't any more helpful than "not smooth". Why not just "non-Maxwellian" with a link to Maxwell-Boltzmann distribution?
- Double layers: Do you really need a current to produce a double layer? Won't you get a double layer at any boundary between two plasmas with different temperatures? A separate entry for this topic is probably a good idea.
- Instabilities: Like I said, neutral gases also show a number of instabilities (Rayleigh-Taylor instability, Kelvin-Helmholtz instability, Baroclinic Instability), so this is not a distinguishing characteristic.
- Art Carlson 13:02, 2005 Jun 11 (UTC)
Thanks for your input:
- I don't want to compare plasmas to wave-particle duality, but to the fact that plasmas exhibit a wave aspect (that can be modelled with MHD), and a particle aspect (when an electric current passes through it, and MHD fails.) Sure, the electric current can be neglected in many models (especially when dealing with MHD and waves). But as soon as you get for example filamentation and double layers, then the electric current must be taken explicitly into account.
- That sounds likes what I would call a fluid description as opposed to a kinetic description. Let's leave quantum mechanics out of it. For the third time, MHD does not automatically fail when current is present. Art Carlson 12:05, 2005 Jun 12 (UTC)
- We could link 'non-Maxwellian' to the Maxwell-Boltzmann distribution page, but it would be nice to express in a word or two, what this really means. In other words, how is it significant and what does it mean.
- True, double layers do not require a current, and are produced between any adjacent plasma regions with different properites.
- Then what makes them explode? Art Carlson 12:05, 2005 Jun 12 (UTC)
- Plasma instabilities formed when an electric current flows through them can have their own characteristics. For example, fluids can produce a Kelvin-Helmholtz instability including plasmas; but pass an electic current through a plasma, and it can produce a diocotron instability which looks remarkably like a Kelvin-Helmholtz instability, but is produced in a different way and has unique characteristics.
- Of course a plasma has different and more instabilities than a gas. But that fact would not normally be enough to classify it as a new phase of matter. Art Carlson 12:05, 2005 Jun 12 (UTC)
- If you want to have a go at tidying up the table/page, please go ahead. I can provide you with some addition information if you want to contact me by email, details at Ian Tresman 22:20, 2005 Jun 11 (GMT)
