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The way heat is transferred from one object to another

by Crazymechanic
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Crazymechanic
#1
Sep16-13, 05:42 AM
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Hello forums.

Would it be fair to say that the way heat is transferred from one object to another is through different types of electromagnetic radiation?
In theory the higher the wavelength of that EM radiation the higher the temperature we can achieve while delivering that radiation to a certain object or piece of matter?

Now why i'm asking this is because vacuum is considered a good insulator of both heat and electricity.
Technically speaking one cannot heat a perfect vacuum as there is nothing to be heated there (no particles no matter) is that correct?

But is vacuum really a heat insulator as the suns given em spectrum still travels through it to reach earth where it can strike air particles etc heat them up and so on.Or is vacuum only good enough for something that doesn' t radiate? As a vacuum chamber would seal off a hot cup of coffee from getting cold that fast but it would still let some gamma and other radiation through from an object which is both hot and radiates like nuclear fuel etc?

so if I have some very hot potato for example which would somehow magically give off fast neutrons in the Mev range would there be any " kettle or pot" that could seal those off without getting hot itself from the interactions with those energetic particles?
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CWatters
#2
Sep16-13, 12:07 PM
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Perhaps go back and look up the difference between heat transfer by conduction and by radiation.

http://en.wikipedia.org/wiki/Thermal_conduction
http://hyperphysics.phy-astr.gsu.edu...heatra.html#c2

http://en.wikipedia.org/wiki/Thermal_radiation
http://hyperphysics.phy-astr.gsu.edu...stefan.html#c2
Crazymechanic
#3
Sep16-13, 02:44 PM
P: 853
since you responded to my last post with only some links which I went through and were good , then I am going to ask , so in the case of vacuum as an heat insulator it only is useful when dealing with thermal conduction as it has to do with very close objects which surface electrons can interact hence vacuum has no electrons being between two such objects the interaction is lost so no heat conduction right?

as for heat radiation or heat transfer via electromagnetic waves which is mediated by photons whic travel through vacuum , hence we feel sunlight and see it. so vacuum in this case would not insulate an object getting hot because it would be struck with high energy photons which would travel through vacuum.right?

Drakkith
#4
Sep16-13, 02:54 PM
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The way heat is transferred from one object to another

That's pretty much correct.
Chestermiller
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Sep16-13, 07:50 PM
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Quote Quote by Crazymechanic View Post
since you responded to my last post with only some links which I went through and were good , then I am going to ask , so in the case of vacuum as an heat insulator it only is useful when dealing with thermal conduction as it has to do with very close objects which surface electrons can interact hence vacuum has no electrons being between two such objects the interaction is lost so no heat conduction right?

as for heat radiation or heat transfer via electromagnetic waves which is mediated by photons whic travel through vacuum , hence we feel sunlight and see it. so vacuum in this case would not insulate an object getting hot because it would be struck with high energy photons which would travel through vacuum.right?
Heat conduction primarily occurs by molecular (collisions) between higher kinetic energy molecules and lower kinetic energy molecules.
CWatters
#6
Sep17-13, 04:09 AM
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Quote Quote by Crazymechanic View Post
since you responded to my last post with only some links which I went through and were good , then I am going to ask , so in the case of vacuum as an heat insulator it only is useful when dealing with thermal conduction...
Correct. But depending on the situation that might be good enough.

...as it has to do with very close objects which surface electrons can interact hence vacuum has no electrons being between two such objects the interaction is lost so no heat conduction right?

as for heat radiation or heat transfer via electromagnetic waves which is mediated by photons whic travel through vacuum , hence we feel sunlight and see it. so vacuum in this case would not insulate an object getting hot because it would be struck with high energy photons which would travel through vacuum.right?
Correct.

