106 Electricity & Electronics We have learned that a potential diff erence or electromotive force is created when electrons are redistributed. A body might assume a charge its polarity is determined by the defi ciency or excess of electrons. People have turned their scientifi c interests and research to the development of machines and processes that cause an electrical imbalance and an electrical pressure. Th ere are six basic sources of electricity or electromotive force. Th ey are friction, chemical action, light, heat, pressure, and magnetism. In this chapter, we will discuss in detail producing electricity from chemical action or batteries. You will also learn how electricity is produced using light, solar batteries, pressure, and heat. 7.1 CHEMICAL ACTION One of the more familiar sources of an electrical potential or voltage is the battery. A battery contains several cells connected in series or parallel, usually contained in a single case. Alessandro Volta, another Italian scientist, invented the electric cell, named in his honor, called the voltaic cell. Th e unit of electrical pressure, the volt, is also named in his honor. Volta discovered that when two dissimilar metals (electrodes) were placed in a chemical that acted upon them (electrolyte), an electrical potential was built up between them. A voltaic cell converts chemical energy into electrical energy. Simple voltaic cells can be constructed to demonstrate this action. Cut a one inch square of blotting paper and soak it in a strong salt solution. Place the wet paper between a penny and a nickel as shown in Figure 7-1. Th e salt solution acts as the electrolyte and the coins act as electrodes to conduct electricity. If a sensitive meter is connected to the electrodes, it will indicate that a small voltage is present. A better cell can be made by placing a carbon rod (these may be removed from an old dry cell) and a strip of zinc in a glass jar containing an acid and water solution, Figure 7-2. When the polarity of the carbon rod is tested, it will be positive. Th e zinc strip is negative. If a wire is connected between these elements, or electrodes, a current will fl ow. A voltaic cell will always have a positive electrode and a negative electrode through which the current will fl ow. In the zinc-carbon example of a voltaic cell, the sulfuric acid (H 2 SO 4 ) and water (H2O) solution is an electrolyte. When the electrodes are placed in this acid electrolyte, a chemical action takes place. Th e sulfuric acid breaks down into positive ions (2H+) and negative ions (SO42–). Th e negative ions move toward the zinc electrode, and combine with it by making zinc sulfate (ZnSO 4 ). Th e positive ions move toward the carbon electrode. Th is action creates a potential diff erence between the electrodes. Th e zinc will be negative. Th e carbon will be positive. Th is cell will develop about 1.5 volts. If a load, such as a light, is connected to the cell, a current fl ows and the light glows, as seen in Figure 7-2. As the cell is used, the chemical action continues until the zinc electrode is consumed. Th e chemical equation for this action is: Zn + H2SO4 + H2O → ZnSO4 + H2O + H2 Zinc, sulfuric acid, and water chemically react to form zinc sulfate, water, and free hydrogen gas. Th is cell cannot be recharged because the zinc has been consumed. 7.1.1 Primary Cells Th e zinc-carbon cell just described is a primary cell. A primary cell is a cell in which the chemical action cannot be reversed. A primary cell cannot be recharged. S A F E T Y When mixing acid and water, always pour acid into water. Never pour water into acid. Acid will burn your hands and your clothing. Wash your hands at once with clean water if you spill acid on them. Acid may be neutralized with baking soda. See your instructor for fi rst aid! Liberty L bi e r t y 0.6 V OFF A COM V Ω A V Ω Blotting paper Penny Nickel Goodheart-Willcox Publisher Figure 7-1. A simple cell is produced using a nickel, a penny, and a salt solution. Light Zinc Carbon Acid and water solution Glass jar Goodheart-Willcox Publisher Figure 7-2. This experimental cell, made with zinc, carbon, and acid, produces enough electricity to power the light. Copyright Goodheart-Willcox Co., Inc.