heat transfer in vacuum

[mopc]

If you are standing near an extremely hot object in an environment of perfect vacuum, even though this object does not emit light, would you feel some of its heat?

[lti]

yes.

there are three mechanisms for heat transfer: convection, conduction, radiation.

in a vacuum heat can only transfer by radiation.

U said that the object doesnt emit light. that is incorrect. light is emitted from any object that has heat energy (everything). Objects with low temperatures, around 290K (things we find on earth) emit light of a lower wavelength than we can see. generally in the Infra red part of the spectrum. Outer space has a temperature of about 3K and emits light in the microwave part of the spectrum.

as things get hotter they begin to emit light in the visible spectrum and so appear to be glowing.

lead melts at quite a low temperature (below the temperature required to emit visible light) and so molten lead is a dull grey colour. Whereas iron melts at a much higher temperature and so molten iron glows.

an object being heated will begin to glow red, then white, then blue, then would not glow any more as u heated it up. Wiens law shows the relationship of spectrum emited to temperature of black body radiation.

so an EXTREMELY hot object as u say may not emit visibly light. it may emit mainly x-rays or gamma rays and would not glow. U would get very hot standing next to it. ofcourse x-rays and gamma rays are such high energy that u would also get very sick from standing next to such a body.

[mopc]

Thank you very much, excellent explanation!

[Maddad]

Yes indeed, it was an excellent answer.

[Ilya]

Excellent answer, except it is not entirely true.

an object being heated will begin to glow red, then white, then blue, then would not glow any more as u heated it up. Wiens law shows the relationship of spectrum emited to temperature of black body radiation.

so an EXTREMELY hot object as u say may not emit visibly light. it may emit mainly x-rays or gamma rays and would not glow.

That last part is wrong. Extremely hot objects emit MOST of their radiation in x-rays and gamma rays, but they still emit visible light -- and rather more of it than the objects whose radiation PEAKS OUT in visible light. No matter which wavelength you choose, a hotter object will always emit more of it than a cooler one:

http://hyperphysics.phy-astr.gsu.edu/hbase/bbrc.html

[JHotz]

yes.

there are three mechanisms for heat transfer: convection, conduction, radiation.

in a vacuum heat can only transfer by radiation.

U said that the object doesnt emit light. that is incorrect. light is emitted from any object that has heat energy (everything). Objects with low temperatures, around 290K (things we find on earth) emit light of a lower wavelength than we can see. generally in the Infra red part of the spectrum. Outer space has a temperature of about 3K and emits light in the microwave part of the spectrum.

as things get hotter they begin to emit light in the visible spectrum and so appear to be glowing.

lead melts at quite a low temperature (below the temperature required to emit visible light) and so molten lead is a dull grey colour. Whereas iron melts at a much higher temperature and so molten iron glows.

an object being heated will begin to glow red, then white, then blue, then would not glow any more as u heated it up. Wiens law shows the relationship of spectrum emited to temperature of black body radiation.

so an EXTREMELY hot object as u say may not emit visibly light. it may emit mainly x-rays or gamma rays and would not glow. U would get very hot standing next to it. ofcourse x-rays and gamma rays are such high energy that u would also get very sick from standing next to such a body.

Maybe you can confirm a theory of mine. I am a glassblower. When I blow a glass bubble the same quantity of glass with the same amount of energy at the same distance from a body part heats it up much more as the bubble gets bigger. I think it is because radiant heat has a greater range of angles to hit the body part. Have you heard of this effect and does it have a name.

[Evan]

I have done some glass work as well. The larger bubble has a greater surface area and radiates heat faster. It also cools faster as you know.

[JHotz]

I have done some glass work as well. The larger bubble has a greater surface area and radiates heat faster. It also cools faster as you know.

Thank you for responding to my post

You are right I did know that. I just had reasoned that as the reason for the greater heat. Thanks.

So if we shrink the bubble again it has been cooled faster. Same principle as gas coolant in an air conditioner really.

[Evan]

To the various statements about color temperature vs radiation frequency, that assumes black body radiation. It changes if the object isn't a black body. Molten iron radiates intensely in the visible spectrum but molten aluminum does not. Molten aluminum has a highly reflective surface with about 92% reflectivity if clean. According to QED that surface is both externally reflective and internally reflective. This accounts for the fact that molten aluminum at a smelter does not appear to glow under normal shop lighting even though it is hot enough to glow red. It actually does glow red but only very dimly since most of the radiation is reflected internally.

