| MadSci Network: Physics |
Hello Micah,
Thank you for your question. I do hope you have good safety measures in place-such high voltages can be extremely dangerous! I can't quite pin the exact answer to your question from the information you supplied, but I'll explore a few possibilities.
First, let's take a look at the cause of the color of a plasma. A plasma is a gas in which a significant fraction of the molecules and atoms are split into free electrons and ions[1]. Now, this fraction of electrons might be rather modest-for the arcs you describe, an ionization degree of perhaps 1% is fairly typical. These free electrons get jostled around in the electric field that is caused by the voltage difference. This gives them a temperature that can reach about 10,000 K (this is about 10,000 degrees centigrade)[2]. These electrons are more energetic and essentially hotter than the background gas. Many plasmas have electrons that are a lot more energetic than the background gas[3].
These electrons start hitting other particles with their large energies. Some of these particles are atoms. It is possible to "hit" such an atom so hard that one of the electrons in the atom gets knocked into a higher orbit [4]. Such an atom can emit light when it falls back to the ground state, and we can see this light. The color of this light is particular to the gas that is causing the plasma: neon gives its characteristic red glow, while sodium gives the familiar orange of street lights. When this light is put through a prism, you see a single or a few lines; hence, this light is called line emission.It happens that for nitrogen, the principle component of air, the dominant line emission for typical atmospheric arc plasmas is orange.
A second mechanism that can cause plasmas to produce light is the interaction of electrons with the ions. The electrons, which are negative, speed towards the positive ions and make an orbit around them. The resulting acceleration and deceleration produce a kind of light that is called Brehmsstrahlung [3,5,6]. This light has many different colors and appears whitish for most plasmas.
These two processes happen in each plasma, but often, one is far more important than the other. In general, higher power densities (more power!) tend to favor the second mechanism, while lower power densities favor the first mechanism. In a nutshell, lower power densities mean a lot less electrons, which do have a little bit more energy per electron. More energy per electron favors line emission, while more electrons cause a lot more collisions between electrons and ions and hence more Brehmsstrahlung.
There is one more thing to consider that is important. For the types of arcs we are discussing, Ohm's Law [7] does not apply. The resistance of the plasma is a strong function of the power of the plasma, and is in general low-a few Ohms, perhaps. This means that the actual voltage in the plasma is a lot less than 40.000 Volts. The limiting factor is not the voltage of the transformer, but rather the maximum current that can be supplied through it. This depends on the control electronics, the thickness of the wires, etc. This does mean that there might be a very large difference in plasma power-and this might make line emissions more important than Brehmsstrahlung.
Finally, the material of the transformers might play a small role. The electrode tips can be damaged, and the removed material can be ejected into the plasma streams. These metal impurities can lead to relatively high amounts of line radiation. If one of the transformers has a metal tip that produces orange light when it is in a plasma state similar to that in the arcs, and the other does not, this could explain the difference in color.
In summary, we have considered two possible mechanisms for the color difference: the difference in power delivered, which might be a lot bigger than the voltage difference would suggest, and possible contamination with electrode material. I hope this clarifies matters.
Regards,
Bart Broks
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