If incandescent lightbulbs have a vacuum inside, why do they get so hot on the outside?
If incandescent lightbulbs have a vacuum inside, why do they get so hot on the outside?
Shouldn't the vacuum insulate the glass from the heat of the burning filament?
Why does the sun heat the Earth if space is a vacuum?
God's will
Ooooh risky
Maybe we should create a community that is called c/nostupidanswers instead of c/nostupidquestions
Who?
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.
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.
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)?
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.
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
for better insulation you can put more reflecting layers inside. i've heard of insulation for liquid helium pipe, it used thousands of layers of aluminized mylar between two tubes in vacuum. it's one barrier between 4.2K and room temperature, and it works good enough to be used in helium manufacturing plant
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.
Vacuum is an absence of stuff. What would be insulating the glass? Air would be an insulator; a vacuum wouldn't do shit.
Think of heat like a physical object. It can travel through a vacuum unimpeded. There is nothing within a vacuum to stop the heat traveling through it. But if there was even just a thin atmosphere, it would collide with some of those molecules and not reach the glass, taking longer for the glass to get hot.
It's a bit more complicated than that since in reality the air will also be heated and transfer that heat, but just for the purpose of how vacuums work, we can ignore all that.