Science Buddies: "Ask an Expert"

Hi. I have a math project (which is kind of like a science project). I need to make a parabolic (rather, parabolodial) solar cooker from cardboard and aluminum foil, and nothing else. Only wood for the base and adhesives. How do I make the parabolodial shape?

Thanks,
blueswim

Hi,

Here is a site that will allow you to construct parabola graphically:

http://jwilson.coe.uga.edu/EMAT6680/Dun ... nt6_KD.htm

Here is a book showing several techniques to construct parabolas

http://books.google.com/books?id=I2HdPn ... as&f=false

Here is a third technique:

http://mathdemos.gcsu.edu/mathdemos/conic_via_locus/

And yet another variant:

http://www.csames.illinois.edu/document ... n_Plan.pdf

And more:

http://www.atm.org.uk/resources/dynamic ... namic.html

Someshere in all of this I am sure you can find a method you like.

Have fun!

Best regards,

Barrett Tomlinson

You might also be interested in this related idea:

http://www.freepatentsonline.com/3186128.pdf

This paper may describe exactly the technique you need:

http://ashokk_3.tripod.com/srinivasan.htm

I think you have gotten my question wrong.
I'm fine on the graphing parabolas part.
I need help on building a parabolic/parabolodial solar cooker.
I read about the last site given but I can't access it, unfortunately.
All the sites I have seen so far don't result in true parabolic shapes.

Thanks,
blueswim

Hi

I checked the bottom link that describes how to cut a flat surface to form a parabolic surface, and the link works correctly. I have no idea why you can’t access it Here are some other links:

http://www.humboldt.edu/~ccat/solarcook ... on-pg1.htm

http://images2.wikia.nocookie.net/solar ... ctions.pdf

http://www.angelfire.com/80s/shobhapard ... sided.html

http://www.sunspot.org.uk/Solar1000/solar1000.htm

http://www.angelfire.com/80s/shobhapard ... ooker.html

http://solcooker.tripod.com/solar5.htm

http://solcooker.tripod.com/solar4.htm

Hope this helps.

Barrett Tomlinson

Thanks.
Would I need to get an equation for the parabola before making the paraboloidal solar cooker? (the equation of the parabola is the largest cross section of the paraboloid)
If not how do you derive an equation?

The purpose of the parabolic reflector in a solar cooker is to reflect as much solar energy and focus it on what you want to cook. If what you want to cook is larger than a "point", then the accuracy of how precise the reflector matches a paraboliod quickly diminshes and knowing the precise mathmatical equation really doesn't matter.

The fundamental physics properties that your reflector needs to consider are:
1) the incident angle equals the reflectance angle, and
2) the sun is a sufficiently long way away so that you can consider all of the energy waves coming from it to be roughly parallel, and
3) the largest dimension of what you want to cook

A practical design is going to require the area at the nominal focus point where the food is located has to be at least twice as large as the food. If the food blocks more than half of the incident waves at the focus plane the back side cooking power will be less than what you get in terms of front side cooking without a reflector.

Once the reflector is more than three focal distances deep, the amount of additional energy concentrated per square inch of reflector goes down significantly.

Hope this gets you thinking about the fundamentals involved.

Okay. How about if you have an equation of the line?
How would you build it with that equation?
thanks,
blueswim

blueswim wrote:Okay. How about if you have an equation of the line? How would you build it with that equation?

I would probably plot the parabola and its focal point on a piece of cardboard. If the size of the food to be cooked placed at the focal point is a appropriate for that parabola equation (see my earlier post on sizing hints), then I would cut out the inside of the parabola. I would then replicate that piece of cardboard and cut the replicas on their centerline (line that goes through the focal point and the minima). Using a square piece of cardboard as the base, stand the pieces up to form spokes alligning the centerlines vertically from the center of the cardboard base and spacing them equally around in a circle. These pieces are then used as a form for the reflector material to hold it in the shape of the paraboloid. I'm sure there are other ways. If you are careful in how you cut out the first piece, it can be used as a check piece inside the paraboliod for forming the reflector between the webs.