I have always been fascinated with the conic sections. They are what makes mathematics so interesting to me. A shape that bends and focuses the invisible. They are almost magical. And to some that do not understand the fundamentals behind them could be construed as some type of witchcraft.
We are done with state testing and I wanted a project that was fun, easy, and applicable to the math we were studying, so why not have students build solar cookers. I kept my rules simple. It has to be less than 2 feet by 2 feet. I did not limit their depth. It had to be made from tape, cardboard, tongue depressors, ice cream sticks and aluminum foil for the reflective surface. It had to in some way incorporate the parabolic design. Then I gave them eight 50 minute class periods for planning and building.
The results on a 20 degree Celsius day where nothing short of astounding for my first time with this project. Four groups used the properties of the parabolas in their design and construction. These groups reached temperature of 31, 76, 84, and 105 degrees at the focus. One group did not use parabolas at all and their temperature reached 52 degrees.
When designing, three groups used geogebra to map out their parabolas so that they would have reference points to measure and cut. Some used the projector in combination with geogerba as an overhead to draw templates on their cardboard. In all I was pretty impressed with the designs of all groups.
The lowest temperature group had a good design (trough), but the 2 X 2 limitation I put on their trough design allowed for a very small surface area. I think that was the biggest factor in temperature. It was how much surface area was exposed to the sun. The group that won had a design that fit in a 2X2X3 prism. While the group that placed second with a temperature of 84 degrees C actually could have fit their cooker in a 2 by 2 by 2 cube. In all groups that used the parabolic reflective surface when you reached your hand into the cooker the heat change was noticeable even the lowest temperature group (collecting heat from a long thin trough focus is hard) and when we tried to cook marshmallows it was easy to find the focus as the marshmallow would almost light up in brightness at the focus. Safety issue to consider for next time. . . . They were all so bright that it was almost like watching someone weld. Probably should have eye protection when looking into the cookers. My eyes are still burning and had to take a pain killer to stop headache 8th hour.
Next post will be The Marshmallow Test For Solar Cookers.