If I recall the classic calculation of blackbody radiation is done with a
hollow sphere with a tiny hole.
The inside of the sphere is perfectly reflective and energy that is in it is
captured except for the small bit
that escapes thru the hole. There is a calculation of all the oscillation modes
that exist in the sphere.
But that is all that I can recall. It is an exersize done in basic Quantium
Mechanics classes.
I acutally have a black body calibration source that is a graphite furnace
and temperature
controller. Used to calibrate IR detectors. I sometimes use it to calibrate
thermocouples that
I just poke thru the hole. It takes hours to stabilize. Got it cheap at a
hamfest. Along with some crystal impedance
meters.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Jim Garland [4cx250b@muohio.edu]
Sent: Sunday, December 19, 2010 9:59 PM
To: amps@contesting.com
Subject: Re: [Amps] Black Heat Shields
I've really enjoyed this thread. It's interesting how very practical issues,
such as how to cool an SB-220, eventually regress to questions about science
and basic physics. That's one of the things that makes this forum especially
interesting, in my opinion.
A couple of comments about the science in this threae. First, the notion of
a "black body" (usually written by physicists as one word: blackbody) is a
theoretical construct. There ain't no such thing. It is rather an imaginary
object which is assumed to be a perfect absorber of electromagnetic
radiation. (A graphite box is a pretty good approximation, but it's not
perfect.) A blackbody is also an ideal emitter of electromagnetic radiation,
which you can approximate by drilling a small hole in the graphite box to
let the radiation out.
Now, here is the first key point: a blackbody absorbs radiation of whatever
frequency impinges upon it (e.g., 20 meter CW, a laser, whatever), but it
does not emit the same frequency of radiation that it absorbs. Rather it
emits a broad spectrum of radiation known, naturally, as a blackbody
spectrum. If you were to look at this spectrum on a bandscope, it would look
like a big, broad lopsided peak that rolls off to zero both at high
frequencies and low frequencies.
The second key point is that the frequency at the top of the peak (and
details about the shape of the peak) are determined solely by the
temperature of the blackbody. Basically, what happens is that the blackbody
absorbs photons, which then become "thermalized" and turned into other
photons, which are emitted. Of course, since the blackbody spectrum is
determined only by its temperature, it doesn't really matter how it got hot.
It could be by absorbing EM radiation, or by friction, or convection, or by
chemical burning, or even a nuclear reaction.
(A quick digression. You've all heard about the 2.7K background temperature
of the universe, left over from the big bang. That temperature is determined
by measuring the cosmic background RF noise as function of frequency, and
then fitting the curve to the theoretical blackbody radiation curve. The
curve fits best when a temperature of 2.725K is used, and the agreement with
the curve is amazingly accurate. In fact, the object that comes closest to
being a perfect blackbody is the universe itself! Incidentally, the peak of
the curve occurs at about 160 GHz, but as the universe expands the peak will
shift downward. Who knows, in a few hundred billion years, it may wipe out
the 20 meter band.)
One final comment, for those wonder how something that absorbs radiation can
also emit radiation. Imagine what would happen if that didn't happen.
Suppose, for example, we take a graphite box and set it in outer space so
that the sun shines on it. In outer space, there's no convective heating or
cooling (let's ignore the solar wind, meteor dust, and other minor
complications), so the only way heat can enter or leave the graphite box is
by radiation. If the graphite didn't emit radiation, then it would keep
absorbing sunlight and get hotter and hotter. Its temperature would keep
rising, until it became white hot and then blue hot, and then finally so hot
the carbon atoms would fuse and it would explode like a hydrogen bomb (in
this case a carbon bomb). Of course, that doesn't happen and the reason is
that the graphite box eventually reaches thermal equilibrium; the energy it
absorbs exactly equals the energy that it emits. If we replace the graphite
with, say, a block of wood, then the same thing will happen. But because
wood isn't as good an absorber of radiation as graphite, it won't get as hot
when it reaches equilibrium. In other words its equilibrium temperature will
be lower. And, if we put a glass mirror in outer space, it won't heat up
much at all, because it will absorb only tiny bit of energy. However, even
that tiny bit gets reradiated, and if one measures the spectrum of the
radiated energy, it will look very much like a blackbody spectrum. So the
bottom line is that all things absorb radiation to some degree, thermalize
it, and reradiate it back out. A theoretical blackbody does this perfectly,
everything else less so, to varying degrees, but the basic concept is the
same. And it's a good thing Mother Nature works this way, because if she
didn't the universe as we know it wouldn't exist and we wouldn't be having
this conversation.
End of lecture. Tnx for the bandwidth!
73.
Jim W8ZR
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