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Name: Glenn
Status: other
Grade: other
Location: Outside U.S.
Country: USA
Date: Summer 2012


Question:
I was reading an answered question about why an object looks black and understand the answer completely. My original question which lead me here in the first place is: If there exists a room, which has zero light but is at a very high temperature, and we place two objects (of identical size and material), one being black and the other being light in color, inside this room, do both exhibit Identical heat absorption? OR, to put another way: Is the radiation of light the single determinant factor for black objects absorbing more heat?

Replies:
Glenn, If the room has an atmosphere, and surfaces are identical except for color, heat entering through conduction and convection will be the same. Radiation will not be an initial part of absorption. As the objects heat up, radiation will be part of heat loss. If both objects react with radio waves, microwaves, and infrared waves the same, then they will emit these forms of radiation equally as the objects heat up. If hot enough to emit some visible light, then the black object will emit more, causing it to lose more energy. The black object may be a little bit cooler.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


Glenn

A comment of yours, sets an alarm off (for me, at least). You say, “… understand the answer completely.” Be very careful about understanding something “completely”. That may mean that you do not understand the answer completely, at all. Do not be too, too sure about making such a claim. You claim total understanding, but what about electromagnetic radiation that is outside the range (a very narrow range) of visible radiation? That is a very narrow sampling of electromagnetic radiation extending from radio waves and longer wavelengths, and electromagnetic radiation with wavelengths shorter than vacuum ultraviolet wavelengths and shorter?

At sufficiently short wavelengths the distinction between electromagnetic radiation and “particles” becomes diffuse. So that an electron, a proton, etc. are “just” short wavelength radiation. You need to think through your assumptions very carefully.

Vince Calder


Glenn,

Answering your question is a challenge. "Zero light but at high temperature" is a little distracting because the high temperature will have a wavelength(s) somewhere between 750 nm to about 10,000 nm. Radiative heat is an electromagnetic phenomenon. Of particular interest is the heat room(an oven). An oven at a constant temperature is not a dynamic system, it is at equilibrium.

Inside the heat saturated room, the black object will initially absorb the radiation faster than the white, which will be reflecting energy. However, the room and both objects will come to thermal equilibrium rather quickly through vibrational transfer of energy. Black objects, white objects and the oven will be hot. In the hot room, there is no way to demonstrate the effects of black vs white.

Radiative heat, in a dynamic(chaotic) environment will be preferentially absorbed by a black body as compared to a white body.

Hoping this helps. Peter E. Hughes, Ph.D. Milford, NH


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