How could heat be concentrated ?

[Externet]

As light can be focused to a small area with the use of a lens, how could heat -say from a household radiator- be concentrated into a smaller area ?

 

As to focusing invisible heat from a large source to raise temperature of a smaller body-

Miguel

[cameron marical]

Well, the only {not very efficient} way that I can think of is use the heat to make light, and then have that light input directly to a laser, and the beam of photons from the laser would heat whatever they were hitting, but as said, that is not very efficient.

[abskebabs]

If I understand your question correctly, I guess it is possible to concentrate heat with the use of a lens, or possibly with some kind of radiating construction approximating a blackbody surrounding the object you'd like to heat up. I guess this body could be insulated from the outside, with some convenient material. I guess the blackbody could be heated with some kind of energy source from the "inside", perhaps a chemical reaction or something of the sort, lor perhaps an electrical plug.

 

The problem with all kinds of schemes is you have to take into account the second law, which tends to discourage all such constructions that "concentrate" heat, and ones that do would have a large entropy cost associated with them.

[cameron marical]

Second Law of what?

[iNow]

Thermodynamics... Think "entropy."

[swansont]

As light can be focused to a small area with the use of a lens, how could heat -say from a household radiator- be concentrated into a smaller area ?

 

As to focusing invisible heat from a large source to raise temperature of a smaller body-

Miguel

 

[KingArthur] A duct. [/KingArthur]

 

i.e. physically channel the hot air.

 

However, it depends on what you mean by "concentrate." If the radiator is at 350K, you won't get air that's hotter than that, you'll just get more of it. But that's true of a lens system, too — you can't get a spot hotter than the source (Brightness theorem). If you could, you would be able to spontaneously transfer heat from a cold object to a hot one, in violation of the already-mentioned second law of thermodynamics.

 

If you want to get something hotter, you have to use your energy differently and come up with a better heat source.

[Externet]

Let me try another wording/example.

 

If a person is at a focus of an elliptical mirror; would the body heat reach the other focus and warm up a little object placed there to same or more than 37 C ?

 

As a lens and light. The light gets brighter at the focal point of a lens because all rays merge there.

 

If radiated heat behaves as radiated light but in another wavelenght, how to harness/route/handle that heat radiation into a 'spot' which will get hotter than the heat source?

 

Miguel

[proton]

As light can be focused to a small area with the use of a lens, how could heat -say from a household radiator- be concentrated into a smaller area ?

 

As to focusing invisible heat from a large source to raise temperature of a smaller body-

Miguel

Heat is defined as the energy transferred between a system and its environment as a consequence of a temperature difference between them. I don't think that's what you're referring to here. Perhaps you're wondering how to focus the thermal energy from the radiator, correct? If I recall correctly, most of the thermal energy comming from a radiator is blackbody radiation, most of which is IR radiation in this case. So if you know of a way to focus IR radiation then you've accomplished your task.

 

In the present case there is no problem with the second law of thermodynamics because the energy flow is not in the form of heat since the flow is not due to a difference in temperature.

[swansont]

 

If radiated heat behaves as radiated light but in another wavelenght, how to harness/route/handle that heat radiation into a 'spot' which will get hotter than the heat source?

 

 

No, as I mentioned before, you can't do this. You would then be spontaneously transferring thermal energy from cold to hot. The entropy cops will be unhappy.

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Merged post follows:

Consecutive posts merged

Heat is defined as the energy transferred between a system and its environment as a consequence of a temperature difference between them. I don't think that's what you're referring to here. Perhaps you're wondering how to focus the thermal energy from the radiator, correct? If I recall correctly, most of the thermal energy comming from a radiator is blackbody radiation, most of which is IR radiation in this case. So if you know of a way to focus IR radiation then you've accomplished your task.

 

In the present case there is no problem with the second law of thermodynamics because the energy flow is not in the form of heat since the flow is not due to a difference in temperature.

 

How is the blackbody radiation from the radiator not heat flow from a difference in temperature?

[proton]

How is the blackbody radiation from the radiator not heat flow from a difference in temperature?

Perhaps you're right.

 

In any case focusing IR radiation won't violate the second law of thermodynamics. For example; heat a body such that it becomes white hot. Now use a lens to focus some of the optical light comming off the body, like focusing the light from the blackbody we call the sun. How is this violating the second law of thermodynamics? We can't focus the light from the sun to heat a body to a temperature higher than the sun.

Edited June 25, 2009 by proton

[insane_alien]

the key is not the focusing but transferring enrgy from cold to hot.

