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Q & A: Plasma balls and X-rays?

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Most recent answer: 10/22/2007
Q:
I read in your site that plasma balls are safe to touch since the electricity given out by them is very small. However, I wonder if there are any other harmful effects of plasma balls. For instance, do they emit harmful amounts of X-rays?
- Rohan (age 39)
Massachusetts, USA
A:
No, I would not expect a plasma ball to emit X-rays in any detectable amount (if at all). While the voltages inside the plasma ball are fairly high, there is also a good deal of gas inside the ball that the electrons collide with. The electrons speed up in the electric field until they get enough energy to knock an electron in a gas atom (or molecule) into another state, or out of the atom altogether. If an electron has too little energy to do this, then the collisions the electron undergoes with the gas atoms are "elastic" -- the gas atoms are unable to absorb the energy from the flying electron. If the electron has more energy than this, the probability that it will suffer an "inelastic" collison with a gas atom goes way up, and you get the characteristic glowing of the gas. If an electron has more energy than this, it quickly loses this energy to collisions with the gas molecules.

This process limits the energy of emitted radiation to that of the lowest available transition energy of the gas molecules in the plasma ball. If you took all the gas away, then the electrons can freely accelerate in the electric field and then radiate X-rays when they strike something. But if there's a lot of gas around, they bump into the gas atoms and lose their energy before they get going that fast.

It's certainly no different from a neon sign or a fluorescent light fixture -- it's just shaped differently so that it looks cool.

Tom

(published on 10/22/2007)

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