Chapter 7 Sources of Electricity 107 Defects in the Primary Cell One might think that the chemical action of the zinc-carbon primary cell would continue to produce a voltage as long as the active ingredients of the cell were present. In studying the equation for the discharge of the cell, you will observe the formation of free hydrogen gas. Since the carbon electrode does not enter into chemical action, the hydrogen forms gas bubbles. Th ese collect around the carbon electrode. As the cell continues to discharge, an insulating blanket of bubbles forms around the carbon. Th is reduces the output and terminal voltage of the cell. Th e cell is considered polarized. Th e action is called polarization. To overcome this defect in the simple voltaic cell, a depolarizing agent can be added. Compounds that are rich in oxygen, such as manganese dioxide (MnO2), are used for this purpose. Th e oxygen in the depolarizer combines with the hydrogen bubbles and forms water. Th is chemical action appears as: 2MnO2 + H2 → Mn2O3 + H2O Th e free hydrogen has been removed, so the cell will continue to produce a voltage. One might assume that when current is not being used from the cell, the chemical action would also stop. However, this is not true. During the smelting of zinc ore, not all impurities are removed. Small particles of carbon, iron, and other elements remain. Th ese impurities act as the positive electrode for many small cells within the one large cell. Th is chemical action adds nothing to the electrical energy produced at the cell terminals. Th is action is called local action. It can be reduced by using pure zinc for the negative electrode, or by a process called amalgamation. With amalgamation, a small quantity of mercury is added to the zinc during manufacturing. As mercury is a heavy liquid, any impurities in the zinc will fl oat on the surface of the mercury, causing them to leave the zinc surface. Th is process increases the life of a primary cell. 7.1.2 Types of Primary Cells Th ere are many diff erent primary cells. Although the primary cell has been described as a liquid cell, the liquid type is not in common use. Rather, the primary cell is often a dry cell. In a dry cell, the electrolyte is in a paste form as opposed to a liquid form. A dry cell averts the danger of spilling liquid acids. Zinc-Carbon Cell Zinc-carbon cells are an example of a dry cell. Th e dry cell consists of a zinc container that acts as the negative electrode. A carbon rod in the center is the positive electrode. Surrounding the rod is a paste made of ground carbon, manganese dioxide, and sal ammoniac (ammonium chloride), mixed with water. Th e depolarizer is the MnO2. Th e ground carbon increases the eff ectiveness of the cell by reducing its internal resistance. During discharge of the cell, water is formed. Alkaline Cell Th e alkaline cell uses manganese dioxide for the positive activating substance. Zinc powder is used as the negative activating substance. A caustic alkali is used for the electrolyte. Progress in electronic product design has demanded more compact supply sources. Th e number of products needing a large current and a long battery life have increased. Th is required the development of more advanced batteries. Cylindrical alkaline batteries are now widely used to supply power for electronic products, Figure 7-3. Yeti studio/Shutterstock.com Top plate (+) Insulation ring Bottom insulator Collector Separator Gelled anode Steel can PVC tube Outer metal jacket Sharp steel strip (–) Insulation ring Bottom plate Panasonic Battery Sales Division Figure 7-3. AA size alkaline cells. S A F E T Y Improper battery use can cause leakage and explosion. Therefore, obey the following precautions. ■ Install the batteries with the positive (+) and negative (–) polarities in the proper direction. ■ Do not use new and old batteries together. ■ Never attempt to short circuit, disassemble, or heat batteries. Do not throw batteries into a fi re. ■ Batteries contain dangerous materials that should be recycled or disposed of properly. Contact your local recycling facility or fi re department for more information. Copyright Goodheart-Willcox Co., Inc.