Section 20.2
20.2 Electric Current and Ohm’s Law
1 FOCUS Objectives 20.2.1 Describe electric current and identify the two types of current. 20.2.2 Describe conduction and classify materials as good electrical conductors or good electrical insulators. 20.2.3 Describe the factors that affect resistance. 20.2.4 Explain how voltage produces electric current. 20.2.5 Calculate voltage, current, and resistance using Ohm’s law.
Key Concepts
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What are some examples of conductors and insulators?
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Reading Strategy
L2
a. Electric current is the flow of electric charge. b. Electric current is the continuous flow of electric charge.
2 INSTRUCT
Electric Current Use Visuals
L1
Figure 7 Emphasize that electric current needs a continuous path through the flashlight. Ask, How is the direction of current related to the direction of the flow of electrons? (Electrons flow from the negative terminal of one battery to the positive terminal of the other; the direction of current is in the opposite direction.) Then ask, What is the function of the switch? (Flipping the switch to the “on” position completes the path for the flow of charge) Visual, Logical
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What causes an electric current?
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How are voltage, current, and resistance related?
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electric current direct current alternating current electrical conductor electrical insulator resistance superconductor potential difference voltage battery Ohm’s law
Reading Strategy Predicting Copy the table below and write a prediction of what electric current is. After you read the section, if your prediction was incorrect or incomplete, write what electric current actually is. Electric Current Probably Means
Electric Current Actually Means
a.
b.
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I
f you’ve ever tried to fix a flashlight, you know there are several parts to check. The batteries may be dead, or the bulb may have burned out. The switch could be broken, or the spring might be corroded. If even one part isn’t functioning, the flashlight won’t light.
L2
LINCS Have students: List the parts of the vocabulary that they know, such as potential and difference. Imagine what a potential difference might look like and how the terms might fit together. Note a reminding sound-alike word, such as potential energy. Connect the terms, perhaps in a long sentence or a short story. Self-test (quiz themselves).
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What factors affect electrical resistance?
Reading Focus
Build Vocabulary
Vocabulary
What are the two types of current?
Electric Current As you can see in Figure 7, the parts of a flashlight form a continuous path through which charge can flow. This continuous flow of electric charge is an electric current. The SI unit of electric current is the ampere (A), or amp, which equals 1 coulomb per second. The two types of current are direct current and alternating current. Charge flows only in one direction in direct current (DC). A flashlight and most other battery-operated devices use direct current. Electric current in your home and school is mostly alternating current. Alternating current (AC) is a flow of electric Switch Flow of current charge that regularly reverses its direction. Negative terminals In a flashlight, electrons flow from the negative terminal of one battery to the positive terminal of the other battery. But notice that + + – – the current is in the opposite direction. This is because scientists define current as the direcSpring Positive terminals tion in which positive charges would flow.
Figure 7 A complete circuit is required for charge to flow in a flashlight. Batteries must be placed so that charge can flow from negative to positive, passing through the bulb. Interpreting Diagrams What purpose does the spring at the base of a flashlight have?
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Section Resources Print • Laboratory Manual, Investigation 20A • Guided Reading and Study Workbook With Math Support, Section 20.2 • Transparencies, Section 20.2
Technology • iText, Section 20.2 • Presentation Pro CD-ROM, Section 20.2 • Go Online, NSTA SciLinks, Conductors and insulators
Conductors and Insulators L2
thin wire
thick wire metal ions electron
Conductors and Insulators Why is a metal wire usually coated with plastic or rubber? The metal wire is an electrical conductor. The rubber and plastic are electrical insulators. An electrical conductor is a material through which charge can flow easily. A material through which charge cannot flow easily is called an electrical insulator. The coating around a wire helps to control the current and keep it where it is needed. A metal is made up of ions in a lattice. The ions are not free to move. But each ion has one or more electrons that are not tightly bound to it. These free electrons can conduct charge. Most materials do not easily conduct charge because they don’t have free electrons. Metals such as copper and silver are good electrical conductors. Wood, plastic, rubber, and air are good electrical insulators.
