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Electric Fish, Eels, and Rays Current, Potential Difference, and Resistance

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Electric Fish, Eels, and Rays Current, Potential Difference, and Resistance ist9_ch11.qxd 7/21/09 3:17 PM Page 432 11.1 Current, Potential Difference, and Resistance Here is a summary of what you will learn in this section: • An electrochemical cell generates a potential difference by creating an imbalance of charges between its terminals. • Potential difference is the difference in electric charge between two points that will cause current to flow in a closed circuit. • Current is the rate of movement of electrons through a conductor. • An electric circuit is a path along which electrons flow. • Resistance is the ability of a material to resist the flow of electrons. • Resistance in a wire depends on wire length, material, temperature, and cross- sectional area. Figure 11.1 The elephantnose fish has tiny electric sensors in its nose that help it find food. Electric Fish, Eels, and Rays You probably know that when it comes to electrical safety, it is very important to keep electrical devices away from water. For some animals, this safety concern about electricity is not a problem. In fact, they survive because they can use electricity in the water. The elephantnose fish from central Africa has an extended nose that contains about 500 electric sensors (Figure 11.1) These sensors are used to help this tiny fish find food. The elephantnose fish hides for protection during the day and comes out to feed at night. The electric sensors help it find smaller living things crawling along the bottom of the river or swimming in the water. Research has shown that these electric sensors are so sensitive that they can detect chemical pollutants. Further research will Figure 11.2 The electric eel uses determine if this type of sensor can be used to monitor the levels electricity to defend itself and to stun its prey. of pollutants in rivers. 432 UNIT D The Characteristics of Electricity ist9_ch11.qxd 7/21/09 3:17 PM Page 433 The electric eel in Figure 11.2 lives in the murky waterways of the Amazon and Orinoco river basins of South America. It’s really a fish and not an eel, but it really is electric — and dangerous. The eel’s electricity comes from a special organ in its long tail that contains thousands of muscle cells that work like tiny batteries. Each cell can produce only a small amount of electricity, but by working together all the cells can produce controlled bursts of electricity equal to five times the energy of a standard wall socket. These electrical bursts are used to stun prey Figure 11.3 A Pacific electric ray can send out a powerful electric when the electric eel is hunting for food. Some electric eels also shock. generate an electric signal to attract a mate. The Pacific electric ray, found along the west coast of North America, has an electric organ located in its head (Figure 11.3). This organ can generate enough electricity to knock down a human. Other types of electric rays use these electric shocks for defense when they are attacked. Rays belong to a category of animals called Torpedo. The name for this category comes from the Latin word torpidus, which means numbness. This term describes what happens to a person who steps on an electric ray. D12 Quick Lab Light the Lights In this activity, you will use a combination of wires, Procedure light bulbs, and an electrochemical cell to investigate how a steady, controlled flow of electrons can cause 1. Use wire and the dry cell to make one bulb light the bulbs to light up. up. Record your arrangement. 2. Use wire and the dry cell to make two bulbs light Purpose up. Record your arrangement. To discover how to make flashlight bulbs light up 3. If time allows, try other arrangements for step 1 using a standard battery and step 2. Questions 4. Explain how to use wire and a dry cell to make Materials & Equipment one bulb light up. Include a labelled sketch in • 1 D dry cell your answer. • 5 insulated copper wires with both ends bare 5. Explain how to use wire and a dry cell to make • two 2.0 V-flashlight bulbs two bulbs light up. Include a labelled sketch in your answer. CAUTION: Disconnect the wires if they get hot. Do not use dry cells if they show any sign of corrosion. Current electricity is the continuous flow of electrons in a closed circuit. 