When the weather is warm and sunny, I am reminded of just how much energy is in circulation every day. Most energy takes the form of heat, and this is quite apparent during the summer - cars get hot when left in the sun, puddles on the sidewalk evaporate, and dark hair becomes scorching to the touch. Wouldn't it be neat to capture some of this fascinating, elusive energy?
By building a solar oven, the sun's heat can be directed to a single point. With enough sunshine, this heat can be used to cook food and provide students with a tangible result of this redirection (and a snack).
Make sure to use foods that are safe to eat prior to placement in a solar oven, such as marshmallows, chocolate, or hot dogs. This is to avoid accidental food poisoning.
Materials:
Pizza box
Aluminum foil
Black construction paper
Plastic wrap
Tape/glue
Pre-cooked food (Ex: marshmallows, hot dogs)
Heat-safe dish
Directions:
Open the pizza box and make sure it is clean. Leftover pizza may go bad and begin to smell.
Cut eight [measurement] strips from the aluminum foil. Cut one [measurement] square. Tape four of the strips to the inside edges of the box.
Tape a piece of black construction paper to the bottom of the pizza box. Take the four leftover aluminum strips and tape them around the black construction paper.
The box lid will reflect energy into the bottom of the box. Tape the aluminum square to the inside of the lid.
Stretch the plastic wrap over the box so the food will be protected from dust and insects. Reference the above image if needed. Use tape to secure the plastic wrap to the sides of the box.
Put the food into a heat-safe dish and slip under the plastic wrap. The dish should be resting on the black construction paper.
Wait fifteen to thirty minutes, then go back and check on your food. Be careful - the dish may be hot!
If the food has not fully cooked/melted, set the timer for another fifteen to thirty minutes before checking on it again.
The Science:
Black paper absorbs more heat than white paper, making it an optimal color to use in the solar oven. To understand this, we must first understand the composition of light.
Visible light is a wave. Similarly to ocean waves, a light wave's length and frequency can be measured. Typically, the wavelength ranges from 380 nano-meters (purple) to 750 nano-meters (red) and encompasses all colors of the rainbow. When all these colors are combined together, it appears white. Glass prisms show rainbows because they reflect each wavelength by a different amount, separating the colors and making them easy to differentiate.
This is why white paper appears white - all the colors on the light spectrum reflect off of it, and bounce back into your eyes. None of the light is absorbed, and therefore, no heat is transmitted. On the other hand, black paper is dark because most of the light is absorbed - very little bounces back into your eyes, but a lot of heat is taken in. Since so much light is absorbed by the paper, the energy in the light is absorbed as well.
Aluminum foil reflects light as well. Using the lid reflects direct sunlight into the bottom of the box, and the strips glued to the sides help to evenly distribute the heat.
To make these concepts clearer to students and children, teachers might consider the following ideas:
Use a glass prism and flashlight to demonstrate the idea that white light is composed of many colors. What happens if a green/red laser is shined on the prism? Does it split into different colors as well?
Use aluminum foil and a flashlight to reflect the light around the classroom. If the flashlight shines directly at the foil, where does the light end up? What happens if the foil is moved to a different angle?
Put two cups of water in the sun, and place a thermometer in each one. Wrap one cup in black paper, and the other in white paper. Which one heats up faster? Why does this occur?
Put a marshmallow into the completed solar oven. What happens to the volume? Gather ideas about why this occurred.
Did you try this activity? Tell me what you think in the comments! Sixteen similar activities can be downloaded for free, and they include activity directions, related careers, a photo, and explanations of the science behind the reaction.
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