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Re: [Phys-l] paint your roof?



At last! A note of appropriate skepticism....

If I do a 1 dimensional model of the topic in hand, I can use a series arrangement of two resistors. I will suppose that one end is connected to a voltage of 293 volts
and the other to a sink at 3 volts. If the equivalent electrical conductivity representing the roof to the ceiling is somewhat high compared to the sky equivalent conductivity, the potential will divide to provide a junction near 293V.
If the reflectivity of the roof coat is high, the equivalent resistance of the sky resistor is high. If the 4 volt end is now raised to several thousand volts, again the junction is not much higher than the 293 volt end.
If I want to draw some conclusion from this simple metaphor, I can suppose that a rough black gritty roof shingle surface provides a more "conductive" sky path (via radiation) so that it will be hotter in sunlight, and colder at night than a reflective roof. We have only heard that the reflective roofing material is colder than the ambient night air, and this should be true for the best reflectors AND the worst, surely?
(I didn't even get into the excellent thermal resistivity offered by white roof membrane which is thick and plastic, along with its relatively low thermal capacity. These properties would seem to have the effect of increasing both the ceiling resistance and the sky resistance and making the roof surface temperature more agile.)

Brian W

curtis osterhoudt wrote:
That's what I figured, but it makes a lot more sense to me in terms of dew and chemical potentials rather than *just* temperature equilibration. The effect happens all the time with camping under the open sky vs. tent, too, but I've never been very comfortable with the "radiating to space" explanation (though it may be totally right and my intuition fails me).
In short, I have a hard time seeing how metals (or whatever) can sense a nice big heat sink out there *somewhere*, and end up at a lower temperature than the surroundings. Do they continuously suck heat out of the air and re-radiate it to the vacuum? Taking that to the extreme, if I put a piece of metal into a "transparent" container and put it under vacuum, the metal would eventually reach the temperature of the background radiation in the universe, unless something is coupling to the metal (conductively, convectively, or radiatively) and warming it back up. It wouldn't just hold on to its heat energy. At which distance do I need to put an insulator to keep the communication from taking place? Around Earth's atmosphere? Outside the galaxy? If so, what relativistic effects contribute to the phenomenon?
Is it because conductors have such a near continuum of phonon and electronic modes that they can "talk" this way? Is the communication affected by the Casimir effect?

/************************************
Down with categorical imperative!
flutzpah@yahoo.com
************************************/




________________________________
From: "Edmiston, Mike" <edmiston@bluffton.edu>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, May 27, 2009 1:44:11 PM
Subject: Re: [Phys-l] paint your roof?

If it's not coupling (radiating) to the air, and not coupling to your
hand, what *is* it coupling to?

I reckon that would be "outer space." I suppose the membrane has high
radiance in infrared as well as visible.

Astronomer hobbyists know this also happens to telescope objectives and
that's why we need "dew shields," especially in a humid area like Ohio.
If I am out at night in a field or lawn, high-humidity air is rising
from the field. The objective lens is radiating heat to space and is
colder than the ambient air. As the humid air comes into contact with
the lens, it can condense and fog the lens. A dew shield helps channel
the air past the lens (which is recessed by the shield) and helps
prevent the condensation.

On numerous cold nights when the ambient air temperature was in the
forties, I had the corrector plate on my Schmidt-Cassegrain telescope
frost over, even with a dew shield. I finally made a heater with series
carbon-film resistors sewn into elastic fabric and stretched around the
perimeter of the corrector end of the scope. It only requires a couple
watts or so to drive it. Just need it warm enough to prevent
condensation.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l