Plasma Ball Colors

In reference to the plasma ball, why does the electricity in the globe change colour, from blue in the centre to pink towards the glass?
- Aaren (age 18)
newcastle, Australia

Plasma balls work by electrical discharges through low-pressure gases. Voltage differences between the central electrode and the outer glass sphere are created by cycling the voltage on the central electrode rapidly from large negative voltages to large positive voltages, taking advantage of the capacitance between the central electrode and the inner surface of the glass sphere.

Electrons travel from the electrode outwards half of the time, feeling the push of the electric field. The energy they get after traveling a short distance depends on the local electric field and the distance they manage to travel before colliding with a gas molecule (this is called the "mean free path" and depends quite a lot on the pressure of the gas inside the ball. The lower the pressure, the bigger the mean free path.)

Different gases are used inside plasma balls, and these gases glow differen colors. Neon has the famous bright orange glow, argon is a deep purple, nitrogen is a reddish purple. Other gases glow with different colors. Why, you may ask? The reason has to do with the different energy levels of the electrons in orbit around these atoms (or molecules, as in the case of nitrogen). An atom changes its electronic energy configuration most often when the energy of the electron striking it is "just right" (this is called the Franck-Hertz effect). The energies of electrons bound inside atoms and molecules is quantized, that is, it can only take on certain values.

If the mean free path is more or less the same everywhere in the gas, then the speed of the electrons flowing through the plasma depends on the local field strength. Slower electrons will preferentially collide and change the energy state with atoms which prefer exactly that electron energy. Electrons may still collide with all the other gas components, but they will not change the atomic energy state if the energy is wrong.

That way, if a mixture of gases fills the plasma ball, then different gases will glow in different parts due to the falling electric field strength as you go away from the central electrode.

Tom

(published on 10/22/2007)

That answer does not really answer the question. Perhaps you have a different plasma ball?I think I have the same on your previous questioner has. The answer given, does not, to me, answere why the plasma changes colour along the plasma discharge route. It is pink immediately next to the inner electrode, then changes to a more intense blue as it forms a plasma path to the inside of the glass ball, whereupon it changes again to the neon pink colour as it diffuses outward to the glass surface.Can you explain the colour change for the single plasma discharge path as it progresses from origin to destination? Is it to do with energy concentration along the path?
- Steve (age 70)
Dunedin, NZ

Yes, the old answer had the ingredients but didn't quite finish in putting them all together. The key ingredients were that different gases emit different colors and that different energy electron collision tend to excite different atoms to the states which then emit light. So the question becomes: why do the typical electron collison energies vary from place to place? 
If the color variation was smooth and uniform as we look from the middle out toward the edge, you could imagine a simple explanation involving electrons gradually speeding up or slowing down on their pathways. You can see from the wiggly little lit-up paths, hwever, that the electron flow is not close to being smooth and uniform.Electrons tend to slow along paths where some of the gas is already ionized, but those paths heat up and tend to drift up, etc. So it's a very complicated effect, and there's no rule that the tyical collison strength will be the same everywhere. That allows some regions to tend to light up one gas and other regions to tend to light up another. Since they give off different color light, that's what you see.

Mike W.

(published on 06/27/2020)