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Name: Al A.
Status: other
Age: 30s
Location: N/A 
Country: N/A
Date: 8/31/2004


Question:
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 virtual particles 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:
For normal temperatures the color of a heat radiator makes no difference so long as the color is not metallic, like silver.

In physics, people learn about "blackbodies." They learn that a blackbody absorbs 100% of the light striking it, and it also emits the theoretical maximum of heat energy.

They learn that a white object does not absorb or emit light as well. A perfect white body emits nor absorbs any radiation.

Only when one takes additional physics classes does one learn that the absorption and emission depend on the wavelength of the light. In the visible spectrum, black objects absorb light well, but white objects do not absorb light well. That is not true at other wavelengths.

The wavelength corresponding to hot (but not glowing) radiators is far into the infrared, with a wavelength of perhaps 5 micrometers. At this wavelength, all paint colors, including white, are "black" and emit and absorb heat well.

You will get very warm standing next to a hot black wood stove. You will get very warm standing next to a hot white wood stove too. But white stoves are difficult to keep clean.

In a hot climate, the best color to paint a roof so that it stays cool is white. The white color does not absorb much visible light from the sun, but the white color is also good at emitting heat when it gets hot.

In the ultraviolet light range, some metals are transparent, and in the infrared, some semimetals are transparent. Plastics and many types of glass are essentially "black" in the ultraviolet.

Silver paints are a special case. These pigments contain metal particles which have peculiar optical properties. They usually are poor absorbers or emitters of radiation, either optical or heat. So do not use silver paint on a radiator.

This has nothing to do with black holes.

Bob Erck


Dear Al,

If the zeroth law of thermodynamics is correct, there must be a relationship between the ability of an object to absorb heat and its ability to emit heat. The zeroth law says that all objects in thermal equilibrium are at the same temperature. If an object could absorb radiation and not emit any, it would gradually increase its temperature and so become warmer than its surroundings. This, incidentally, could solve our energy problems, but does NOT happen. For example a mirror reflects all the radiation striking it and absorbs none. Therefore a mirror cannot emit radiation.

Similarly, a white object reflects all wavelengths with equal efficiency (that's what makes it white), which means it does not absorb any wavelengths, and so must not emit any wavelengths (if it is perfectly white). On the other hand, a black body absorbs all wavelengths completely and does not reflect any (that is what makes it black), so it must emit all wavelengths equally efficiently.

The ideal black box consists of a large non-transparent container with a small hole in it. The hole is an almost perfect black body since any light striking it will enter the container and bounce around inside with very little chance of leaving the container (since the hole is so small compared to the size of the container). Obviously, if light is incident on the hole, the gas inside will warm up until as much radiation leaves through the small hole as enters it.

So black is a very good radiator precisely because it is a good absorber. A black hole, though black, actually does radiate energy, although not very much because its effective temperature is VERY low and radiation intensity goes like the fourth power of the temperature.

Best, Dick Plano



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