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Chapter 34 Electric Current 681 681

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Chapter 34 Electric Current 681 681 ELECTRIC CURRENT Objectives • Describe the flow of electric ELECTRIC CURRENT charge. (34.1) 34 • Describe what is happening inside a current-carrying wire. Electric current is related to the (34.2) THE BIG voltage that produces it and the • Give examples of voltage sources. (34.3) IDEA .......... resistance that opposes it. • Describe the factors that affect the resistance of a wire. (34.4) • Describe Ohm’s law. (34.5) he previous chapter discussed • Explain the causes of electric the concept of electric potential, shock. (34.6) Tor voltage, in terms of energy per • Distinguish between DC and AC charge. We’ll see in this chapter that (34.7). voltage can be thought of as an “elec- • Describe how AC is converted tric pressure” that produces a flow of to DC. (34.8) charge, or current, within a conductor. • Describe the drift speed of The flow is restrained by the resistance conduction electrons in a it encounters. When the flow takes current-carrying wire. (34.9) place along one direction, it is called • Identify the source of direct current (DC); when the charges conduction electrons in a circuit. (34.10) flow to and fro, it is called alternating • Relate the electric power used current (AC). The rate at which energy by a device to current and is transferred by electric current is voltage. (34.11) power. These ideas are better under- stood if you know how they relate to one another. Let’s begin with the flow discover! of electric charge. MATERIALS paper towel, vinegar (or salt solution), dime, penny, galvanometer discover! EXPECTED OUTCOME The cell How Can You Make a Simple Voltage Analyze and Conclude produces a voltage. Source? 1. Observing Describe the galvanometer read- ANALYZE AND CONCLUDE 1. Soak a piece of paper towel in a salt solution ing when the leads were brought in contact 1. Reversing the leads reverses or vinegar and place it between a dime and a with the coins. What happened when the the deflection of the penny. leads were reversed? galvanometer needle. 2. Attach one lead from a galvanometer to each 2. Predicting What do you think would happen 2. Connecting a number of the of the coins. if a number of these dime-and-penny cells cells in series would increase 3. Now attach the lead that was originally were connected in series? (That is, placed end the voltage produced. attached to the dime to the penny, and vice to end with dimes touching pennies.) 3. A battery consists of versa. 3. Making Generalizations How do you think electrical cells connected in voltage sources such as the batteries used in series. portable electronic devices are constructed? 680 680 0680_CP09_SE_CH34.indd 680 11/29/07 4:36:27 PM 0680_CP09_SE_CH34.indd 681 11/29/07 4:36:33 PM 00680_cp09te_CH34.indd680_cp09te_CH34.indd 680680 11/30/08/30/08 10:50:1210:50:12 AMAM The purpose of this chapter is 34.1 Flow of Charge to build a good understanding of electric current and to dispel some of the common Recall that heat flows through a conductor when a difference in tem- Electrons in a wire are perature exists between its ends. Heat flows from the end of higher misconceptions about electricity. like water in a pipe; If you are teaching only one temperature to the end of lower temperature. When both ends reach whenever a little water chapter on electricity, this the same temperature, the flow of heat ceases. enters one end, almost should be it. Charge flows in a similar way. When the ends of an electric immediately the same conductor are at different electric potentials, charge flows from one amount of water exits the other end. end to the other. Charge flows when there is a potential difference, or difference in potential (voltage), between the ends of a conductor. The flow of charge will continue until both ends reach a common 34.1 Flow of Charge potential. When there is no potential difference, there is no longer Key Term a flow of charge through the conductor. As an example, if one end potential difference of a wire were connected to the ground and the other end placed in ᭤ Teaching Tip Explain the contact with the sphere of a Van de Graaff generator that is charged to lighting of a fluorescent lamp a high potential, a surge of charge would flow through the wire. The or neon discharge tube by flow would be brief, however, for the sphere of the generator would the Van de Graaff generator quickly reach a common potential with the ground. from the previous chapter in To attain a sustained flow of charge in a conductor, some arrange- terms of current