Chapter 7 Sources of Electricity 117 5. True or False? A storage battery stores electricity. 6. Connecting cells in parallel increases the _____. A. voltage B. life of the battery C. current D. All of the above. 7. True or False? The capacity of a battery is its ability to produce a current over a certain period of time. 7.2 OTHER SOURCES OF ELECTRICAL ENERGY Batteries are a very common source of electrical energy however, there are many other sources of electrical power. Devices that convert energy in the forms of light, heat, and mechanical pressure are found everywhere. If you have used a solar powered calculator or a crystal microphone, you have seen these conversions in action. 7.2.1 Electrical Energy from Light Thanks to advancements in technology, we can convert the sun’s light directly into electricity. The invention of the photovoltaic cell, also called solar cell and photocell, made this conversion possible. The first photovoltaic cell was made from selenium, but today crystalline silicon is used. Photovoltaic cells made from crystalline silicon have a much higher efficiency than the original selenium cells. A photovoltaic cell, Figure 7‑16, is constructed of two thin layers of crystal- line silicon each injected with impurities to form a negative and a positive semiconductor material. When secured together and then exposed to light, an electrical potential is developed. The layers are connected to thin wires that allow the photovoltaic cell to be connected to an electrical circuit. See Figure 7‑17. While a typical photovoltaic cell produces approximately 1 watt and Photovoltaic Effect Flow of current Electron P-type semiconductor N-type semiconductor Hole Sunlight Load sivVector/Shutterstock.com Figure 7‑17. Two semiconductor materials are sandwiched together to form a photovoltaic cell. Electrical energy is produced when light shines on the cell. + Photovoltaic cell Goodheart-Willcox Publisher Figure 7‑16. Schematic symbol for a photovoltaic cell. Copyright Goodheart-Willcox Co., Inc.