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Natural Science Forum / Physics / Optics / December 2004Light, Heat, Vacuum
Hello, While a light bulb filament is powered, the glass bulb is very hot. I would expect very little heat transfer from the filament to the glass through the small amount of gas within the bulb. Is the glass heating due to absorption of infrared? I am wondering about the energy loss of a light bulb filament after the power is removed. Visible and IR light are emitted so that is some of the loss. Heat transfer from filament to glass bulb should be small due to vacuum. Heat transfer from filament ends to external electrical contacts will occur. Eventually the filament reaches ambient temperature and equilibrium. If the unpowered hot filament were surrounded by a perfect vacuum it would continue to emit light for a while. As energy is lost, the filament would cool and emit less light. At what point does this process stop? Tom Hubin thubin@earthlink.net > Hello, > [quoted text clipped - 8 lines] > Visible and IR light are emitted so that is some of the loss. Heat > transfer from filament to glass bulb should be small due to vacuum. Yeah and heat transfer from the Sun to the Earth should be small "due to vacuum". Oh, and light transfer too. THINK, MAN. "Conduction, convection and radiation" > transfer from filament ends to external electrical contacts will occur. > Eventually the filament reaches ambient temperature and equilibrium. [quoted text clipped - 3 lines] > filament would cool and emit less light. At what point does this process > stop? Just as hell freezes over. > While a light bulb filament is powered, the glass bulb is very hot. I > would expect very little heat transfer from the filament to the glass > through the small amount of gas within the bulb. Is the glass heating > due to absorption of infrared? Most modern incandescant lamps have an inert gas such as nitrogen filling the envelope. It is not a vacuum as used by early carbon filament lamps. Bill > As energy is lost, the filament would cool and emit less light. > At what point does this process stop? The process would stop at a temperature of absolute zero, but that is never reached. Before it reaches absolute zero, it will reach an equilibrium temperature determined by it's surroundings. At that point, it both radiates energy, and absorbs energy that is being radiated by it's surroundings; the two processes balance each other --> no further temperature change happens. > > As energy is lost, the filament would cool and emit less light. > > At what point does this process stop? [quoted text clipped - 7 lines] > it's surroundings; the two processes balance each other --> no > further temperature change happens. The rate at which the filament emits is given by P/A = sigma*T^4 where: sigma is a constant and T is the absoluite temp and P/A is the power density. This is Stefans Law. This shows that it emits considerably more for even a small increase in temp. Most of this is in the IR spectrum. The frosted bulb serves to scatter the visible portion of the light and substantial amounts of light (IR and visible) are absorbed by the bulb thus heating it. Very little of the bulb heating is done by conduction through the glass, most of it is by radiation. dboh...@mindspring.com wrote: > The rate at which the filament emits is given by P/A = sigma*T^4 Don't forget the emissivity factor on the right hand side. Which, if memory serves, is around 0.3 for tungsten at 3000 K (a typical bulb filament temperature). (Your conclusions are still right on the money.) -- Mark > dboh...@mindspring.com wrote: > [quoted text clipped - 4 lines] > filament temperature). > (Your conclusions are still right on the money.) Is a tungsten filliment at 3000K still the same colour as it is at 300K? (obviously usually swamped by self-generated light) Nope, they're different. 3000 K is yellow or yellow-orange, while 300 K would be a deep red color (assuming you have a visible sensor with enough sensitivity to "see" its radiation) See Fig. 2 at http://www.ledproductstore.com/colorimetry.htm If you're unfamilar with this diagram, it's the CIE Chromaticity Diagram and shows all colors that can be perceived by the human eye. Colors associated with single-wavelengths appear around the outer edge of the diagram, and colors associated with blackbodies of varying temperatures lie along the curve that is shown in the figure (red for cooler temperatures, blue-white for the hottest temperatures). While it's not clear from the figure just where the color converges to at low temperatures, the color is noticeably changing even at 1000 K. The figure shows 2850 K to be in the yellow-orange portion of the diagram (and 3000 K would of course be close to this). It may be that for cooler temperatures the blackbody curve approaches the "pure red" point on the diagram, but if somebody else could confirm or refute that I'd be interested. Regards, Mark > Nope, they're different. 3000 K is yellow or yellow-orange, while 300 > K would be a deep red color (assuming you have a visible sensor with > enough sensitivity to "see" its radiation) Sorry, unclear. I was meaning the colour as viewed by reflected light, not emitted. > I was meaning the colour as viewed by reflected light, not emitted. Sorry. It seems that there is a difference in color, but perhaps a very small one. My CRC Handbook of Chemistry and Physics (76th ed.) lists emissivity of tungsten at various temperatures for two wavelengths, 467 nm and 650 nm. At 300 K, the emissivity is 7% higher at 467 than 650 (.505 vs. .472) At 3000 K, it's 9% higher at 467 (.455 vs. .418) This suggests a shift toward the blue at higher temperatures, but you'd probably need to look at emissivity in the entire visible spectrum to make definitive statements like that. Suffice it to say that there is a difference. -- Mark p.s. Of course, viewing reflected light from a 3000 K source would be a real problem. >At what point does this process >stop? Well, when the filament cools to 3.6 degrees Kelvin, it's still emitting microwaves similar to the cosmic microwave background... so I think you're in for a long wait. John Savard http://home.ecn.ab.ca/~jsavard/index.html |
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