Figure 8 Using a thick straw to drink a milkshake is easier than using a thin straw. Similarly, electrons flow more easily through a thick wire than they flow through a thin wire, assuming the wires are made of the same material. Applying Concepts Why should the wire in light bulb filaments be very thin?
Resistance As electrons move through a conducting wire, they collide with electrons and ions. These collisions convert some kinetic energy into thermal energy. Because less energy is available to move electrons through the wire, the current is reduced. Resistance is opposition to the flow of charges in a material. The SI unit of resistance is the ohm. When you drink a milkshake as shown in Figure 8, it is easier if you use a thicker straw. In the same way, resistance is lowered if you make a wire thicker because more electrons can flow through a thicker wire. A material’s thickness, length, and temperature affect its resistance. Resistance is greater in a longer wire because the charges travel farther. As temperature increases, a metal’s resistance increases because electrons collide more often. If resistance increases as temperature increases, what happens as you cool a conductor? Could you reduce the resistance to zero? This is the idea behind superconductors. A superconductor is a material that has almost zero resistance when it is cooled to low temperatures. The best superconductor found thus far must be cooled to about 138 K.
For: Links on conductors and insulators Visit: www.SciLinks.org
Resistance Integrate Earth Science
L2
Although early research in superconductivity concentrated on metals, the best superconducting materials known to date belong to the broad class of minerals called perovskites. Perovskites are composed of barium, lanthanum, copper, and oxygen. Certain ceramics made from clays containing perovskites were found in 1986 to become superconducting at the relatively high temperature of 35 K. This was 12 K higher than the highest temperature at which any metal alloy was superconducting. By altering some of the components in perovskites, higher superconducting temperatures have been reached. At present, the highest temperature for a superconducting material is 138 K. Have students research the history of superconducting ceramics and report on their findings. Verbal
Web Code: ccn-2202
Electricity
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Customize for Inclusion Students Visually Impaired Analogies, such as drinking a milkshake through a straw, are useful tools for improving the comprehension of visually impaired students. Develop similar analogies for other electrical processes. For instance, have students consider the insulating qualities of a winter ski jacket. Just as an electrical insulator coating a wire helps to
Students may think that only metals make good conductors. Point out that many solutions containing ions are good conductors. The acid inside a battery is one example. Charges also flow freely in ionized gases, such as the gases inside a fluorescent light or a neon light. In contrast to metals, where electrons are the only moving charges, both positive and negative charges move freely in ionized gases and in solutions containing ions. Logical
control electric current, the jacket contains thermal insulation that reduces the flow of thermal energy to help a person stay warm. Be sure that students understand that such analogies are not exactly the same as the processes that they help to illustrate, but have enough similarities in certain physical properties to make them useful.
Download a worksheet on conductors and insulators for students to complete, and find additional teacher support from NSTA SciLinks.
Answer to . . . Figure 7 It provides firm contact to the negative terminal of one battery and pushes the other battery into contact with the light bulb. Figure 8 Thin wire will heat up and become hot enough to glow.
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Section 20.2 (continued) Modeling Resistance in a Wire
Modeling Resistance in a Wire
Materials
L2
Objective After completing this activity, students will be able to • describe how the thickness of a conductor affects its resistance. Skills Focus Observing, Measuring, Controlling Variables, Using Models Prep Time 15 minutes
white paper, metric ruler, number 2 pencil, multimeter
3. Repeat Step 2 using the wide rectangle.
Procedure
Analyze and Conclude
1. Draw a narrow rectangle 1 cm wide by 5 cm long on the paper. Draw a wide rectangle, 3 cm by 5 cm. Use the pencil to completely fill in both rectangles with graphite.
1. Observing In which rectangle is the resistance greater? How does resistance change as the electrodes move together?
Voltage If you remove the batteries from a flashlight, the light will not shine. Why? Because there is resistance in the wires and the bulb, charges do not flow on their own without a source of energy. In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy.