433 ist9_ch11.qxd 7/21/09 3:17 PM Page 434 During Reading Current Electricity Illustrations The electricity of the electric eel and the electric ray is similar to Support Understanding of the static charges you have felt from a sweater or the huge static Vocabulary charges of lightning. Unfortunately, static charges are not useful As you read the text, be aware for operating electrical devices. They build up and discharge, but of how the photos, diagrams, or they do not flow continuously. other illustrations support your understanding of unfamiliar To operate electrical devices, you need a steady flow of vocabulary. What term or electrons. Unlike static electricity, a flow of electrons moves concept is illustrated by the continuously as long as two conditions are met. First, the flow of photo or diagram? How does the electrons requires an energy source. Second, the electrons will not illustration make the concept flow unless they have a complete path to flow through. This path easier to understand? If you get is called an electrical circuit. The continuous flow of electrons in stuck on unfamiliar terminology, a circuit is called current electricity. check the illustrations as one way to improve your understanding. Electric Circuits A circuit includes an energy source, a conductor, and a load. An electrical load is a device that converts electrical energy to another form of energy. For example, in Figure 11.4, the light bulb is the load. It converts electrical energy to light and heat. WORDS MATTER Many electric circuits also include a switch. A switch is a The word “circuit” comes from a device that turns the circuit on or off by closing or opening the Latin word meaning to go around. The word “circuit” can also be used circuit. When the switch is closed, the circuit is complete and to describe a complete journey of electrons can flow. An open switch means there is a break in the people or objects, such as the circuit of Earth around the Sun. path, so the electrons cannot flow through the circuit. The circuit is turned off when the switch is open. energy source + electrical load conducting wires – switch Figure 11.4 An electric circuit 434 UNIT D The Characteristics of Electricity ist9_ch11.qxd 7/21/09 3:17 PM Page 435 Electrochemical Cells One simple and convenient energy source is a battery. A battery is a combination of electrochemical cells. Each electrochemical cell is a package of chemicals that converts chemical energy into electrical energy that is stored in charged particles. A simple electrochemical cell includes an electrolyte and two electrodes. • An electrolyte is a liquid or paste that conducts electricity because it contains chemicals that form ions. An ion is an atom or a group of atoms that has become electrically charged by losing or gaining electrons. Citric acid is an example of an electrolyte. • Electrodes are metal strips that react with the electrolyte. Two different electrodes, such as zinc and copper, are used in a battery. As a result of the reaction between the electrolyte and electrodes, electrons collect on one of the electrodes, making it negatively charged. The other electrode has lost electrons, so it is positively charged (Figure 11.5). copper electrode (+) zinc electrode (–) F D B Figure 11.5 The citric acid in the grapefruit is the electrolyte. Electrons collect on the zinc C electrode, leaving positive charges on the copper electrode. The meter measures the flow A of electrons. Wet Cells and Dry Cells E An electrochemical cell that has a liquid electrolyte is called a wet A – zinc powder and electrolyte, cell. Wet cells are often used as an energy source for cars and where electrons are released other motorized vehicles. An electrochemical cell that uses a paste B – electron collecting rod C – separating fabric instead of a liquid electrolyte is called a dry cell (Figure 11.6). D – manganese dioxide and carbon, You use dry cells in flashlights, hand-held video game devices, where electrons are absorbed cameras, and watches. Each electrode in a dry cell or battery can E – negative terminal, where electrons leave also be called a terminal. Terminals are the end points in a cell or F – positive terminal, where electrons return battery where we make a connection. Figure 11.6 An alkaline dry cell Current electricity is the continuous flow of electrons in a closed circuit. 435 ist9_ch11.qxd 7/21/09 3:17 PM Page 436 Recycling and Recharging Dry Cells Eventually, the chemicals in a dry cell are used up and can no longer separate charges. When you are finished using a dry cell, you should recycle it rather than discard it (Figure 11.7). Dry cells can contain toxic materials, such as the heavy metals nickel, cadmium, and lead. Household dry cells and batteries are responsible for over 50 percent of all the heavy metals found in landfills. Some dry cells are rechargeable cells. Chemical reactions in a rechargeable cell can be reversed by using an external energy source to run electricity back through the cell. The reversed flow of electrons restores the reactants that are used up when the cell produces electricity. Since rechargeable dry cells can be reused many times, they have less impact on Figure 11.7 During recycling, the chemicals in a dry cell are the environment than non-rechargeable dry separated and can be reused.
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