There is actually a calculated index of refraction for molten metals even if they are not transparent to visible radiation. Aluminum has about the same index as water so at the critical angle total internal reflection occurs. This further reduces the amount of radiation.

[lti]

That last part is wrong. Extremely hot objects emit MOST of their radiation in x-rays and gamma rays, but they still emit visible light -- and rather more of it than the objects whose radiation PEAKS OUT in visible light. No matter which wavelength you choose, a hotter object will always emit more of it than a cooler one:

thanks for correcting that Ilya. I wasnt sure of the graphs.

To the various statements about color temperature vs radiation frequency, that assumes black body radiation.

thanks for clarifying that. i did mention black body (in a some what garbled and in retrospect incorrect sentence) but didnt bother explaining further.

[swansont]

To the various statements about color temperature vs radiation frequency, that assumes black body radiation. It changes if the object isn't a black body. Molten iron radiates intensely in the visible spectrum but molten aluminum does not. Molten aluminum has a highly reflective surface with about 92% reflectivity if clean. According to QED that surface is both externally reflective and internally reflective. This accounts for the fact that molten aluminum at a smelter does not appear to glow under normal shop lighting even though it is hot enough to glow red. It actually does glow red but only very dimly since most of the radiation is reflected internally.

And that has nothing to do with Al having a melting point of 660 C and Fe at 1535 C?

[Swift]

To the various statements about color temperature vs radiation frequency, that assumes black body radiation. It changes if the object isn't a black body. Molten iron radiates intensely in the visible spectrum but molten aluminum does not. Molten aluminum has a highly reflective surface with about 92% reflectivity if clean. According to QED that surface is both externally reflective and internally reflective. This accounts for the fact that molten aluminum at a smelter does not appear to glow under normal shop lighting even though it is hot enough to glow red. It actually does glow red but only very dimly since most of the radiation is reflected internally.

And that has nothing to do with Al having a melting point of 660 C and Fe at 1535 C?

I can't say it has nothing to do, but I suspect the optical properties are a relatively minor effect and the stength of the atomic bonding has much more.

[Evan]

And that has nothing to do with Al having a melting point of 660 C and Fe at 1535 C?

No, I'm talking if the aluminum and iron are at the same temp. The boiling point of aluminum is a suprising 2467° C. (4454°F)

[Avatar28]

At about what temperature do you start getting a glow from hot iron? I'm just wondering because my parents have a cast iron stove in the basement. There have been times when the fire has really been roaring that I'm SURE I've seen a (very) faint reddish glow coming from it. It wasn't very visible and if you looked directly at it, you couldn't really notice. But I'm sure that it was there, that same internal glowing that you get from really hot iron like has been in a forge, except that it was just on the edge of perception.

[Evan]

It begins to glow at around 900F, just a faint deep red. At 1050F it is very noticeable red and at 1350F it is red/orange. White heat is at 2550F and up.

BTW, you wern't imagining it. It is easily possible to make an iron wood burning stove glow. I do.

[swansont]

And that has nothing to do with Al having a melting point of 660 C and Fe at 1535 C?

No, I'm talking if the aluminum and iron are at the same temp. The boiling point of aluminum is a suprising 2467° C. (4454°F)

Ok. Just wanted to make sure we were comparing apples to apples here.

[Fortis]

At about what temperature do you start getting a glow from hot iron? I'm just wondering because my parents have a cast iron stove in the basement. There have been times when the fire has really been roaring that I'm SURE I've seen a (very) faint reddish glow coming from it. It wasn't very visible and if you looked directly at it, you couldn't really notice. But I'm sure that it was there, that same internal glowing that you get from really hot iron like has been in a forge, except that it was just on the edge of perception.

To work out the wavelength for the peak in the blackbody curve (in microns), divide 3000 by the absolute temperature in K. It's not quite right, but definitely good enough for government work, e.g. ambient temperature is roughly 300 K, so we get a peak at ~10 microns which corresponds to the region where some of the better thermal imagers work. Now all you need to know is that the visible region is roughly 0.4-0.7 microns, an away you go.