Emissivity quest.

[DrRichand1]

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EK 1001 chem #318

An object is 10 degrees warmer than the room in which it sits. In which of the following circumstances will the object reach room temp the fastest?

A. The room is 0C and the object is white.
B. The room is 0C and the object is black.
C. The room is 10C and the object is white.
D. The room is 10C and the object is black.

Answer: D

can someone explain y in layman terms.

 

[MedPR]

The equation is

Where P = rate, sigma = constant, e=emissivity (1 for black, 0 for mirrors), and T is temperature. So the highest rate is when all of those are maximized.

Basically molecules move faster at higher temperatures (more kinetic energy) so that's why you want 10deg instead of 0deg. Emissivity is the ability to radiate heat. As you know, black absorbs and emits heat very well (hence not wearing black in the summer).

 

[chiddler]

i knew that black absorbs heat well, but why does it also emit heat well?

 

[MedPR]

i knew that black absorbs heat well, but why does it also emit heat well?

Conservation of energy I think. If your black shirt didn't emit all the heat it absorbed, it would melt or burn or something.

 

[Morsetlis]

At higher temperature, molecules are more energetic. Thus, at 10C and 20C, there is more energy for mvoement of molecules to collide into each other and equalize the temperature, compared to 0C and 10C.

Emissivity = 1 - Reflectivity. A white surface is reflective, so therefore a black surface is emissive.

It's not just in one direction. The more that could go in, the more it could emit. Think of a garden hose: it's a hole, that lets light is (is dark) and water out. If you plug the garden hose, light could not come in, and water could not come out. There no longer a communication between the world and the inside of the object.

 

[DrRichand1]

So how does this relate to people in the desert wearing black robes instead of white.

 

[MedPR]

So how does this relate to people in the desert wearing black robes instead of white.

No clue, never heard of that before.

Edit: I guess the inner layer of black absorbs and keeps their body temp constant (below the heat of the desert air) while the outer layer transfers heat somewhere else. Not sure though.

 

[chiddler]

So how does this relate to people in the desert wearing black robes instead of white.

if you're referring to arabs, they usually wear white.

 

[Morsetlis]

So how does this relate to people in the desert wearing black robes instead of white.

They're insane.

 

[MedPR]

i knew that black absorbs heat well, but why does it also emit heat well?

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/absrad.html#c1