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If incandescent lightbulbs have a vacuum inside, why do they get so hot on the outside?
(self.nostupidquestions)
Why does the sun heat the Earth if space is a vacuum?
this post was submitted on 17 Jul 2023
113 points (97.5% liked)
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Because of me. Whenever I look up at the sun, I think about the inevitable supernova which the sun sooner or later will turn into. This in turn gives me anxiety and makes me sweat a lot, which heats the earth.
Correct me if I'm wrong but isn't the sun too small to turn into a supernova? and basically will just die a boring death after swallowing all the planets in the solar system and fade into nothingness
A new worry has been unlocked: We're all going to die but not in a cool way and all the other solar systems will think we're lame.
Now I'm worrying even more, and glabal warming will become even worse!
Don't worry, it has zero effect on global warming. These are timescales so vast, humanity will have either wiped itself out or evolved into something unrecognisable long before the sun starts expanding.
Global warming is something that operates on the order of decades or centuries.
If we manage to stop global warming and maintain or better yet repair the state of our climate afterwards, it will take roughly a billion years for the sun to get 10% hotter and boil off our oceans regardless of what we do and longer yet for it to start its red giant phase.
To put that in perspective, a billion years ago life on Earth was all single celled.
Not sure if you're serious, or expanding further on the running joke.
Thank you for the explanation, but outside of the joke I am fully aware of the time scales and origin and consequences of global warming. :)
global warming is definitely something it makes sense to worry about and which there's still some chance to mitigate the worst effects of.
The sun expanding - or even the much earlier effects before that happens, as the sun gets hotter - will happen on such long time scales that there simply won't be any humans at all; most species only last about a million years or so, vastly less time than we're talking about.
We might well make the planet nigh uninhabitable in considerably less than one-millionth of the sun-being-a-major-problem time. It's like worrying about the bridge maybe rusting dangerously a few decades from now, while not paying attention to the truck that has just veered onto your side of the road and will surely hit you in the next few seconds. You need to take evasive maneuvers, not worry about the bridge.
@[email protected] like teenagers, all solar systems think other solar systems are cooler than them and know what they’re doing
The vast majority of solar systems have significantly smaller suns with equally lame or lamer endings.
That said, because the sun is slowly getting hotter over the ages the older it gets, the Earth's oceans will have boiled off before the sun starts expanding.
basically will just die a boring death after swallowing all the planets in the solar system
Not all the planets, no. Mercury and Venus, sure. The earth's orbit will move somewhat further out when the sun expands, and probably won't be swallowed but it will at least be well baked.
Not quite that large. At the peak of its red giant phase, the sun's size will reach just about Earth's current orbit. Quite possibly the Earth will remain just slightly outside the sun due to the orbit becoming larger to compensate for the sun's decreasing mass, but the Earth's oceans will have boiled off before the sun even enters its red giant phase, because between now and then the sun will get progressively hotter over the millenia (well, technically this slow increase in average sun temperature has been going on for ages already, it's just really slow and masked by several cycles).
God's will
Ooooh risky
Maybe we should create a community that is called c/nostupidanswers instead of c/nostupidquestions
Who?
Vacuum means no convection heat. It however does not mean no radiation heat. The filament radiates heat through vacuum the same way the sun does.
Additionally, theres not vacuum in buldbs, but inert gas, like argon.
There no longer is a vacuum in bulbs, but there was for a very long time.
"Halogen" bulbs are just incandescent bulbs filled with inert gas and a small amount of bromine or iodine. Very interesting chemistry https://en.wikipedia.org/wiki/Halogen_lamp
In addition to the stuff everyone else is saying, most modern bulbs don't have a vacuum at all.
Most modern bulbs are filled with an inert gas like argon or xenon. Usually at a lower pressure (around 70% of standard atmospheric pressure), but nowhere near a vacuum.
This has, while inert to chemical reaction, is more than capable of transferring heat.
Heat is infrared. Light. Vacuum doesn't have much effect.
Also many bulbs are filled with inert gases rather than being vacuums.
Heat is infrared. Light.
All light heats up anything that absorbs it. This includes infrared, but it also includes visible light, microwaves, radio waves, etc. You can get a nasty burn from putting your hand near a live radio transmitter antenna, for example, even though it's emitting in RF, not infrared.
In addition, all physical objects glow with a light that is determined by their temperature. This includes your body. You are, right now, emitting light. As it happens, because of your body's temperature, that light is mostly in the infrared.
Why do kids' science books leave you with the impression that "heat is infrared"? Because you can see body heat with an infrared camera. Infrared is light that you can't see with your eyes — but with the right tool, you can use to see body heat. This rounds off to "heat is infrared".
