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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