Sorry Adrian, but I respectfully disagree with your perceived "flaw" in my
reasoning. (When did I post that, anyway? Don't remember doing it!) A
"blackbody" (one word) is a well-understood physics concept, and is defined
as an object which absorbs radiation at all wavelengths. Obviously, no real
material satisfies that theoretical criterion exactly (with the possible
exception black holes) but most materials come pretty close. Transparent
glass, for instance, which absorbs very little radiation in the visible
spectrum (but may do so in the infrared or UV spectrum), when it gets hot
enough (red hot, white hot, etc.) will radiate like crazy. Objects, like
graphite, which appear black, are absorbing visible light. They reradiate
the absorbed energy in the form of infrared radiation, which we can't see.
There's a famous and well-verified law of physics known as the
Stefan-Boltzmann law, which says that the power radiated by a hot object
varies as the fourth power of its temperature. Even the universe itself
satisfies the law of "blackbody radiation), even though the background
temperature of the universe is only four degrees Kelvin. None of this is to
discount the importance of convective cooling, or it's applicability in
real-life sitiuations, but that's a separate process and always exists in
parallel with radiative cooling. No matter how hot or cold something is,
it's always radiating its energy.
73,
Jim W8ZR
_____
From: Adrian [mailto:vk4tux@bigpond.com]
Sent: Monday, June 08, 2015 4:49 PM
To: Amps@contesting.com; 4cx250b@muohio.edu
Subject: Re: [Amps] Fan for SB220
The flaw in your argument is that the object does not remain black as it
heats to a point where it starts radiating
thermal radiation.
The fact that evacuated solar HWS tubes work so well at preventing heat loss
via air (replaced by a vacuum between the two glass envelopes, one within
the other)
proves that heat radiation from a black material is nonsense, as if so,
would cause heat loss via vacuum, for which only radiated heat can cross.
The original point is regarding a black surface collecting radiated heat and
re dissipating via convection, and not 'radiating', as many suggest.
radiated heat would pass through a surrounding vacuum, if there was one,
whereas convectional heat transfer will not, and convectional heat transfer
(air required)
is what occurs from a black surface dissipating heat within an amplifier
enclosure.
Adrian ... vk4tux
"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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