 

you can focus the IR light no bother but you can't get it hotter than the temperature of the source momatter how much focusing you do.

[proton]

(never mind)

Edited June 25, 2009 by proton
changed my mind

[CaptainPanic]

The normal way to focus heat is a "heat pump", which uses the same principles as your fridge. However, a heat pump has absolutely nothing to do with focusing IR.

 

About concentrating IR radiation, some questions:

1. What type of solid does NOT absorb infrared? Because if we want to build some IR lens, it should not absorb IR. But I am no expert in this field.

2. Does IR radiation bend in a lens (if it's not absorbed) just like ordinary light? We need to be able to bend it at least a bit in order to focus it.

[swansont]

About concentrating IR radiation, some questions:

1. What type of solid does NOT absorb infrared? Because if we want to build some IR lens, it should not absorb IR. But I am no expert in this field.

2. Does IR radiation bend in a lens (if it's not absorbed) just like ordinary light? We need to be able to bend it at least a bit in order to focus it.

 

Various types of glass transmit IR to some extent, as do other transparent media like sapphire. IR is EM radiation, so it will bend. However, the index of refraction is typically wavelength-dependent, so the focal length will be different.

[CaptainPanic]

Various types of glass transmit IR to some extent, as do other transparent media like sapphire. IR is EM radiation, so it will bend. However, the index of refraction is typically wavelength-dependent, so the focal length will be different.

 

Is that transmit as in: "absorb and emit" or as in "allow to pass through"?

 

If it is the second one, should be possible to make a lens for IR? With a focal point? Wouldn't that be what was asked in the 1st post?

 

Somehow there seems something wrong with the concept (putting all practical issues aside - looking only at theory: thermodynamics and optics). Because every object emits IR radiation, with an IR-lens it should be possible to always heat something up, regardless of the orientation of the lens or the temperature of the IR emitter.

[swansont]

Is that transmit as in: "absorb and emit" or as in "allow to pass through"?

 

Allow to pass through (if we're thinking classically). i.e. no resonant absorptions.

 

 

If it is the second one, should be possible to make a lens for IR? With a focal point? Wouldn't that be what was asked in the 1st post?

 

Well, IR is a large range of wavelengths, and so you run into the same problem, but you don't need to focus the light down to its minimum radius to concentrate the light if the target is large enough.

 

Somehow there seems something wrong with the concept (putting all practical issues aside - looking only at theory: thermodynamics and optics). Because every object emits IR radiation, with an IR-lens it should be possible to always heat something up, regardless of the orientation of the lens or the temperature of the IR emitter.

 

Ah, but the target emits IR, too, and the lens works both ways. The hotter the target, the more it emits. If the target is hotter, it's sending more energy back to the cooler object. You can't win. The best you can do is an equilibrium where they are at the same temperature.

[Externet]

OK.

Let's say that infrared lenses do exist. Where in the spectrum heat falls, am unsure if those lenses be functional:

 

http://www.3dlens.com/infraredfresnellens.htm

 

http://www.thorlabs.com/navigation.cfm?Guide_ID=132&gclid=CMbxi9fwr5sCFSIuagodb1nWCg

 

A pot boiling water on your stove top, 20cm size, 100 C.

If an infrared lens is placed in front of the pot, its 'projected' infrared focused point to a 1 cm spot would never be over 100 C, as I understand from what has been explained.

 

Miguel

Edited June 29, 2009 by Externet

[Sisyphus]

OK.

Let's say that infrared lenses do exist. Where in the spectrum heat falls,

 

Heat does not have a fixed place in the spectrum. Infrared is associated with heat only because most things radiate primarily in infrared around the sorts of temperatures that don't kill us. Lightbulbs, having very hot filaments, give off a lot of visible light. Even human beings give off a very, very small amount of visible light.

[swansont]

A pot boiling water on your stove top, 20cm size, 100 C.

If an infrared lens is placed in front of the pot, its 'projected' infrared focused point to a 1 cm spot would never be over 100 C, as I understand from what has been explained.

 

Miguel

 

You got it. If you want to heat something hotter, you can't limit yourself to "passive" thermodynamics (i.e. letting thermal energy move around on its own). You have to convert other forms of energy into thermal energy.

Edited June 29, 2009 by swansont

[CaptainPanic]

Ah, but the target emits IR, too, and the lens works both ways. The hotter the target, the more it emits. If the target is hotter, it's sending more energy back to the cooler object. You can't win. The best you can do is an equilibrium where they are at the same temperature.

Assumptions

For the sake of the discussion, let's assume the perfect IR lens to exist, and to work at 100% efficiency (meaning no absorption and only transmission (as in: allow the IR to pass through)).

