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Name: Al A.
Status: Other
Age: 30s
Location: N/A
Country: N/A
Date: August 2004


Question:
Why are most radiators painted white, and not black?

Was asked on your site and the answer given was that a black radiator would make a better radiator than a white one.

However, I need to understand this further because to me the answer is ridiculous. Is my understanding of black completely wrong? - black must indicate an inability to radiate energy - its black like a black hole. Black holes can only radiate heat through the process of virtualparticles created at the event horizon - therefore they are not truly black either. By what process would my truly black radiator transfer energy to the air around it?

If a radiator is painted with black paint the pigment in the paint gives the radiator colour. Black paint after all is an illusion. Therefore the question can only be true if we use the words black paint rather than black colour (which in English is an oxymoron - black is not a colour)

Can you please let me know our thoughts on this as the topic has caused a major discussion in our office?



Replies:
Several points are being confounded here:

1. Is there data on what the dominant color of radiators is? (I assume you mean automobile radiators, not the ones in older houses.)

2. The color "black", and in the sense used in paints and coatings it IS considered a "color", has nothing to do with "black holes". In the case of "black holes" the term is used metaphorically.

3. The idea behind painting a radiator white, if in fact they are, is that white paint would reflect incident light making the radiator more efficient. However, in the "real world" you could paint it pink, orange or green, because the dominant mechanism of heat exchange of an automotive radiator and the atmosphere is convection not radiation. It just does not get hot enough to radiate a lot of heat by radiation. Fans are installed in most cars models to increase air flow to increase convection, a car will quickly overheat if the fan belt breaks, a car in stop-and-go traffic for a long time will overheat too.

4. "Black body" radiation does not depend upon the color of the body. If you heat a white bar or a black bar to sufficiently high temperature that it "glows" red, you get the same "glow" because that depends only on the temperature. At room temperature a "black body" is still "glowing" but it is "glowing" in the infrared, not the visible region of the electromagnetic spectrum so we cannot see the "glow" with our eyes.

Vince Calder



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