being directly proportional to a difference in ment must be provided to keep one end at a higher potential than voltage. One end of the lamp the other. The situation is analogous to the flow of water from a was in a stronger part of the higher reservoir to a lower one, as shown in Figure 34.1a. Water will energy field than the other. flow in a pipe that connects the reservoirs only as long as a difference There was more energy per charge on one end than the in water level exists. The flow of water in the pipe, like the flow of other and so more voltage at one charge in the wire that connects the Van de Graaff generator to the end than the other, providing the ground, will cease when the “pressures” at the two ends are equal. In potential difference. order that the flow be sustained, there must be a suitable pump of some sort to maintain a difference in water levels, as shown in Figure 34.1b. Then there will be a continual difference in water pressures and a continual flow of water. The same is true of electric current. CONCEPT What happens when the ends of a conductor are at CHECK ...... different electrical potentials? CONCEPT When the ends of a ᭣ FIGURE 34.1 CHECK ............ conductor are at different electric potentials, a. Water flows from higher charge flows from one end to pressure to lower pressure. the other. The flow will cease when the difference in pressure ceases. b. Water continues Teaching Resources to flow because a difference in pressure is maintained • Reading and Study with the pump. Workbook • PresentationEXPRESS • Interactive Textbook • Next-Time Question 34-1 CHAPTER 34 ELECTRIC CURRENT 681 681 0680_CP09_SE_CH34.indd 680 11/29/07 4:36:27 PM 0680_CP09_SE_CH34.indd 681 11/29/07 4:36:33 PM 00680_cp09te_CH34.indd680_cp09te_CH34.indd 681681 11/30/08/30/08 10:50:1610:50:16 AMAM 34.2 Electric Current Key Terms 34.2 Electric Current ampere, electric current Electric current is the flow of electric charge. In solid conductors ᭤ Teaching Tip Explain that the the electrons carry the charge through the circuit because they are ampere is named after physicist free to move throughout the atomic network. These electrons are André-Marie Ampère (1775–1836). called conduction electrons. Protons, on the other hand, are bound ᭤ Teaching Tip Have your inside atomic nuclei that are more or less locked in fixed positions students imagine a tube completely filled with Ping-Pong within the conductor. In fluids, such as the electrolyte in a car battery, balls. As one more ball is pushed positive and negative ions as well as electrons may compose the flow into one end, one ball comes of electric charge. out the other. This should help them understand that a current- Measuring Current Electric current is measured in amperes, for carrying wire has no net charge. which the SI unit is symbol A.34.2 An ampere is the flow of 1 coulomb ᭤ Teaching Tip Write current ~ of charge per second. (Recall that 1 coulomb, the standard unit of voltage difference (I ~ V) on the charge, is the electric charge of 6.24 billion billion electrons.) In a wire board. This is the lead-in to that carries a current of 5 amperes, for example, 5 coulombs of charge Ohm’s law. Make sure students understand that a potential pass through any cross section in the wire each second. That’s a lot of difference can cause a flow of electrons! In a wire that carries 10 amperes, twice as many electrons charge. pass any cross section each second. Figure 34.2 shows a simplified view of electrons flowing in a wire. Strictly speaking, the voltage term in Ohm’s law implies the difference in potential, so voltage difference is redundant, but it underscores a point that may be FIGURE 34.2 ᭤ missed, so go for it. When the rate of flow A high moment in my life was of charge past any cross a conversation with Richard section is 1 coulomb Feynman about teaching (6.24 billion billion elec- physics conceptually. The trons) per second, the topic of redundancies in current is 1 ampere. teaching electricity came up. Feynman advised me to put concepts above grammar, and go for redundancies when they Net Charge of a Wire A current-carrying wire has a net underscore a point—so it’s electric charge of zero. While the current is flowing, negative elec- okay to say “current flows in trons swarm through the atomic network that is composed of posi- a circuit!” tively charged atomic nuclei. Under ordinary conditions, the number of electrons in the wire is equal to the number of positive protons in the atomic nuclei. When electrons flow in a wire, the number enter- ing one end is the same as the number leaving the other.
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