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Make an economic and environmentally friendly oven for roasting poor little marshmallows. Create a solar-powered oven that uses light from the sun to warm food. Learn about how light can be transmitted, absorbed and reflected.
What you need
- small cardboard pizza box
- black paint and paint brush
- pencil or pen
- craft knife
- 2 sheets of thick, clear plastic, cut to the roughly same size as the pizza box lid (overhead transparencies work well)
- aluminium foil
- sticky tape
- nail or small pencil
- food to ‘cook’ (e.g. marshmallow, chocolate, cheese on biscuits)
What to do
Safety: This activity requires adult supervision or assistance to use the craft knife and black paint.
- Paint the inside and outside of the pizza box with the black paint and let it dry overnight.
- Close the pizza box and sit the pizza box on a table so that the lid of the pizza box is at the top.
- Use a ruler to measure a point 5 cm from one edge of the lid of the box. Draw a straight line on that point, parallel to the edge.
- Repeat Step 2 for the other 3 edges of the lid of the box. (Hint: When you’ve finished you should have drawn a square that has a 5 cm thick frame of box surrounding it.)
- Use a craft knife or scissors to cut 3 sides of the square. Do not cut one of the sides of the square as this will be the hinge for the oven.
- Lift the square flap and make a crease at the hinge.
- Use a pencil or pen to trace the shape of the square flap on a sheet of aluminium foil.
- Use the scissors to cut out the square of aluminium foil. (Hint: It should be identical in size to the square flap.)
- Glue the aluminium foil square to the underside (the side that faces into the box) of the square flap. Make sure there are no creases in the foil.
- Lift the square flap and tape one piece of plastic across the top side of the frame.
- Open the box and tape one piece of plastic across the underside of the frame.
- Roll up some newspaper and tape this around the inside edges of the box.
- Close the box.
- Carefully push a nail into the back edge of the box (the edge that runs parallel with the hinge of the square flap).
- Tape one end of the string to the top of the square flap and tie the other end of the string to the nail.
- Open the flap as much as is needed by changing the length of the string. (Hint: The square flap needs to be adjusted so that sunlight can be reflected off of the aluminium foil and into the box.)
- Open the box to place the food (ie. marshmallow or chocolate) inside the box.
- Close the box and align the square flap so that the sunlight is reflected into the oven.
- Measure how long it takes to heat the food so that the food is warm.
- Think about and try to answer the following questions:
- Will the aluminium foil absorb or reflect light?
- Will the black paint absorb or reflect light?
- What will the plastic do?
- Why were there two layers of plastic?
- How will the newspaper insulate the box?
- How long does it take to warm your food?
- How can you improve your oven so that it heats food more quickly or so that it makes the food warmer?
Did you know?
Do you have a calculator that works without a battery? Have you ever seen street lights that work without a battery and without being plugged in? Do you know someone with large, black rectangles (solar panels) on the roof of their house? These are examples of solar cells using solar power. Solar power is a renewable energy source, meaning that it can be easily replaced or re-used. Solar power is considered a ‘clean’ energy resource because it doesn’t create as much pollution as the burning of fossil fuels such as coal and natural gas. Solar cells are photovoltaic cells, meaning they collect sunlight and convert sunlight into electricity. This electricity can then be used to power things, like a calculator, road sign, pool, hot-water unit or light globe.
In August of 2008, 23 year-old, Nicole Kuepper, a PhD student and lecturer in the School of Photovoltaic and Renewable Energy Engineering at UNSW, won two Eureka Prizes for her research in solar cells. The Australian Museum Eureka Prizes “reward outstanding achievements in Australian scientific research, leadership and innovation, communication and journalism and school science.” Ms. Kuepper has developed and patented a solar cell that can be made at low temperatures using an inkjet printer, a pizza oven and nail polish. She hopes that the solar cells can be used to bring cheap, clean electricity to two billion people lacking electricity in developing countries.
This solar-powered oven uses sunlight to warm the food. Light is energy that travels in waves. When light hits an object, a couple of things can happen; light can pass through the object, light can be absorbed by the object or light can be reflected by the object.
If light hits a transparent (see-through) object like glass, water or clear plastic, the light can pass through the object. Some of the sunlight was able to pass through the air and through the sheets of plastic into the oven.
However, if light hits an opaque (not see-through) object like people, wood and stones, the object doesn’t let light pass through it, instead the object absorbs the light and becomes warm. The cardboard box and the food became warmer because they absorbed the light energy.
Shiny objects, like metals, absorb some light and become warm, but they also reflect light. The aluminium flap reflected the light from the Sun off its surface and into the oven. This meant more sunlight could be absorbed by the food inside the oven than would have been achieved by simply having a plastic window in the oven.
Some objects absorb more light than others. Objects that appear black to us absorb all of the light that shines on them and don’t reflect any light back. Objects that appear white don’t absorb the light that shines on them and instead reflect all of the light back. The box was painted black to increase the amount of light it would absorb; this resulted in the box became warmer.
Heat transfers from warmer objects to cooler objects. When the box became warmer than the air and objects surrounding it, the heat energy from the box transferred into the cooler air and objects nearby. Some objects, like metals, are good at transferring heat; they are called conductors. Other objects, like wood, paper and straw, aren’t good at transferring heat; they are called insulators.
We didn’t want the solar-powered oven to transfer its heat easily, because then the heat energy would have moved away from the food that we were trying to ’cook’. Newspaper was used as an insulator to stop the transfer of heat from the box to the outside air and to other nearby objects.
Change the design of your solar-powered oven to see which design warms food the best. You could try: different colours of paint, different sizes of ovens, different types of insulators, different sizes of windows and more square flaps with aluminium foil.