Class Time 25 minutes Safety Students should wear safety goggles and aprons during the lab. Be sure students wash their hands after completing the lab.
Analyze and Conclude 1. Resistance is greater in the narrower rectangle; it decreases. 2. There are more available electrons for current within the cross-sectional area of a thick wire than of a thin wire. As wire length decreases, the distance electrons travel decreases, so the number of collisions that cause resistance decreases. Logical, Group
For Enrichment
Potential Difference Recall that potential energy is related to Figure 9 A water fountain has a pump inside that lifts water to the top, increasing the gravitational potential energy of the water. In the same way, a voltage source increases the electrical potential energy of electric charges.
L3
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Voltage L1
Relate Text and Visuals Refer to page 190D in Chapter 7, which provides the guidelines for relating text and visuals. Have students read the paragraphs about potential difference while referring to Figure 9. Emphasize that the figure provides an analogy, so that the difficult concept in the text will be easier to visualize. Ask, How are gravity and an attractive electric field similar? (Work must be done against each to increase the potential energy of an object.) Logical
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position. In Figure 9, water at the top of the fountain has more gravitational potential energy than water at the bottom. That is why water falls spontaneously from a higher to a lower height. In the same way, charges flow spontaneously from a higher to a lower potential energy. The potential energy of a charge depends on its position in an electric field. Potential difference is the difference in electrical potential energy between two places in an electric field. Potential difference is measured in joules per coulomb, or volts. Because it is measured in volts, potential difference is also called voltage.
Voltage Sources How does water get to the top of the fountain? A pump inside the fountain does work on the water to increase its potential energy. In the same way, a source of voltage such as a battery does work to increase the potential energy of electric charges. Three common voltage sources are batteries, solar cells, and generators. A battery is a device that converts chemical energy to electrical energy. Batteries, like other voltage sources, have terminals that can connect to wires in a circuit. One terminal is positive and the other is negative. A voltage drop, or potential difference, is maintained across the terminals. In a 9-volt battery, for example, the voltage drop is about 9 volts.
Have students repeat the experiment with different metal strips of identical dimensions and make a table listing the resistances. Kinesthetic
Build Reading Literacy
2. Using Models Explain why a thick wire has lower resistance than a thin wire if all else is equal. Why does resistance decrease as a wire’s length decreases?
2. Place the multimeter electrodes at opposite ends of the narrow rectangle. Record the resistance. Keep one electrode in place and
Materials white paper, metric ruler, number 2 pencil, multimeter
Expected Outcome Students will learn that electrical resistance decreases as the width of the resisting material is increased or the length of the resisting material is decreased.
slowly drag the other one toward it. Record your observations.
Facts and Figures Diamond As a rule, good conductors of heat are also good conductors of electricity. This is because most conductors are metals, and the electrons in metals can move with ease, transferring energy through the metal, regardless of whether the energy is thermal or electrical. However, one substance that is a good thermal conductor (in fact, the best thermal conductor) is an electrical insulator:
diamond. Diamond is a form of carbon in which all carbon atoms are bound to other carbon atoms in a tetrahedral crystal. These strong bonds give diamond its great hardness, and make it possible for kinetic energy to pass easily through the crystal by heat. However, because there are no free electrons to move throughout the crystal, diamond cannot conduct electricity.
Ohm’s Law
Ohm’s Law The unit of resistance, the ohm, is named after the German scientist Georg Ohm (1789–1854). It was Ohm who first determined how resistance and current affect voltage. He discovered that voltage is not the same everywhere in a circuit. Ohm hypothesized that resistance reduces the voltage. He published his research in 1826, but his findings were so controversial that he lost his job. Eventually his work became widely accepted. Ohm found a mathematical relationship between voltage, current, and resistance. This relationship became known as Ohm’s law. According to Ohm’s law, the voltage (V) in a circuit equals the product of the current (I) and the resistance (R). Ohm’s Law
Build Science Skills
Calculating Help students use Ohm’s law to calculate the voltage when the current is 4.0 amps and the resistance is 3.0 ohms. Figure 10 A multimeter can be used to measure current, voltage, or resistance. Here the voltage of a 9-volt battery is measured.
