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Why do light bulbs don't explode?

  1. Mar 25, 2009 #1

    fluidistic

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    I'll soon be seeing the famous formula [tex]PV=nRT[/tex] in my course although I already learned it years before when I was in high school. So don't be too hard on me for my question please.
    My question is : if there is some gas (krypton or any other gas) in light bulbs and that the tungsten in it is really hot (around 4000K if I remember well), why don't the bulb explode under the gas' pressure? I guess it's because the choice of the gas is such that the gas don't expand that much. But still, 4000K seems really hot to me.
    Or is it because the bulb is really tough? Or both?
    Thanks.
     
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  3. Mar 25, 2009 #2

    mgb_phys

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    There is almost no gas inside a light bulb - it's a vacuum.
    The small amount of gas is just to stop tungsten evaporated from the filament coating the inside of the glass.
     
  4. Mar 25, 2009 #3

    fluidistic

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    It explains it all. Thanks a lot.
     
  5. Mar 25, 2009 #4
    Exploding light bulbs sounds a nice idea for April 1. Is there a simple way make light bulbs that contain gas at atmospheric pressure?
     
  6. Mar 25, 2009 #5

    mgb_phys

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    I doubt they would explode. If you filled them with air the filament would burn out, if you filled them with an inert gas they would probably only reach 200deg C which wouldn't even give you 1 atmosphere of extra over-pressure - since they are built to stand one atmosphere of under-pressure they would probably survive.
     
  7. Mar 25, 2009 #6

    Danger

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    Since there is a significant public hazard involved, I suggest that this thread be locked.
     
  8. Mar 25, 2009 #7
    I see! So, perhaps a bit of water would do the trick. It would evaporate raising the pressure considerably...
     
  9. Mar 25, 2009 #8

    Danger

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    That is so not a good idea...
     
  10. Mar 25, 2009 #9

    Astronuc

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    I believe the OP was referring to normal lightbulbs. Let's not take this down the path of finding ways to make lightbulbs explode. That is a safety and fire hazard.

    If a lightbulb gets a leak, the tungsten filment burns out quickly because WO3 forms a very porous oxide and pure W oxidizes rapidly when very hot.
     
  11. Mar 25, 2009 #10

    Danger

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    Thanks for the save, Astro. In keeping with the original post, it's because argon, nitrogen, and the like are non-reactive gases. They do not promote degradation of the filament as would happen in an oxygen atmosphere.
     
  12. Mar 25, 2009 #11

    fluidistic

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    Yeah I knew. If I haven't been clear enough : my doubt was that as the volume remains constant and the temperature of the gas increases a lot, I thought that pressure would also increase in such a way that the bulb should explode. But as mgb_phys pointed me out there's a very few quantity of gas so pressure is not sufficient to make the bulb explode.
    Hence the thread is "solved".
     
  13. Mar 25, 2009 #12

    Lok

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    Lightbulbs were usually made with a vacuum inside, but for better lifetime they are filled with an inert gas. Noble gases are best, nitrogen isn't as it will react with tungsten at 2000'C.

    The thing is that the amount of gas in a lightbulb is very small so that when the fillament reaches it's functional temperature the pressure will be smaller than 1 atm. So if gas is present, while the lightbulb is off, it will have a pressure of 0.075 atm ( close to that).

    Btw , some bulbs do explode if u raise the power ( voltage*amp), but mostly they just burn out. Don't expect a glamorous explosion, just a few glass shards blown very close to the initial bulb.
     
  14. Mar 25, 2009 #13

    Danger

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    I was totally unaware of that. Thanks for the heads-up.
     
  15. Mar 25, 2009 #14

    Andy Resnick

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    Although this thread has been limited to discussing incandescent bulbs, I should mention that high pressure arc lamps used in microscopy, are near 1 atmosphere pressure and they can explode if used improperly.
     
  16. Mar 26, 2009 #15

    Redbelly98

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    Another factor is that the 3000 K is the temperature only at the surface of the filament, it is not the temperature of the entire gas volume.
     
  17. Mar 26, 2009 #16

    fluidistic

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    Ah I see. But after about 5 minutes of use the temperature of tungsten' surface shouldn't differ that much from the temperature of the gas, right? Or is argon a very very bad thermic conductor? It's hard for me to imagine that argon's molecules are very fast close to the filament and not really fast in bulb' surface. Ah but... this is true. I think I once read that bulb surface is about 200°C. In a few years I should be studying statistical thermodynamics. I'm sure this could help me understand well the light bulb at a molecular scale.
     
  18. Mar 26, 2009 #17

    mgb_phys

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    Which case are we talking about?
    In a regular bulb (near vacuum) yes the scavenger gas touch the filament will heat up to 2000K or so and head off to the walls at reasonably high speed. It will hit the cold glass envelope and dump most of it's heat before drifting back randomly to hit the filament again. the amount of energy trasnferred to the glass this way is small compared to radiation heating from the IR absorbed in the glass.

    If you filled the bulb with argon at one atmopshere then there would be a much efficent convective heat transfer, just because there are lots more atoms/second hitting the filamane. Argon has a poor thermal conduction becuase it's relatively heavy and therefore slow - this is why you use it in eg. dry suits.
     
  19. Mar 26, 2009 #18

    russ_watters

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    If the surface is 200C and is much larger than the surface area of the filament, then the temperaturre of the gas inisde is going to be much closer to 200 than the filament temp.
     
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