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From: colonel@gloria.UUCP (George Sicherman)
Newsgroups: net.physics
Subject: Re: black pots
Message-ID: <690@gloria.UUCP>
Date: Tue, 20-Nov-84 11:13:22 EST
Article-I.D.: gloria.690
Posted: Tue Nov 20 11:13:22 1984
Date-Received: Sun, 25-Nov-84 08:46:05 EST
References: <2559@dartvax.UUCP> <642@watdcsu.UUCP> <380@x.UUCP>
Organization: SUNY-Buffalo Confuser Science
Lines: 57

["... 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