Recently, I found an exercise in which a graph was shown with the variation of temperature of two painted cans as they received sunlight. One of the questions was "Which of these two bodies will cool down faster?". I thought it was the white can and for my surprise i found out I was wrong. Can someone explain me why?
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sammy gerbil
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have a look here, emissivity and absorptivity https://en.wikipedia.org/wiki/Emissivity https://en.wikipedia.org/wiki/Emissivity#Absorptivity – anna v Jun 16 '18 at 14:18
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Thank you. Can I ask why does it exists this equivalence in emission and absorption (in terms of particles interactions)? – Ana Helena Vieira Jun 16 '18 at 14:28
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it is a long thermodynamic argument that has to do also with black body radiation see http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/absrad.html .black body is a quantum phenomenon . the argument in the link ends up also with photon arguments – anna v Jun 16 '18 at 15:20
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Your intuition is right. White paint has an emissivity that is very similar to that of black paint in the wavelength region where it matters - in the thermal infrared, with wavelengths around 10 um. So both radiate approximately the same amount, and the black absorbs more from visible light, so the black one stays warm longer. – Jun 28 '18 at 12:38
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Pieter, I'm sorry. I did not quite understand your explanation. It contradicts the info on the comments below. Can yoy clear your point to me? – Ana Helena Vieira Jun 28 '18 at 21:23