2nd law question (Can Solar radiation be focused with mirrors to heat above Sun's temperature?)
ANSWER to your question below: Thermal radiation goes both ways, A to B and B to A (if body A sees B then B sees A too) and each radiates energy proportional to T^4. Net thermal radiation from sun A through mirrors to a body B will take place until TB=TA=5800K (or whatever it is) and then B will radiate back to sun A the same radiation (in ideal case of mirrors, etc.) and there will be no more energy exchange between A and B, they will come to the equilibrium (thus B cannot have higher temperature than the source A by thermal radiation heat transfer alone; only if there is another cold reservoir to produce work we may heat to higher temperature with Carnot part of original heat, see photocell below). Even in non-ideal case some equilibrium will establish (that is the 2nd Law), and body B, if in equilibrium, will radiate the same energy it receives (say from sun A), but it may not only radiate to the sun A but also in part (dissipate) elsewhere to its surroundings.
As for photocell, it will produce electricity (which is work) only if it is (maintained) colder than radiation source, up to the Carnot efficiency, and the 2nd Law is satisfied. You can make electricity from any two different-temperature thermal reservoirs (A and B) and heat any other body (C) at arbitrary low or high temperature, but the 2nd Law will be satisfied for the three isolated bodies (A, B, C), i.e., the total entropy will increase or stay the same in ideal case, and work potential within the three (isolated) bodies (A, B, and C) will ideally stay the same as original (Carnot work potential) or be reduced due to irreversibility. (Kostic, 4/12/2006; see more at: http://www.kostic.niu.edu/energy ).
I've got a question about the 2nd Law. It states heat does not spontaneously flow from cold to hot bodies . What about, say, magnifying solar power here on Earth with lenses/mirrors/whatever -- would the maximum temperature that one could achieve on Earth merely by focusing solar power be at most the surface temperature of the Sun (5800K, which is what the blackbody radiation curve of the Sun corresponds to)? I've heard, unreliably, on a few sites that this is the case, though no one indicates why.
(For example, this site claims that a large solar magnifier should attain "the surface temperature of the Sun).
This all makes sense from the POV of the 2nd law, but it seems like if you could concentrate, say, 100m^2 of solar energy (at 1kW/m^2) into a tiny area, you'd have an enormous amount of energy in a small area, and I don't understand why it would be limited by the temp. of the Sun. Also, it's obvious that you could convert the same light into electricity, and use that electricity to make temperatures as high as you want.
Could anyone shed some light onto this puzzle? Schmiddy 19:23, May 11, 2005 (UTC).
See ANSWER above.