Electrons—the negatively charged particles that surround atomic nuclei—are the basis of electricity. In some materials, including most metals, electrons are only loosely bound to nuclei; in these electrical conductors, electrons can move freely from atom to atom. The electricity that powers familiar devices such as computers, televisions, radios, and light bulbs is simply the flow of electrons. But where does electricity come from?

The electricity we use is a form of energy derived from other energy sources. Power plants, driven by the burning of fossil fuels (such as coal, natural gas, and oil) generate the majority of the world's electricity by taking advantage of the relationship between electricity and magnetism. The combustion of fossil fuels produces the heat to boil water, which drives turbines that use steam to create rotational motion. In this process, thermal energy transforms into mechanical energy, which generates electricity as it causes a magnet to spin within a coil of wire; the spinning magnet causes electrons to move within the nearby conductor, creating a flow of electrons through the wire. Nuclear and geothermal energy are also commonly used to generate electricity in a similar manner, and wind and flowing water can also be used to drive turbines.

The Sun also provides a source of energy from which to produce electricity. Solar cells, or photovoltaic cells, convert sunlight directly into electricity using the photoelectric effect—when certain materials absorb light, electrons are set free. Most solar cells are composed of several layers, including silicon, metal conductor strips, and an anti-reflective coating. When the silicon absorbs sunlight, the energy knocks electrons loose from the silicon atoms; the electrons then flow out of the cell (along the metal conductor strips) as electrical current. The anti-reflective coating increases the cell's efficiency by reducing the amount of sunlight that is reflected.

Solar power is one of the most environmentally friendly sources of electricity—it does not produce any harmful emissions, and the Sun provides an essentially inexhaustible source of energy. However, the materials for traditional solar panels are expensive. As technology has improved, the efficiency of solar cells has gone up and manufacturing costs have come down, but the cost of producing electricity from solar energy remains relatively high compared to conventional sources. New solar technologies, such as the possibility of a "solar paint," may be able to reduce the costs of solar electricity to make it more competitive and accessible. Researchers are using nanotechnology (technology at a very small scale) to develop nanoparticles made from cheap materials that can convert sunlight to electricity in new ways. When sunlight strikes these tiny structures, electrons are knocked loose, similar to the way traditional silicon solar cells work. However, instead of installing expensive solar panels on a roof, the entire roof would be "painted" with this inexpensive technology.

Use this NOVA classroom activity to design, build, and test a solar cooker that will pasteurize water.

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