If you look in a vacuum flask designed to keep coffee hot you will probably find the glass is also silvered to reflect radiated heat.

http://home.howstuffworks.com/thermos2.htm
sophiecentaur
#7
Sep17-13, 04:39 PM
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Quote Quote by Chestermiller View Post
Heat conduction primarily occurs by molecular (collisions) between higher kinetic energy molecules and lower kinetic energy molecules.
Conduction in gases and insulators can be described that way but thermal conduction in metals is due to the highly mobile electrons - just the same as for electrical conduction, in fact, but totally random. Metals are excellent thermal conductors compared with most other substances. Diamond is one exception.
Chestermiller
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Sep17-13, 05:42 PM
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Quote Quote by sophiecentaur View Post
Conduction in gases and insulators can be described that way but thermal conduction in metals is due to the highly mobile electrons - just the same as for electrical conduction, in fact, but totally random. Metals are excellent thermal conductors compared with most other substances. Diamond is one exception.
That's very interesting. What about liquids?
D H
#9
Sep17-13, 05:57 PM
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Quote Quote by CWatters View Post
If you look in a vacuum flask designed to keep coffee hot you will probably find the glass is also silvered to reflect radiated heat.
Reflecting incoming radiative energy keeps cold contents cold. To keep hot contents hot one needs to cut down on outgoing radiative energy. So what does that? A mirrored surface. Emissivity and reflectivity go hand in hand. A highly reflective surface has a low emissivity, and vice versa.
Crazymechanic
#10
Sep18-13, 03:41 AM
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As to what sophie said , that I guess is the reason why good electrical conductors (metals) are also good heat conductors.Gold and silver and copper being among the best.
Actually speaking about good conductors , silver was the best both for thermal and electricity.


Maybe someone can point out that if I had a thermos with a vacuum chamber inside and a silver plated side , and say I would have electromagnetic wave generating coffee , then how high frequencies the silver would reflect and what frequencies it would either let through or which would start to compromise the silver itself and heat it up.
I guess somewhere until infrared the silver would reflect but higher like x rays or gamma it would either change it' s structure , heat up etc ?
Drakkith
#11
Sep18-13, 03:59 AM
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The silver would absorb all frequencies to some extent. I think it's pointless to really discuss UV, X-Rays, and Gamma rays in this situation. You're pretty unlikely to find your coffee emitting those.
Crazymechanic
#12
Sep18-13, 04:54 AM
P: 853
Who said I have coffee in my thermos? :D:D Maybe I'm transporting plutonium and hiding it in a thermos.
Well it' s not about the thermos or coffee rather what happens to a material which reflects mostly at lower frequencies and does what I don' t know exactly at higher ones...
Drakkith
#13
Sep18-13, 05:03 AM
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Well, once you get into the X-Ray range, it doesn't really matter that much what the material is. It just starts to go right through everything, and if absorbed the radiation knocks electrons out of their orbitals and stuff. Super duper hugely high energy radiation can do weird things like create particle-antiparticle pairs when it interacts with matter.
Crazymechanic
#14
Sep18-13, 11:13 AM
P: 853
and hence antiparticles tend to annihilate quite fast then does that mean that not only the short wavelength radiation is destroying my silver shield slowly it also heats it up and makes a little bit of antimatter reactions in that material?
Integral
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Sep18-13, 12:51 PM
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Good lord man, lets stick to things that matter. You are interested in infrared, not high energy particles.

Think of it like this, in both conduction and radiative heat transfer we are concerned about the temperature differential. In your shiny thermos the liquid inside sees its own reflection in the sides of the container, since it sees itself it is seeing the same temp so heat transfer from both radiation and conduction is theoretically zero. Of course in the real world we cannot achieve 100% reflection so there is some heat loss ie it sees a slightly different temperature in the reflection. Any contamination of the surface increases the losses.
gmax137
#16
Sep18-13, 01:25 PM
P: 861
Quote Quote by Integral View Post
Good lord man, lets stick to things that matter. You are interested in infrared, not high energy particles...
Really! when we say "radiation heat transfer" that doesn't imply radiation like gamma and x-rays. The warmth you feel from the sun is basically blackbody radiation at 5,800 K (if I recall correctly), it isn't gamma energy you're feeling.
Crazymechanic
#17
Sep19-13, 04:50 AM
P: 853
I know it isn't gammas I'm feeling from the sun, cmoon :D Otherwise I would long don't feel anything at all....

Yes integral I am interested in low wavelength radiation when speaking about a thermos but I also wanted to think about what happened in sch or similar combination trying to contain something of a higher energy.
No need to get that excited.But thanks for the reply, it was also worthwhile.


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