Intro to Chap 5 ‘Sources of Electricity’

Intro to Chap 5 ‘Sources of Electricity’ • Every student knows the story of Benjamin Franklin and his kite. For centuries before Franklin, scientists and philosophers had observed lightning. It was through the experimentation and research of Franklin that the relationship between lightning and static electricity was confirmed. What is electricity and where does it come from?

Years before the discovery of the electron theory by J. J. Thomson, it was suggested by Franklin that electricity consisted of many tiny particles. or electric charges. He further theorized that electrical charges were created by the distribution of electrical particles in nature.

We have learned that a potential difference or electromotive force is created when electrons are redistributed. A body might assume a charge; its polarity is determined by the deficiency or excess of electrons. People have turned their scientific interests and research to the development of machines and processes that cause an electrical imbalance and an electrical pressure.

There are six basic sources of electricity or electromotive force. They are friction, chemical action, light, heal, 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.

5.1 CHEMICAL ACTION • One of the more familiar sources of an electrical potential or voltage is the battery. In 1790, the Italian scientist, Luigi Galvani, observed a strange phenomena during the dissection of a frog supported on copper wires. Each time he touched the frog with his steel scalpel, its leg would twitch. Galvani reasoned that the frog's leg contained electricity.

Alessandro Volta, another Italian scientist, invented the electric cell, named in his honor, called the voltaic cell. The unit of electrical pressure, the volt, is also named in his honor. Volta discovered that when two dissimilar elements were placed in a chemical that acted upon them, an electrical potential was built up between them. Thus, electricity can be produced by chemical action.

The student can construct several voltaic cells 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 5-1- If a sensitive meter is connected to the coins, i[ will indicate that a small voltage is present.

In Figure 5-2 electricity is created using a grapefruit. Make small cuts in the skin of a grapefruit. In one cut, place a penny. In the other cut, place a nickel. Once again, a meter 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 5-3. Fallow all safety precautions when performing experiments.

Nickel • Figure 5-1. A simple cell is produced using a nickel, a penny, and a salt solution. • Figure 5-2. A grapefruit can be used to produce enough electricity to operate a small radio.

Lesson in safety: • 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 first aid!

Acid and water solution • Figure 5-3. This experimental cell, made with zinc, carbon, and acid, produces enough electricity to power the light.

When the polarity of the carbon rod is tested, it will be positive. The zinc strip is negative. If a wire is connected between these elements, or electrodes, a current will flow. A voltaic cell can be described as a way of converting chemical energy into electrical energy. • In the zinc-carbon example of a voltaic cell, the sulfuric acid (HiSO,) and water (H=0) solution is also known as an electrolyte. When the electrodes are placed in this acid electrolyte, a chemical action takes place. The sulfuric acid breaks down into positive ions (2H+) and negative ions (50,=-).

The negative ions move toward the zinc electrode, and combine with it by making zinc sulfate (ZnSO,). The positive ions move toward the carbon electrode. This action creates a potential difference between the electrodes. The zinc will be negative. The carbon will be positive. This cell will develop about 1.5 volts.

If a load, such as a light, is connected to the cell, a current flows and the light glows, as seen in Figure 5-3. As the cell is used, the chemical action continues until the zinc electrode is consumed. The chemical equation for this action is:

Zn + H=50, + H=0 - ZnSO,+ H,O + H= f • Zinc plus sulfuric acid plus water chemically reacts to form zinc sulfate and water and free hydrogen gas. This cell cannot be recharged because the zinc has been consumed.

Primary Cells • The zinc-carbon cell just described is what is known as a primary cell. A primary cep is a cell in which the chemical action cannot be reversed. A primary cell cannot be recharged.

Defects in the primary cell • One might think that the chemical action of the zinccarbon 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. These collect around the carbon electrode. As the cell continues to discharge, an insulating blanket of bubbles forms around the carbon. This reduces the output and terminal voltage of the cell. The cell is said to be polarized. The 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 (MnO,), are used for this purpose. The oxygen in the depolarizer combines with the hydrogen bubbles and forms water. This chemical action appears as:

2Mn0z + Hz - MnzO,+ Hi0 • The 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. These impurities act as the positive electrode for many small cells within the one large cell.

This chemical action adds nothing to the electrical energy produced at the cell terminals. This 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 [o the zinc during manufacturing. As mercury is a heavy liquid, any impurities in the zinc will float on the surface of the mercury, causing them [o leave the zinc surface. This process increases the life of a primary cell.

Types of Primary Cells • There are many different primary cells. What follows are details on some of most common primary cells you might encounter.

Zinc-carbon cell • 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 x 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.

Flashlight batteries (cells) are examples of dry cells. The dry cell consists of a zinc container that acts as the negative electrode. A carbon rod in the center is the posi tive electrode. Surrounding the rod is a paste made of ground carbon, manganese dioxide, and sal ammoniac (ammonium chloride), mixed with water. The depolarizer is the MnO,.

