Light emission as wire rope breaks

  • #1
I knew the one inch wire rope was under severe strain, that's why I was watching it when it parted (broke). First, a few of the individual steel wires broke with some small time interval between each break. Then more wires broke at a faster rate until the remainder broke all at once. It was possible to see when each wire broke because it was dark and as each wire parted a flash of neon blue light was emitted at the break. I have some fuzzy idea of how electrons/photons might be "shaken loose" as a wire was stretched to its breaking point, but that's not my question. The question is: Why blue light? It has been suggested to me that this was because the emitted radiation was traveling faster than the local speed of light through a transparent medium (air) ie it was Cherenkov radiation. Is this a realistic explanation?
 

Answers and Replies

  • #2
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That's a very interesting phenomenon. I have never seen nor heard of anything like that. I guess what could be happening is the crystalline structure change, and that would release some energy.

Now if somebody knows (I certainly do not) the base energy levels of iron's atoms in all its crystalline forms, we could then check whether any transition would have energy corresponding to blue light or greater.
 
  • #5
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Having not seen what you describe, I can only make an educated guess. Just before wire yields, it pinches. If there is some way for the wire to gain an electric charge, the field at the point of yield becomes large and you will get a corona. The corona will look like you describe, and is unlikely to last very long.
 
  • #6
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If you take an asperin tablet and snap it in two inside a dark room you get a similar result.
You see a blue spark it has something to do with the piezo electrical effect though I ain't sure it's quite the same phenomenom.
 
  • #7
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Probably quiet similar to the effect of producing x-rays with tape.
http://www.nature.com/news/2008/081022/full/news.2008.1185.html
Check out the video link there.

Of course that requires a vacuum to get sufficient charge separation before discharge. Basically when you stress the wire it resist because of the bound electrons. So the stress actually separates the bound electrons slightly and stores that stretching energy in the band gap. Then when it breaks the electrons accelerate back into a ground state releasing that energy as electromagnetic radiation, light.

Note that this is an effect of accelerating electrons whether the acceleration is caused by changing atomic orbitals or merely some free electrons jumping some band gap.
 
  • #8
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When metal is machined, heavy currents flow as the 'electron sea' is split. What you see is an electrical spark when the strands part. Sparks in air are generally blue. Might be yellow in a sodium rich atmosphere, which would be an interesting experiment. Suffice to say, if you place a scope probe across the pending break, make sure it is HV rated, or use a ballast resistor in shunt.
 
  • #9
Thank you all for your responses. What none of you have been unkind enough to say is "No dummy, it ain't got nothin' to do with Cherenkov radiation." Now that I think on it the light emited by some candies, when broken, is blue. Buckleymanor says aspirin also sparks blue. DarioC says that black friction (electrical?) tape gives off blue light when separated. My take is that these emissions from different materials are not completely understood. The question that remains is : Why blue? Why not longer wavelengths instead of shorter?
 
  • #10
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There's a fairly straight forward explanation of why blue light is so ubiquitous in so many materials stressed this way. When an electron jumps a gap, to equalize charges, the wavelength generated depends on the energy required to jump this gap. That is turn is determined by the dielectric constant of the medium the electrons must jump through, in this case air. Hence, the one thing all these blue light producing materials have in common is the dielectric constant of the air in the gap the electrons are crossing.

This also explains why in order to get x-rays instead of blue light from scotch tape you have to put it in a vacuum. This effectively reduces the dielectric constant such that the energy required to jump the gap is greater. Hence you get much higher energy x-rays, instead of blue light.
 

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