 

Let's also assume two bodies: A and B.

A is significantly larger than B.

A is hotter than B at t = 0.

 

The lens is designed and positioned such that it will focus all IR from A, and focus that on B. It therefore focuses IR from a large area on a smaller area. And at the same time, of course, IR from B will diverge and hit A.

 

We're also assuming that (somehow) no heat is lost to anywhere else (all IR emitted from A or B will pass through the lens).

 

Reasoning

The lens will now catch more IR from A than from B.

This will cause B to heat up. This in turn increases the IR radiation back from B to A.

 

Radiation (or better: IR-flux) will only be equal in both directions if a similar amount of IR originates from B as from A.

 

That is only possible if the temperature on B is higher than A.

 

Problem

Now, I also see that this goes straight against all I learned about thermodynamics. But I fail to find the flaw in the reasoning. There should be something fundamentally wrong here (not just the fact that a perfect IR lens does not exist, or that perfect insulation does not exist).

[swansont]

If A and B are the same size, then the IR flux will be equal when they are at the same temperature. B will never be hotter than A. That is fully compliant with thermodynamics.

 

If A and B are different sizes, then you have to worry about other restrictions — you can't focus light down to an arbitrarily small spot, and you can't capture all of the radiation. But you end up with the same result that B will never be hotter than A.

[CaptainPanic]

If A and B are the same size, then the IR flux will be equal when they are at the same temperature. B will never be hotter than A. That is fully compliant with thermodynamics.

 

If A and B are different sizes, then you have to worry about other restrictions — you can't focus light down to an arbitrarily small spot, and you can't capture all of the radiation. But you end up with the same result that B will never be hotter than A.

 

I'm afraid that I didn't understand your 2nd paragraph.

Why can't you concentrate light down to an arbitrary small spot using a lens? I mean, that's what lenses do, right? They either concentrate light, or diverge... wikipedia lists all the options here.

 

I know that capturing all the radiation is impossible in practice, which is why I suggested to look at a simple theoretical (fictional) case.

 

I agree that B cannot be hotter than A, but I still haven't understood exactly where my reasoning goes wrong.

[swansont]

I'm afraid that I didn't understand your 2nd paragraph.

Why can't you concentrate light down to an arbitrary small spot using a lens? I mean, that's what lenses do, right? They either concentrate light, or diverge... wikipedia lists all the options here.

 

Yes, lenses can concentrate light, but the profile at the focus is a Fourier transform of the profile as it hits the lens. The profile which corresponds to an infinitely small spot is a plane wave, which is the ideal case used in approximations, but in reality cannot be achieved. Many beams are described by a Gaussian profile, which gives you a finite-sized spot.

 

You can exploit this if you want to spatially filter the light by filtering out higher-order modes. You put a pinhole at the focus, and get a nice zero-order Gaussian beam output (or something a lot closer to that)

 

http://en.wikipedia.org/wiki/Spatial_filter

 

All of that assumes one frequency. You have the additional problem of chromatic aberration — the focus will depend on the wavelength, so any broadband IR will have different focal points for different wavelengths. Another effect that will prevent an arbitrarily small spot.

[CaptainPanic]

Hmm...

 

I still would like to see a better explanation... but hopefully we're getting close I'm feeling a bit stubborn because I keep asking, but I honestly still don't understand why a 2nd object cannot become warmer than another object that radiates photons (IR, light, whatever radiation), when you use a lens.

 

The radiation originating from any object includes, as was already mentioned, pretty much all wavelengths. Let's assume a perfect black body (see: graph), which does radiate all wavelengths.

 

I still don't see why with a perfect lens you cannot concentrate the light so much that it will heat up an object to a temperature higher than the source of the photons. Just focus it on a really really tiny spot. The flux of photons should become so great that a higher temperature can be achieved?

 

I understand perfectly that the energy balance will not be breaking the 2nd law of thermodynamics. The total energy of all photons originating from the source of photons will always be greater than on the "really really tiny spot".

 

But why wouldn't it reach a higher temperature? That does not necessarily go against the 2nd law (look at heat pumps and your fridge: it's possible).

 

I think that for determining the temperature that is achieved somewhere we must look at the flux (photons / receiving surface area) and the wavelength. This flux can be concentrated immensely using lenses.

[insane_alien]

That does not necessarily go against the 2nd law (look at heat pumps and your fridge: it's possible).

 

that is possible because energy is put into the system in the case of heatpumps(entropy is increased elsewhere in order for a local decrease in entropy)