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Re: [Phys-L] Solar Constant Puzzle.



Interesting input! Though it is tempting to equate the radiated energy input to the convected output from the can's surface, Timothy is asserting that convection is not the only loss path from the can, and that reradiation may be "trapped" by an injudicious sun shade. It seems to me that a shade whose lower surface is not appreciably different from ambient air temperature, and which is arranged to redirect reradiated IR from the can, away from the can, would minimize this heating path. Perhaps a smallish sheet of foam polystyrene surfaced on both sides with aluminum foil, and canted so as to deflect rays from the can to one side would serve the purpose.

Brian Whatcott Altus OK Indian Territory.

On 5/27/2014 12:12 PM, Folkerts, Timothy J wrote:
Don't forget the 'greenhouse effect' (ie 'backradiation') in the calculations. There is a handy online calculator called MODTRAN (http://climatemodels.uchicago.edu/modtran/) for atmospheric IR radiation.

In particular, for 'Midlatitude Summer' looking up from Altitude=0, there is 310 W/m^2 of incident IR. If you block the sunlight with some sheet of material, that would be akin to adding a layer of clouds to the model. Low clouds (0.33 km) increase the backradiation to 393 W/m^2, or an extra 83 W/m^2. Direct noon sun is ~1000 W/m^2, so this correction is relatively small, but certainly not trivial (especially as the sun gets away from the zenith). Other details (like geometry, emissivity, temperature of the blocking sheet) would all make a difference.

Tim

(PS This is more of a general comment, not specifically addressing the most recent riddle).


________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] on behalf of rjensen@ualberta.ca [rjensen@ualberta.ca]
Sent: Tuesday, May 27, 2014 11:22 AM
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] Solar Constant Puzzle.

Sure, I'll bite.

I see two factors: heat flux and heat capacity.

Given identical exteriors in steady state, the heat flux is constant.
Thus the temperatures are constant.

The heat capacities of the solid cylinders is greater than the cans,
so the tin cans will reach steady state faster. The cans are more
responsive to changing solar irradiation.

Dr. Roy Jensen
(==========)-----------------------------------------¤
Lecturer, Chemistry
E5-33F, University of Alberta
780.248.1808




On Mon, 26 May 2014 09:57:56 -0500, you wrote:

Today's physics puzzle:

Let's suppose you wanted to do a determination of the Solar Constant
using a two cylinders of aluminum blackened with candle soot, one of
them shaded from direct sunlight and provided with an electrical heater.

And suppose I wanted to replicate your determination of the Solar
Constant using two empty tin cans of the same size and surface area.

Which cylinders would develop the higher surface temperature in the
steady state: the solid aluminum ones, or the empty blackened tin-cans?

Brian Whatcott Altus OK Indian Territory.