Heat is not infrared. All physical objects emit light; objects around human body temperature glow mostly in the infrared; which we can't see with our eyes, but can see with scientific instruments. And when an object absorbs light (including infrared), it gets hotter.
Wow, I fucking learned something today. Thank you, stranger.
Go see what happens if you lick a radio transmitter aerial and report back
If I super heat a metal and it turns visibly red what is happening? Was it already emitting infrared and as it gets hotter the frequency shifts up? Or is it still emitting infrared but has a wider band of frequencies it is emitting as well (i.e. is it emitting frequencies below infrared as well as visible red)?
Yes, as you heat something up to "red hot", the glow shifts from infrared to being partly in visible red frequencies. This is why a blacksmith can use the color of a piece of hot iron to tell how hot it is.
https://en.wikipedia.org/wiki/Black-body_radiation
(This isn't the only way hot things make light, though — for instance, flames can glow with odd colors like green or blue due to specific chemicals burning.)
Same way sun heats the earth
I’m no lightbulb expert but I can tell you a vacuum still allows heat transfer via radiation. This is how we get warm from the sun.
I also doubt the inside of a filament lightbulb is a near perfect vacuum, but maybe a bulb expert will come along to shed some light on that.
Heat radiation in a vacuum is also an important aspect of space travel. If heat could not radiate in a vacuum, we would not be able to dump excess heat from space craft and, at some point, the combination of electric devices operating within the pressure vessel and human heat output would eventually roast the people inside. We need heat to radiate outwards, and, from my understanding, it’s actually a somewhat difficult problem to solve in a vacuum. We take air and evaporative cooling for granted sometimes when on Earth and in space, where air cooling isn’t going to happen, you have to practice other methods of heat transfer.
3 forms of heat transfer
Conduction. Transfer of heat from one medium to another.
Convection. Transfer of heat within the same medium.
Radiation. Non-contact transfer of heat.
In addition to what others have said, they're not a vacuum inside. They're filled with 0.7 atm of argon gas. That would slow the transfer of heat, but there'd still be plenty of heat transfer through convection.
Why not 1 atm?
Several reasons. But I would guess a big part is that air pressure drops with altitude. 1atm is the pressure at sea level. According to my google-fu the air pressure on Mt Everest is a mere 0.33 atm.
You don't want your light bulb exploding when it breaks, especially if part of the reason you put a special gas in it was to prevent it from imploding when it breaks.
Now of course most people live significantly closer to sea level than to the peak of Mt Everest, but if a gas is heated in an enclosed space, its pressure rises.
Also, if you have to choose between shards tending to go inwards or outwards when the bulb shatters, you'd probably prefer them tending to go inwards, provided it's not so fast they shoot past the middle as they would with a much lower pressure.
Lastly, it saves on gas.
Assuming an ideal gas, going from 20 C to 100 C at constant volume brings you from 0.7 atm to 0.9
Tracks better with my assumption than expected.
The filament is heated by electrical resistance. That heat energy comes out as photons in a wide band in the visible and infrared parts of the spectrum. Some of those photons are intercepted by the glass bulb, the metal housing, etc.; their energy heats these materials up.
Even though a vacuum prevents conduction of heat energy, it doesn't prevent radiation of that energy in the form of photons. That's how the light gets from the filament to the room; and that's how the heat gets to the surface of the bulb too.
More then you ever thought you needed to know about lightbulbs! (Including the fact that inconsistent light bulbs are not vacuumes, they just have no oxygen, so they heat up via normal connection)
One of my favorite things about Reddit was that any link in a thread about light bulbs, heaters, EVs, or other appliances was almost certainly to a Technology Connections video. I'm glad to see that trend continuing on Lemmy.
How did I know before even clicking that this would be a Technology Connections video
I literally watched this video about 10 hours ago. He’s the Internet’s favorite Chicagoan.
Maybe they are thinking of how mugs and thermoses can be labeled “vacuum sealed,” and that the marketing implies that the vacuum between the walls insulates the outer wall (where the hand touches) against the heat.
And those are usually made out of some metal, that'll do pretty good reflecting radiation heat transfer
It does do that though. My vacuum thermos keeps things hot or cold for like 6 hours
There's no perfect vacuum. But as others have mentioned already, most of it is electromagnetic radiation. A very small part of this radiation is the visible light you see, most of it is invisible to the human eye.
Heat radiates as infrared light. Infrared doesn’t pass through glass well, so the glass absorbs the heat radiated from the filament.
Heat passed through molecular collisions is conduction, and that is the part blocked by a vacuum.
So the filament is emitting visible and infrared and the bulb is designed to let the visible pass through.
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