radiation

[superduper12]

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is it better to paint your house black or white?

Ans: white is better. in summer your house is cooler than the environment and reflects away heat. in winter, your house is warmer than the environment and white radiates away less heat.

why does it radiate away less heat in winter...I don't understand.

 

[engineeredout]

A surface that absorbs the most radiation also releases the most radiation. This is why a black house will absorb the most heat and also release the most heat given the opportunity. The opposite will be true for a white surface. White will not absorb heat, and it will also not radiate heat.

The formula for radiative heat transfer is Q/A = theta*e*(Tx^4 - Ty^4) where theta is a constant, e = emissivity, and Tx is the higher temperature, and Ty is the lower temperature.

e ranges from 0 to 1. In ideal conditions it is assumed that a black body has an emissivity of 1 and a white body has emissivity 0. You'll notice that with the black body, since the maximum emissivity is 1, the black body has the greatest possible heat transfer. If Tx is higher than Ty heat will flow one direction, but if Ty is higher than Tx it will flow in the opposite direction.

Now use the white body with emissivity -> 0. It is never actually 0 but imagine it getting close enough that the heat transfer in either direction becomes negligable compared to the heat transfer with the black body.

 

[superduper12]

oh, i was trying to figure out what it had to winter and it losing less heat. I guess it's just white both absorbs and emits less heat....any time of the year.

 

[engineeredout]

oh, i was trying to figure out what it had to winter and it losing less heat.

No, the physical laws of the universe don't change just because the earth is suddenly in a different position in its orbit around the sun.

 

[SKation]

I was trying to understand the difference between radiation and reflection from this section of Examcrackers as well however I couldn't see exactly what the difference was between the two. If anybody could explain it that would be really appreciated 🙂