The ground carbon increases the effectiveness of the cell by reducing its internal resistance. During discharge of the cell, water is formed.

You may recall having difficulty removing dead cells from a flashlight. This is because the water produced caused the cells to expand. Although this problem has been solved by improved manufacturing techniques, it is still not advisable to leave cells in your flashlight for long periods of time. You should keep fresh cells in your flashlight, so it will be ready for emergency use.

Lesson In safety: • Improper battery use can cause leakage and explosion. Therefore, obey the following precautions. • 1. Install the batteries with the positive (+) and negative (-) polarities in the proper direction. 2. Do not use new and old batteries together.

3. Never attempt to short circuit, disassemble, or heat batteries. Do not throw batteries into a fire. • 4. Batteries contain dangerous materials that should be recycled or disposed of properly. Contact your local recycling facility or fire department for more information.

Alkaline cell • The alkaline battery 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. Recent progress in electronic product design has demanded more compact supply sources.

The number of products needing a large current and a long battery life have increased. This required the development of more advanced batteries. Cylindrical alkaline batteries are now widely used to supply power for electronic products. They can be used with common manganese dioxide batteries, Figures 5-4 and 5-5.

Mercury cell • A relatively new type of dry cell is shown in Figure 5-6. It is called a mercury cell. It creates a voltage of 1.34 volts from the chemical action between zinc (-) and mercuric oxide (+). It is costly to make. However, the mercury cell is better in that it creates about live times more current than the conventional dry cell.

It also maintains its terminal voltage under load for longer periods of operation. The mercury cell has found wide use in powering field instruments and portable communications systems.

Figure 5-4. AA size alkaline cells. • Figure 5-6. Mercury cell. It creates voltage by chemical action between zinc and mercuric oxide.

Safety adsorbent sleeve Electrolyte absorbent material • Figure 5-5. Cutaway of AA size alkaline cell.

Lithium cell • Lithium has the highest negative potential of all metals. It is, therefore, the best substance for an anode. Many battery makeups are possible by mixing lithium with various cathode substances. Energy densities of these batteries can be computed by respective reaction equations.

Figure 5-7 shows the energy densities of lithium batteries compared with those of conventional batteries. Lithium is the most suitable anode for production of high voltage and lightweight batteries. Refer to Figure 5-8. • Features of lithium batteries, such as voltage and discharge capacity, are determined by the type of cathode substance used. Fluorocarbon is an intercalation (inserted between or among existing elements) compound. It is produced through reaction of carbon powder and fluorine gas. It is expressed in (CF)n.

Silver oxide cell • Silver oxide cells have several advantages over other types of cells. These advantages include: • Very stable discharge voltage.

Excellent high discharge characteristics. • High energy density per unit volume. • Wide range of operating temperatures. • Compact, thin size.

Compact silver oxide batteries have the highest electrical volume and leakage resistance of any battery of that size. They are commonly used in watches. Two types of silver oxide batteries are made for use in watches. One type uses caustic potash for electrolyte. The other uses caustic soda.

The caustic potash battery has the symbol W on the bottom of the battery. It is for high drain use, where more power is needed. It is used in wristwatches with liquid crystal displays and multifunction analog watches.

The caustic soda battery has the symbol SW on the bottom of the battery. It is for low drain use. It is used mostly in single function analog watches. Figure 5-9 shows a cutaway of a silver oxide cell.

Figure 5-7. Theoretical energy densities of lithium batteries compared with conventional batteries. (Panasonic Battery Sales Division)

Figure 5-8. Cross-sectional view of a cylindrical shaped lithium battery. (Panasonic Battery Sales Division) • Electrolyte to absorbent Barrier • Figure 5-9. Cutaway view of a silver oxide cell. • (Panasonic Battery Sales Division)

Secondary Cells • A secondary cell can be recharged or restored. The chemical reaction that occurs on discharge may be reversed by forcing a current through the battery in the opposite direction.

This charging current must be supplied from another source, which can be a generator or a power supply. Figure 5-10 shows one type of battery charger used for recharging automobile and motorcycle batteries. An alternating current, which will be studied in a later chapter, must be rectified to a direct current for charging the battery.

Figure 510. This type charger is called a trickle charger. It slowly brings a battery back to full charge.

Lead acid cell • A common type of lead acid cell is the car storage battery. A storage battery does not store electricity. Rather, it stores chemical energy, which in turn produces electrical energy.

The active ingredients in a fully charged battery are lead peroxide (P60j, which acts as the positive plate, and pure spongy lead (Pb) for the negative plate. The liquid electrolyte is sulfuric acid (H,50,) and water (H,O). The positive plates are a reddish-brown color. Negative plates are gray.

Intro to Chap 5 ‘Sources of Electricity’