colonel@gloria.UUCP (George Sicherman) (11/20/84)
["... and you know what happens with a Black Pot." --John Lennon] This appeared in net.cooks. Any comment from you in net.physics? > > I still can't believe the ignorance prevailing in journalism today! > > For someone to draw the conclusion that since black cars get hot in > > the sun, then black pots must radiate lots of waste heat is > > incredulous! This person should have at least consulted a basic > > physics text. > > WRONGO, CONS BREATH! > > My basic physics text: > "Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles" by > Robert Eisberg and Robert Resnick, (c) 1974 John Wiley and Sons, New York. > @begin(quote) > Example 1-2, page 7 (boy, this has to be basic): > > Assume we have two small opaque bodies a large distance from one another > supported by fine threads in a large evacuated enclosure whose walls are > opaque and kept at a constant temperature. IN SUCH A CASE THE BODIES AND > WALLS CAN EXCHANGE ENERGY ONLY BY MEANS OF RADIATION. Let (e) represent > the rate of emission of radiant energy by a body and let (a) represent > the rate of absorption of radiant energy by a body. Show that at equilibrium > > e1 e2 > ---- = ---- = 1 (1-5) > a1 a2 > > This relation, (1-5), is known as "Kirchoff's law for radiation." > Proof: > The equilibrium state is one of constant temperature throughout the enclosed > system, and in that state the emission rate necessarily equals the absorption > rate for each body. Hence > > e1 = a1 and e2 = a2 > Therefore: > e1 e2 > ---- = 1 = ---- > a1 a2 > > If one body, say body 2, is a blackbody, then a2 > a1 because a blackbody is > a better absorber than a non-blackbody. HENCE, IT FOLLOWS FROM (1-5) THAT > e2 > e1. THE OBSERVED FACT THAT GOOD ABSORBERS ARE ALSO GOOD EMITTERS IS THUS > PREDICTED BY KIRCHOFF'S LAW. > @end(quote) > > Emphasis mine. > > Good absorbers of radiant energy are good emitters of radiant energy. > > John Woods > B.S. Physics, MIT 1982 > B.S. Electrical Engineering/Computer Science, MIT 1982 -- Col. G. L. Sicherman ...seismo!rochester!rocksanne!rocksvax!sunybcs!gloria!colonel
colonel@gloria.UUCP (George Sicherman) (11/21/84)
["... and you know what happens with a Black Pot." --John Lennon] Sorry for trying to pass the buck. I don't hold a B.S. in physics from anywhere, but I can at least attempt to criticize this: > > I still can't believe the ignorance prevailing in journalism today! > > For someone to draw the conclusion that since black cars get hot in > > the sun, then black pots must radiate lots of waste heat is > > incredulous! This person should have at least consulted a basic > > physics text. > WRONGO, CONS BREATH! > My basic physics text: > "Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles" by > Robert Eisberg and Robert Resnick, (c) 1974 John Wiley and Sons, New York. > Example 1-2, page 7 (boy, this has to be basic): > Assume we have two small opaque bodies a large distance from > one another supported by fine threads in a large evacuated > enclosure whose walls are opaque and kept at a constant > temperature. IN SUCH A CASE THE BODIES AND WALLS CAN EXCHANGE > ENERGY ONLY BY MEANS OF RADIATION. Let (e) represent the rate > of emission of radiant energy by a body and let (a) represent > the rate of absorption of radiant energy by a body. Show that > at equilibrium > > e1 e2 > ---- = ---- = 1 (1-5) > a1 a2 > > This relation, (1-5), is known as "Kirchoff's law for radiation." > Proof: > The equilibrium state is one of constant temperature throughout > the enclosed system, and in that state the emission rate > necessarily equals the absorption rate for each body. Hence > > e1 = a1 and e2 = a2 > Therefore: > e1 e2 > ---- = 1 = ---- > a1 a2 > > If one body, say body 2, is a blackbody, then a2 > a1 because a > blackbody is a better absorber than a non-blackbody. HENCE, IT > FOLLOWS FROM (1-5) THAT e2 > e1. THE OBSERVED FACT THAT GOOD > ABSORBERS ARE ALSO GOOD EMITTERS IS THUS PREDICTED BY > KIRCHOFF'S LAW. > > Good absorbers of radiant energy are good emitters of radiant energy. > > John Woods > B.S. Physics, MIT 1982 > B.S. Electrical Engineering/Computer Science, MIT 1982 The writer makes two assumptions that I find doubtful: 1. The pot is at equilibrium. In practice this means that the pot is at room temperature. If so, the contents have probably congealed, and nobody will want to eat them. 2. The kitchen walls are illuminated only by the pot. Only trolls cook in the dark! And here are two of my own conclusions: 1. The question of "wasted heat" is imprecise. The contents of any vessel that is not a perfect insulator will eventually reach room temperature, dissipating ALL its excess heat. The cook's concern is how fast the heat is radiated. 2. Place a black pot and a white pot in sunlight and let their temperatures stabilize. The black pot will be absorbing energy faster, so by Kirchoff's Law it will be radiating it faster. But this does not mean that the black pot will be cooler! On the contrary, its equilibrium temperature will be higher than that of the white pot. This is borne out by everyday experience. By the same token, this will hold true in any kitchen with its own source of radiation. -- Col. G. L. Sicherman ...seismo!rochester!rocksanne!rocksvax!sunybcs!gloria!colonel
brian@digi-g.UUCP (Merlyn Leroy <Brian Westley>) (11/26/84)
<zzzzzzinggggggggg>
What's all this about black pots? I would think that pots get their
heat from thermal induction, not radiation, so the pot that heats up the
fastest would be a good conductor of heat and a slow radiator.
A lid helps, too.
Merlyn Leroy
"No funny quotes"david@bragvax.UUCP (David DiGiacomo) (11/27/84)
In article <695@gloria.UUCP> colonel@gloria.UUCP (George Sicherman) writes: >> [lots of stuff] >Only trolls cook in the dark! The rest of us leave the refrigerator door ajar... >And here are two of my own conclusions: > >1. The question of "wasted heat" is imprecise. Since my conapt is equipped with resistance heating, I avoid this issue by cooking only in the winter. The rest of the year I eat out or barbecue (I don't know how to save partially used charcoal anyway). -- Welcome to Usenet -- now go home David DiGiacomo, BRAG Systems Inc., San Mateo CA (415) 342-3963 (...decvax!ucbvax!hplabs!bragvax!david)