All,
This question took an interesting turn, so I went for an expert opinion from
my Physics professor. See the following:
Scott
>Good Morning Dr. Otto,
>
>I have been involved in a philosophical discussion concerning Infrared
>radiation spectrums. Some of the questions that have come up that nobody
>can seem to answer are as follows:
>
> 1) At what point does a substance quit emitting an infrared
>radiation? Absolute zero?
>
> 2) Do most substances emit IR radiation?
> I think that it is a relative thing based on the
surrounding
>of the object. In order to achieve equilibrium the object will either
>absorb or give of IR radiation (heat) based on the temperature differential
>between it and its surroundings.
>
>Any comments.
>
>Scott
Hi, Scott:
In answer to question 1): All matter emits blackbody radiation due to
thermal motion, at any temperature above absolute zero. However, the peak
wavelength is inversely proportional to temperature, given by (peak
wavelength in meters) x (temperature in Kelvin) = 2.898 x 10^3 m-K. So,
as the temperature gets smaller, the peak wavelength gets longer. At body
temperature, this peak is around 10 microns, in the Far Infrared. At
higher temperatures, the peak wavelength decreases until an appreciable
amount of light is in the visible region ("red-hot"). At lower
temperatures, the wavelength increases, into the microwave region (1 mm
waves at about 3 Kelvin), and eventually to radio wavelengths, in the
milli-Kelvin region. However, the amount of power emitted is proportional
to T^4, so as the temperature drops, the amount of power drops drastically.
Question 2): Every substance emits IR, although substances which are
"black" (good absorbers) are the best emitters. The "emissivity" is a
measure of how good an emitter a particular substance is. A perfect black
body has an emissivity of 1.0, whereas polished copper has an emissivity
of about 0.02, meaning that it emits only 2% as well as a black body.
You are correct in thinking that it is a relative thing, since the
surroundings are also emitting IR continuously. This is one of the heat
transfer mechanisms that help establish temperature equilibrium. To check
this, all you have to do is stand near a window (out of direct sunlight) on
a cold winter day. You can feel cold "radiating in" at you on the side
nearest the window, because your blackbody radiation is radiating away to
the outside world, and less radiation is radiating back at you from the
colder outside environment.
So, in a nutshell, the rate of heat transfer due to radiation is
approximately proportional to the temperature differential between the
object and its surroundings. This is known as "Newton's Law of Cooling".
Have Fun!
-- Dr. Otto
-----Original Message-----
From: Richard Tilbrook [mailto:[log in to unmask]]
Sent: Thursday, March 16, 2000 9:22 AM
To: [log in to unmask]
Subject: Re: [TN] VOC legislation - evolved to dark
I am not in any way en expert, but I would have thought that the sea emits
IR at a
low level, especially when compared to a human body. I would also have
thought that
a block of dry ice would emit a tiny amount of IR, possibly not detectable
with
current equipment. I would have thought that something like a low
temperature
superconductor, when in superconductive state, would emit no IR. Is this so?
Or
would something have to be at absolute zero to emit no IR?
Richard
[log in to unmask] on 16/03/2000 13:05:35
Please respond to [log in to unmask]@INTERNET@wtgw
To: [log in to unmask]@INTERNET@wtgw
cc:
Subject: Re: [TN] VOC legislation - evolved to dark
*** WARNING ***
This mail has originated outside your organization,
either from an external partner or the Global Internet.
Keep this in mind if you answer this message.
Nope... they wouldn't be dead.. The sea, even if very cold still has an IR
Signature itself... Proof, add an ice cube, do IR detection magnification,
you can sea the cube as colder... Even colder dry ice will give you better
differentiation.
Alain Savard
-----Original Message-----
From: Smith Russell MSM LAPO US [mailto:[log in to unmask]]
Sent: March 16, 2000 8:13 AM
To: [log in to unmask]
Subject: Re: [TN] VOC legislation - evolved to dark
Ah , to throw the proverbial Monkey Wrench into the works
Have you considered submergence in Water, Either Sea of Fresh?
Since IR transmission in Water is minimal , Aquatic plants and
animals exist in the ambient temperature. ( trying to raise body temperature
in such a heat sink would be a tremendous energy cost )
Since there is no discernible infrared signature from the aquatic
life,
then by the logic would they not be considered dead?
Russell Smith
Ciba Specialty Chemicals
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