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Old 30th May 2002, 01:56 AM   #11
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Default Black-Body Radiation

Why does a black heat sink radiate better?
A black object is merely absorbing all radiation in the optical spectrum, correct? While a white object is reflecting optical radiation. The reason a black car is hotter in the sunlight is because it is absorbing energy, and not reflecting it away like a white car will. This doesn't sound like a good trait of a heat sink. Some of you may be getting confused with black-body radiation, which has nothing to do with color. Black-body radiation is a misnomer which merely explains the radiation given off of an object as a result of its temperature, not its color!
It is true that a painted black heatsink will radiate, but a shiny metal heatsink will not only radiate but reflect, thus reducing it's overall temperature.

Check me on this one:
http://www.phys.virginia.edu/CLASSES...radiation.html
This was the first site that I found after an internet search.

Sorry to be preachy,
Dan
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Old 30th May 2002, 02:04 AM   #12
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Default Easy !

A - Ceramic.

Eric.
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Old 30th May 2002, 08:02 AM   #13
Electrons are yellow and more is better!
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Quote:
Originally posted by peranders
Today question is: What material conducts heat very well but is a bad electrical conductor, in fact an isolator? The answer is revealed thursday!
Have you ever thought about daimonds?

Yes, it's true. Daimonds are the BEST heat conductor. It's a typical crossword question but I haven't got it from a trustworthy source. Anyone who knows better?
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
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Old 30th May 2002, 10:00 AM   #14
Geoff is offline Geoff  United Kingdom
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Ref Grey's comment above.

http://www.electro-optical.com/bb_ra...inum%20Samples

http://www.omega.com/literature/tran...sivitya.html#a

Geoff
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Old 30th May 2002, 07:27 PM   #15
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No, a black body is the optimal radiating color.

The principle of reciprocity is at work here; the ability to
absorb photonic heat is the same as the ability to radiate.

The black heat sink will get hotter in the sun, but the chrome
heat sink will run hotter when transistors are mounted to it.

In any case, this is not the main mechanism for ordinary
amplifier heat sinks not running in a vacuum. If they were
underwater, it *really* wouldn't matter.
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Old 30th May 2002, 08:25 PM   #16
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Nelson,
Sorry to be argumentative here, but this does not make sense to me. If a black car radiates heat just as much as it absorbs heat, then why is it hotter than a white car?

It seems to me that the car in the sun is an extreme case, though. One usually doesn't put his or her amp out in the sun. If one did, however, the black heatsink would definitely be hotter than a shiny one, much like the car example. But lets look at another extreme, two heatsinks, one black and one shiny, in complete darkness. What difference does the color make if there is no light to reflect off of the heatsinks. They would both have no color, except for their blackbody radiation (usually in the infrared). The equation for blackbody radiation is only dependent on temperature, and has nothing to do with paint. Hence, both heatsinks would give off the same amount of radiation.

So, what am I getting at?:

In bright light (sunlight) the black heatsink will be hotter.
In no light (darkness) both heatsinks would be the same temperature.

If you are right Nelson, then at some point in the middle (medium light) the black heatsink will have to radiate more heat than it absorbs, as opposed to the shiny one. What is the mechanism for this change? It does not seem clear to me.

I realize that radiative heat loss is not as productive as conductive heat loss and painting a heatsink black will not be too big of a problem. But, unless someone can explain it to me it seems to be worse (however slight) than a shiny heatsink.

I am shooting from my own intuition and a little bit of physics and I may be totally wrong. Can you spot an error or oversight in my logic?

-Dan
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Old 30th May 2002, 09:19 PM   #17
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The equation for blackbody radiation assumes a
black body, one in which there is no reflection.

painting a heat sink black is a better approximation
of a black body than chroming it.
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Old 30th May 2002, 11:00 PM   #18
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As usual, I'm on the run, so maybe I didn't catch the full drift, but Nelson is correct. Black both absorbs and radiates better.
Light and heat (i.e. infrared) are both part of the electromagnetic spectrum, they aren't separate things. Only the frequency changes.
Why is a black car hotter in the sun? Black absorbs more frequencies of light than white does; the albedo is lower. The light is absorbed and 'down-converted' into heat. White absorbs fewer frequencies (and less efficiently) and so receives less energy from the light. Bear in mind that cars aren't a good case, because you get greenhouse effects from all the glass, unpredictable ventillation depending on the surface area underneath (in the shade, that is), and so forth.
Anyway, put a black piece of metal in the sun and it will heat more quickly, but come dusk, it will also cool more quickly.

Grey
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Old 31st May 2002, 04:10 AM   #19
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Default paint=insulation?

Excuse my ignorance, but... painting a heatsink wouldn't nsulate it from contact with air and decrease its capacity to disipate heat?

Ric
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Old 31st May 2002, 05:22 AM   #20
JoeBob is offline JoeBob  Canada
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The black one is hotter, because it heats up more, but dissipates the same amount as the chrome car (ALMOST the same, just slightly more). Because almost all of the heat let off by the car isn't by radiation, but by convection. Radiation really doesn't account for much at all.

I'd think painting a heatsink black would hinder things, sure it would radiate more, but this doesn't account for much heat dissipation, but it would insulate the metal from the air, and well I doubt paint is a good heat conductor (maybe lead based paint? )...
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