STR Micro-Lesson: Crystal Power
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Question
The item pictured is made of the second most common element on the earth. If you treat it right, it might just help you avert a global crisis. What is it?
Answer
This is the 400x-magnified edge of a polycrystalline silicon wafer which has been treated with trace amounts of other elements so as to have the capability of turning sunlight into electricity--more commonly known as a solar cell. (The white stripe is a metallic strip on top of the silicon, to carry away the current generated by the cell.)
Silicon makes up 28% of the earth's weight, second only to oxygen.
As to the global crisis this device might help avert, it's multiple choice according to your perspective: a) global warming, b) diminishing fossil fuel supply, c) the fact that 25 to 30% of the earth's population does not have access to electricity, or d) all of the above.
Micro-Lesson* Plan
Topic: Energy and Sustainability
Download the enlarged version of the image above and show it to your class (using a projector or a color printout). Ask the question just below the image. You may wish to give hints.
Ask students to write down something they know about solar cells. As you discuss their answers, guide the conversation towards the question of whether solar cells are "better" than fossil fuels as a source of energy. Point out that it requires energy to make solar cells, and often that energy comes from the burning of fossil fuels. Ask students to work in small groups to generate answers to this question: What would you need to know to decide whether or not making and using solar cells is a better way to get energy than burning fossil fuels?
In discussing the answers, elicit or present these factors:
1) How much energy does it take to make a solar cell (including all aspects such as mining and transporting the materials used)?
2) How long will a solar cell last before it has to be replaced?
3) How much energy will a solar cell produce during its lifetime?
4) What happens to a solar cell when it no longer works? Can it be recycled? If so, how much energy does that require? If it is discarded, how might that waste material affect the environment?
Explain to students that this kind of thinking about a manufactured item is called "Cradle to Grave", and it means that you try to consider every aspect of the life of the item, from before it is made until after it is no longer of use.
These links may be helpful:
Photovoltaics: Solar Electricity and Solar Cells in Theory and Practice
Futures Channel Movie: Making Sparks
Sustainability Education Handbook
More…
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Comments?
Let us know what you think of this lesson idea, or if you have suggestions for more STR Micro-lessons.
*STR Micro-Lessons™ are designed as "plug-ins" to support and augment standards-based science teaching and learning. For complete lessons using the Scope on a Rope handheld digital microscope, see the STR Teacher Resources page.
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