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Physics/color and heat


I'm building a makeshift solar cooker using a "styrofoam" shipping box, a sheet of glass for the lid, and a cooking pot. I was planning to line the box with something reflective, such as a mylar emergency blanket, to direct more heat to the pot.

YouTube has videos on using solar cookers to melt beeswax. Some people painted the insides of the boxes black, on the theory that this retains more heat for the melting pot. That doesn't make sense to me. The pot is what needs to retain heat, not the lining.

Possibly I would paint the outside of the pot black, although that is not what they were suggesting. And I wouldn't paint the pot if it were Pyrex, on the assumption that clear glass would direct more heat to the contents.

What makes a more efficient cooker, a black lining or a reflective one?
What is a better pot for a solar cooker, dark metal or clear glass?

I'm not sure how a black "reflector" would get much heat to the cooker at all.  This guy had a great idea.  Easy and cheap.  Now, since any good science fair project has to actually answer a question, I would say your question is built right in here.  Which makes a better cooker, a black metal pot or a clear one that will let the sunlight right in to the interior?  The answer is not perfectly straightforward, since there are parts (infrared, UV, etc) of the spectrum that you may not be able to see, some of which will be partly/mostly absorbed by the glass.  However, the Sun's output peaks in the yellow area of the spectrum, so one would expect the best part of the spectrum to get straight to the food in the case of a clear cooker.  The other question is, how much heat goes up outside the metal pot due to the outside heating the air?  But metal pots that are black and have good thermal conductivity should transmit the heat inside relatively efficiently, so if you're cooking white rice (for example), you might be better off with a black metal pot.  If you're cooking black beans, maybe glass.  You'd have to test it to see which of these competing effects (you can measure the temperature of the air above the systems, also, though doing that outside might be difficult) makes one a better cooker than the other.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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