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Voltage & Plasma Globes

  1. Sep 29, 2013 #1
    Hi, I am trying to find a detailed explanation of how a plasma globe works but can't find any detailed info on the web. From what I have read it seems to suggest that the electrode at the center creates an alternating voltage which emits high frequency radio waves. These radio waves ionize the noble gases present in the globe which creates a conductive pathway to the outer glass of the globe. Electrons then flow from the electrode to the glass sphere transferring energy to the ionized gases which emit visible colored light and turn into plasma. I have a few questions which I can't seem to find anywhere:

    Where do the electrons come from that travel through the gases - I would guess they come from ac current from the house supply? So if you touch it does this mean ac current is flowing through you?

    Why is there a potential difference outside the globe (which is proved by bringing a bulb near it) as surely the glass tube would be at zero volts as this is where the plasma sparks end?

    When you hold a fluorescent tube in your hand it FULLY lights up. Why does this do you no harm because if its at the same brightest it would be if it was connected to the mains wouldn't it have the same energy flowing through it (P= VI)?

    Many thanks to any help offered!
  2. jcsd
  3. Sep 29, 2013 #2
    the electrons actually don't come from anywhere they are already in the conductors , copper wires metal electrodes etc. But for them to flow and form current they need something that " starts them" and so yes in your case it is the current coming from your wall socket.

    You don't get shocked from the globe because first of all the globe is encapsulated in a glass sphere , and glass is a very good dielectric or insulator , also because the current is extremely small in such application it is safe to touch it , and when at home you probably have some rubber soles and insulating floor so there is no actual path for the current to go when it touches your finger.
    Well because of the high voltage there may be some very small current traveling through you but that is so small that it is safe to handle it , something similar to measuring the phase wire with a screwdriver type tester that has a little light on it and a high value resistor in series , when you touch it the lamp lights up because current flows through it but because of the resistor the current is so small that you have no problems afterwards.

    There are potential differences in many places in circuitry and life , but the reason why fluorescent bulbs light up in the presence of such globes and also other high voltage equipment is because in the globe case there is not only high voltage but also high frequency and that excites the electrons in the fluorescent lamp , typically mercury vapor as it conducts which further produces UV which makes the phosphor coating of the lamp to shine.you can get the same effect with lower frequency but even higher voltage , like under power lines.

    High voltage especially if high frequency does not stop so easily , the higher the voltage the further it can form a current path through different mediums like air etc.That is the reason why high power transmission cables are high above ground and insulators are thick and long.
    but when you have high voltage and high frequency it gets even worse, the high frequency makes the conductors whatever they may be like wires , traces , rods or current paths through air or gas radiate high frequency EM which may further induce a current into other conductors that come close to it , and by close I mean the higher the voltage the further away from the source this can happen.
  4. Sep 29, 2013 #3
    Many thanks for all the information, very useful! So is it the radio waves created from the electrode responsible for the lighting of the fluorescent lamp? Is the voltage outside the globe also alternating? I did an experiment and you still seem to get equipotentials if you hold the light bulb perpendicular to radial lines.
  5. Sep 29, 2013 #4
    well you can call them radio waves as officially they can go as low as a few Khz but typically we just refer to this as high frequency EMF(electromagnetic field)
    It's not wrong it's just that radiowaves are more like of lower power and much higher frequency but this kind of em radiation is usually of lower frequency but much stronger power as it is radiated off and close to conductors carrying high frequency high voltage electricity.

    Yes they are responsible for that phenomenon.Yes the field outside the globe or current path if you can get one is also alternating , it cannot suddenly change from alternating current inside the globe to static outside , things carry on.

    The thing why this works with fluorescent bulbs and not with the usual incandescent ones is because for a fluorescent bulb to start to illuminate you don't need that much of a current as for a incandescent one.For a filament in an incandescent bulb to radiate EM in the visible spectrum it has to get very hot which is caused by current flowing through the filament , also alot of infrared frequencies are being radiated or in other words heat, the fluorescent lamp is needs much less current to excite the electrons to further produce the luminous effect of the phosphor coating.Even though fluorescent lamps have filaments similar to incandescent bulbs at the ends of the tubes they serve a different purpose than in the incandescent bulb , they do not need to get red hot to radiate visible light , they just need to form current through the mercury vapor in the lamp which further produces the UV radiation which illuminates the phosphor.In your case the filaments don't make the current through the mercury vapor instead the current is induced by the high frequency Em in the vapor.
    the mercury vapor inside the fluorescent lamp is the conductor of electricity through the tube both normally and in your case too as voltage is induced into it from the globe , as soon as enough current is induced the lamp starts to illuminate , the further you take it from the globe the dimmer it gets right ? until it gets all black.
    By the way you can read up on fluorescent lamps and how they work
    Last edited: Sep 29, 2013
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