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electrical arc colors, what causes different colors?

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PresentTeck

Wed Oct 05 2011, 02:12AM

Registered Member #3922 Joined: Thu Jun 02 2011, 06:24AM
Location:
Posts: 23

any idea where i could find any info pertaining to this?

Patrick

Wed Oct 05 2011, 02:13AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ill ask my college physics professors tomorrow.

Are you in high school?

PresentTeck

Wed Oct 05 2011, 02:38AM

Registered Member #3922 Joined: Thu Jun 02 2011, 06:24AM
Location:
Posts: 23

yes, senior year
but i've been doing this kind of stuff since freshman year.
I made a device that can turn wood into simple hydrocarbons that you can run a car off of, and a couple of other fun things :)

Turkey9

Wed Oct 05 2011, 03:13AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I think that it has to do with the heat in the arc, not the voltage. If you put a resistor in series with your MOT I'd be willing to bet the arc would be purple. Steady arcs at a few watts are usually purple because of the blue spectral line of nitrogen and the red line of oxygen.

I think that the very large arcs look orange because of some sort of combustion reaction that is going on inside them, possibly the formation of the different nitrogen based compounds that come out of arcs.

Short powerful arcs (sparks from marx generator) look white because the energy spike is much higher. I bet that they energize the atoms to a higher state and thus more lines from that elements spectrum can be seen. The more colors, the whiter the spark.

That's always what I thought caused the different colors, but there's a huge chance I'm completely wrong.

Mattski

Wed Oct 05 2011, 08:45AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Take a look at the colors possible from electron transitions in gases:

Of most interest are nitrogen and oxygen, the two most plentiful gases in the atmosphere. Both have spectra extending from red to blue and even higher in energy/frequency. The blue emissions require an electron to fall from a higher energy level back down to some lower state, so probably with a higher voltage more electrons get enough energy to reach those high energy levels. The shorter wavelengths require more energy because photon energy is inversely proportional to wavelength.

So I think Turkey9 is on the right track, more voltage gives more colors, gives closer to white. I reckon you could put various compounds on the leads where the discharge happens and get a certain amount of coloration: table salt (sodium and chlorine) would give you yellow from the sodium, boric acid (available as roach killer at the hardware store) I know emits green when burned, I think strontium chloride is red.

I'd also wonder at what point it stops behaving like gas emission spectra, and starts behaving like a radiating black body (like an incandescent bulb), because if it heats up enough that must be what happens. I don't know if the arc gets hot enough for that though.

Patrick

Wed Oct 05 2011, 02:23PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

That does sound credible, the incandecesnt thing.

Proud Mary

Wed Oct 05 2011, 03:12PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

To learn more, Google "arc emission spectroscopy"

Dr. Dark Current

Wed Oct 05 2011, 04:05PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The spectrum has two parts - atomic spectral emission of atoms present in air and sputtered from electrodes, and black body radiation/incandescence.
Not counting the atoms sputtered from electrodes, with low current arcs the temperature is so low that incandescence is almost invisible and atomic emission, mainly nitrogen dominate the visible spectrum. As the current is increased, incandescence becomes significant and the arc color shifts from yellow to white to bluish, depending on current.
At least that's how I see it.

Tetris

Wed Oct 05 2011, 05:48PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

I am not sure about man made arcs, but I know lightning gets its color from what is in the air. I've seen purple, white, blue, and orange lightning. I know there can be "green" lightning, but it is rare. Depending on what chemical is at a high level in the air and where, lightning is different colors. I guess the same applies to all capacitive sparks.

Shrad

Wed Oct 05 2011, 06:20PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

oxygen has transitions in the blue, green and yellow spectrums, according to laser transitions

blue = less energy, then comes yellow, and finally green

a red compound may be due to iron ions getting sputtered and incandescence taking place, giving orange as the resulting colour

there is a nice section about pulsed multiple gas lasers at sams laser faq explaining some experiments with pulsed oxygen laser

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