[wanhafizi (OP)]

Hi there,

I saw a question about plasma globes earlier.


My questions are;

1. What kind of radiation produced by these things? I've noticed that a small florescent bulb could lit up.

2. Why a grounded neon-bulb electrical-tester-pen would also lit up near plasma globes? Is it the same mechanism as the florescent bulb?

3. I've heard that theoretically, a plasma globe could produce X-rays when then the electrons smash to the glass wall. Is it true?

4. How does the glass wall conduct the electricity to the ground? Is it because of the semi-conductivity?

5. Why would the electron streams seem to be dancing? Is it because of the movement of the low density gases inside the globe or it is becoz of the electrons' quantum mechanic property?

Thanks

[exton]

1. What kind of radiation produced by these things? I've noticed that a small florescent bulb could lit up.

Ultraviolet radiation. It's the reason that white shirts seem to glow near them - it's the same kind of light that is produced by a black light.

2. Why a grounded neon-bulb electrical-tester-pen would also lit up near plasma globes? Is it the same mechanism as the florescent bulb?

I think so, yes. The gas is excited by the ultraviolet radiation.

Well, it's not exactly the same. In a neon bulb, it's the gas that gets excited, whereas with the flourescent bulb it's the flourescent coating on the inside of the bulb.

3. I've heard that theoretically, a plasma globe could produce X-rays when then the electrons smash to the glass wall. Is it true?

Sort of. If electrons are moving fast enough when they hit glass, you can get X-Rays, but they don't move anywhere near that fast in a plasma globe.

4. How does the glass wall conduct the electricity to the ground? Is it because of the semi-conductivity?

It doesn't conduct electricity to the ground. The glowing lines that you see aren't electron beams, they're plasma. It's a bunch of charge particles moving about - both charged atoms and electrons. The electrical current in the plasma doesn't move in one direction, it moves back and forth...more or less.

The plasma forms due to a very strong electric field near the spongy metal ball in the middle; it causes the atoms in the gas to become ionized.

Why, exactly, the plasma often takes the form of streamers is somewhat beyond me.

5. Why would the electron streams seem to be dancing? Is it because of the movement of the low density gases inside the globe or it is becoz of the electrons' quantum mechanic property?

The streamers move upwards because of convection; the plasma is hotter than the rest of the gas, and so it's more bouyant and moves upwards.

[daveshorts]

I would doubt that there is a lot of UV getting out of the plasma balls, as otherwise they would be hard to sell to children.

I think the reason that the fluorescent tube lights up is that the plasma balls work at fairly high voltage (1-2kV) and a fairly high frequency (more than 20kHz) this means that there is a very rapidly changing electric field around the globe. This means that any ions in the fluorescent tube are accellerated backwards and forwards, they bash into mercury atoms and they then give off light.

A bit like the rubbing an energy saving light bulb kitchen science http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/lighting-bulbs-without-wires/

The reason it tends to form streames is that ionised air conducts a lot better than unionised air, so electric current tends to pass through it, this current creates more ions so the air becomes more ionised and better conducting, until eventually a small line of ionised air shorts out all the other air.

Two wires with current going in the same direction will repel. So there is also a magnetic effect which causes the streamers to repel one another and even split apart.

[exton]

I don't know how much "a lot" is, but it's more than enough to produce a "black light" sort of effect on (for example) a white t-shirt that's placed close to it.