Dark body vs light body and heat emission

[lomez]

So, I've had several people insist that a dark colored object will radiate heat faster than an identical light colored body. I do not believe this is the case. We know that dark bodies ABSORB more short wave than light colors, and thus become hotter as a result...but that's not the issue. Let's say I take two identical in every way objects, except one is black and one is white. I heat them both to exactly 200 c. and place them both in different rooms which are 0 degrees c. Which will cool down (radiate IR) faster? I say neither, and that both will cool down at exactly the same rate. I don't see how the color of an object would affect its RADIATION of IR.?

If anyone disagrees...please explain the mechanism by which more energy will radiate from either one. Now remember, this is assuming a perfectly dark room. If it were not totally dark, I would expect the WHITE object to cool off faster, because the black object would be also simultaneously absorbing more em than the white colored one.

Either way, I see zero chance that the black object cools faster.

 

[Low-Q]

So, I've had several people insist that a dark colored object will radiate heat faster than an identical light colored body. I do not believe this is the case. We know that dark bodies ABSORB more than light...but that's not the issue. Let's say I take two identical in every way objects, except one is black and one is white. I heat them both to exactly 200 c. and place them both in different rooms which are 0 degrees c. Which will cool down faster? I say neither, and that both will cool down at exactly the same rate.

If anyone disagrees...please explain the mechanism by which more energy will radiate from either one. Now remember, this is assuming a perfectly dark room. If it were not, I would expect the LIGHT object to cool off faster, because it is not being slowed by absorbing as much em energy as the black object.

Either way, I see zero chance that the black object cools faster.

I assume both rooms are equally insolated, and that shape and volume are the same, also that the objects are located at similar spots in those rooms.

I would agree with you that both bodies might cool down equally. However, if these bodies had the same color, but covered with equal blankets with different colors, black and white, I would assume that the body with black blanket cools faster.

However, electric motors that is painted black runs cooler than an equal motor with white color.
The black motor feels hotter on the outside after a race, but the coils inside seems to be cooler.

 

[Andy Resnick]

Absorption = emission, so that answers one of your questions.

Also, you do realize that the appearance (absorbance/emissivity) of an object in the visible waveband is not generally correlated with the object's emissivity in other wavebands? For example, snow is black in the IR band (say, 2-20 microns:

http://www.icess.ucsb.edu/modis/EMIS/images/snowmam01.gif.

 

[Drakkith]

There's a bit of a complication here. The visible color of an object is related to how well it absorbs or emits visible light, but not the rest of the EM spectrum. A dark colored object will emit/absorb more visible light than a light colored one, but it may emit/absorb less radiation in other wavelengths. For example, a black trash bag is transparent to part of the IR band, so if much of the radiation falling on the trash bag is in that part of the spectrum, then a white object can absorb more total energy than the trash bag does.

 

[sardar]

I would like to clarify that the color of an object does not directly affect its rate of heat emission or cooling. This is because heat emission is primarily determined by the temperature of an object and its surface properties such as emissivity, not its color.

Emissivity is a measure of an object's ability to emit thermal radiation, and it is not affected by the color of the object. In fact, objects of different colors can have the same emissivity if they have similar surface properties.

In the scenario described, both the black and white objects are heated to the same temperature and placed in rooms with the same temperature. Therefore, both objects will cool down at the same rate, as they have the same temperature and surface properties.

It is true that darker objects tend to absorb more shortwave radiation and become hotter, but this does not automatically mean they will also radiate more heat. The rate of heat emission depends on the temperature and surface properties of an object, not just its color.

In a perfectly dark room, both objects will cool down at the same rate because there is no external source of radiation to affect their temperature. In a non-dark room, the white object may cool down slightly faster due to its ability to reflect some of the incoming radiation, but this effect would be negligible.

In conclusion, the color of an object does not have a significant impact on its rate of heat emission or cooling. Other factors such as temperature and surface properties play a much more significant role in determining these processes.