Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site x.UUCP Path: utzoo!linus!decvax!genrad!mit-eddie!cybvax0!frog!x!john From: john@x.UUCP (John Woods) Newsgroups: net.cooks,net.flame Subject: Re: Truth in Advertising? Message-ID: <380@x.UUCP> Date: Thu, 15-Nov-84 11:31:10 EST Article-I.D.: x.380 Posted: Thu Nov 15 11:31:10 1984 Date-Received: Sat, 17-Nov-84 04:05:32 EST References: <2559@dartvax.UUCP> <642@watdcsu.UUCP> Organization: Charles River Data Systems, Framingham MA Lines: 94 > < Namin nami nami nami ... > > 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. > Obviously, black absorbs the most heat, as it does not *reflect* light > rays of any wavelength. On the other hand, a silver car will reflect > most light rays (waves? particles?). > On the other hand, an object of colour x can only radiate light (read: > heat) of colour x (i.e. that wavelength). Therefore, a 100% black pot > will radiate *no heat* at all. It will be warm to the touch, though. > However, it is impossible to make a 100% black pot (if it was 100% > black, it would absorb all the light and you wouldn't be able to see > it!), but an almost-black pot is still the best way to go. 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. The only radiation that comes from a blackbody is featureless thermal radiation, which is why it is a "blackbody". Try the following experiment: Build a "cavity blackbody" as follows: Take a cubic hunk of something that won't melt and hollow it out, leaving a small hole in one side (just barely large enough to see in), viz: -------- The hole represents your blackbody. Look into it. | | It should be quite dark (if not, you made it too | large). It helps to paint the inside black, but it | | is not necessary if the hole is small enough. -------- Place it in a high temperature oven and heat it till it glows red (I didn't say it would be easy, did I). Look at the hole. It ought to be glowing red. However, you won't see the inside of the box, you'll only see a featureless red glow. If you can't perform this experiment, don't worry, it was done on a NOVA program a couple of years back. I suggest that you sit down and watch every NOVA program ever made, continuously, until you come across this show. Maybe you'll learn something... "This person" who did that commercial wins a cookie from me for being correct. John Woods B.S. Physics, MIT 1982 B.S. Electrical Engineering/Computer Science, MIT 1982 [ Whether or not the above opinions reflect those of my employer has no bearing upon the radiation of black bodies or the motion of the Earth.] -- John Woods, Charles River Data Systems, Framingham MA, (617) 626-1114 ...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc@MIT-XX.ARPA If your puppy goes off in the next room, is it because of the explosive charge? [y][n]