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what is the highest temperature a 1 ft square piece of copper painted flat black can reach in direct sunlight.
1ft square piece of copper 1/16 inch thick in direct sunlight painted flat black can reach what temperature
what is highest temp and how can that be expressed in watts?
It depends how far away from the sun it is. Also depends what is happening on the other side of the plate.
Assume it is somewhere sunny in the tropics up high on a mountain (also no clouds, pointed at the sun , in a transpaent bell jar with the air pumped out and sat on a superinsulating brick. In this case it gets about 1200 watts per square meter of absorbed sunshine and radiates in a hemisphere into a background temperature of about 4K. (This isn't the best you could do though , you could put the bell jar at the bottom of a long hot tube for example or sit the square on top of a blast furnace - I assume you want to get an idea of the max temperature due to radiation exchange with a near natural environment).
Then look at:
http://www.powerfromthesun.net/Chapter6/Chapter6.htm#6.1.1%20Absorber%20Plate
to decide whether your flat black paint will be a good IR emmitter or bad ( you can get absorbtivity values of about 0.95 but high emmissivity or, slightly lower absorbtivity and low emmissivity in the infra-red)
Once you have the solar weighted emmissivity and absorbtance (http://www.alibaba.com/product-gs/202724506/Solar_Collector_flat_plate_heat_pipe_.html gives very good figures) you work out what the absolute temperature is to radiate 1200*Absorptivity watts into 4K from a body with low emmissivity -- using 90% absorb and 10% emit I get about 660K (about 390C) .
edit: temperature can't be expressed in watts. You want to know what temperature your plate needs to be at to radiate about 1080 watts per m^2 into a low temperature black body.
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