One could think of an object here on Earth as an antenna receiving energy
from the sun. Since the sun is transmitting about 4 × 10**26 watts, it is
not difficult to understand why that Al plate got so hot.
Not just think of, but they actually work that way.
I've forgotten the number of watts per square foot/meter/yard, but it's a
fair amount.
The reason a white object does not get as hot as a dark one is simply that
it reflects a large portion of light energy instead of absorbing it. Also,
an object with more mass stores more energy.
It stores more jouls, just as in capacitance, but capacity does not
translate into higher temperature. That is a function of the color. OTOH
that capacity represents energy so it takes longer to heat the massive
object to a stable temperature and it takes longer for the temperature to
come down. That is a good reason for concrete walls in solar heated homes.
Thermal stability. Surprisningly, Forrest Green seems to absorb more heat
than black.
Imagine the temperature of that huge Aluminum plate had it been painted
black or forrest green.
Even weating leather gloves I burnt my hands, but they provided enough
protection I was able to drop the edge of the plate before the burns were
serious. 32 square feet, 1" thick is 32/12=2.66 Cu feet of Aluminum.
What's that weigh?
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
> Translated into desired
properties of fiberglass used as guy lines: white, thin rod with high
temperature resistance.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Roger K8RI on Tower
Sent: Monday, May 02, 2005 10:35 PM
To: Tom Rauch; k3mm@comcast.net; towertalk@contesting.com
Subject: Re: [TowerTalk] polyrod
Just as a point of interest on color and heat.
In a glass covered solar heater using Forest Green as the color on the
heating element, if the water flow stops, I believe the resting temperature
of the element will be around 340 to 380 F.
I've seen bare aluminum plate 4' X 8' X 1" think (that's heavy!) that had
been left laying on the pavement in front of the shop where I used to work
get so hot that a cup of water thrown on it would hiss as it turned into
steam. It was like throwing water onto a hot frying pan.
Roger Halstead (K8RI and ARRL 40 year Life Member) N833R - World's oldest
Debonair CD-2 www.rogerhalstead.com
_______________________________________________
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_______________________________________________
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Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
_______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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