V I R or I V R
VIR 4.0 amps 3.0 ohms 12 volts Then, help students understand that current is indirectly proportional to resistance by doubling the resistance and calculating the new current while the voltage remains constant: I V/R 12 volts/6.0 ohms 2.0 amps Point out that when the resistance is doubled, the current decreases by one half. Logical
When the current is in amperes and the resistance is in ohms, the voltage is in volts. What is the voltage if the resistance is 3 ohms and the current is 3 amps? V I R 3 amps 3 ohms 9 volts
3 ASSESS
Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current. A multimeter, shown in Figure 10, is a device used to measure current, voltage, and resistance.
Evaluate Understanding
Reteach 1. 2. 3. 4. 5. 6.
List the two types current. Name two good electrical conductors and two good electrical insulators. What variables affect the resistance of a material? What causes charge to flow? According to Ohm’s law, how is voltage related to resistance and current? What is a superconductor?
Critical Thinking
8. Applying Concepts Use Ohm’s law to explain how two circuits could have the same current but different resistances.
Compare-Contrast Paragraph Write a paragraph comparing and contrasting conductors and insulators and the ways in which they might be used. (Hint: Identify materials that are good conductors and materials that are good insulators.)
7. Problem Solving Suppose you have two wires of equal length made from the same material. How is it possible for the wires to have different resistances?
Electricity
Section 20.2
Assessment
1. Direct current, alternating current 2. Good conductors: silver, copper; good insulators: air, plastic 3. Length, thickness, temperature 4. A source of electrical energy causes charge to flow. 5. Voltage equals the product of current and resistance. An increase in voltage increases current. Keeping the same voltage and increasing resistance decreases current.
L2
Ask students to write two questions each about current, resistance, and voltage. Review the questions for accuracy. Then, have students form groups and ask each other their approved questions.
Section 20.2 Assessment Reviewing Concepts
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6. A superconductor is a material that has almost zero resistance when cooled to low temperatures. 7. The two wires may have different thicknesses or different temperatures, which would give them different resistances. 8. If the voltages in the two circuits were such that the ratio of voltage to resistance was the same, both circuits would have the same amount of current.
L1
Have students use Figure 9 to explain how voltage increases the electrical potential of a charge.
Paragraphs should describe conductors as materials, such as copper and silver, through which charges move easily. Insulators should be described as materials, such as wood, plastic, or rubber, through which charges do not move easily. Conductors are used in electrical wiring for buildings, and in electrical appliances, while insulators are used to isolate conductors from unwanted contact. If your class subscribes to iText, use it to review key concepts in Section 20.2.
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Should Car Companies Be Required to Make Electric Cars?
Should Car Companies Be Required to Make Electric Cars? Battery-powered electric cars were first introduced in the late 1880s. They could only be used for short distances at low speeds, but they were quiet and had low maintenance costs. By the 1920s, electric cars were mostly replaced by gasoline-powered cars. Since the 1970s, growing concerns about fuel shortages and pollution from car exhaust has renewed interest in electric cars.
L2
Background In the 1990s, attempts were made by a number of auto manufacturers to develop an electric automobile. Difficulties with the early models include the long amount of time needed to recharge the batteries and the limited distance that such vehicles can travel before they need to be recharged. Such problems may be overcome in the future. Nevertheless, electric vehicles work well enough for driving within a city, and they can be recharged overnight. For this reason, the United States Postal Service is planning to acquire fleets of electric vehicles for local mail delivery. Additionally, the research on the electric vehicle has led to the development of another new type of car: the hybrid automobile (see p. 488). Answers 1. Answers may include the costs of production regardless of demand, legal questions about how much government can or should be involved with business, and the disadvantage of having to manufacture electric cars even if the technology of the cars does not improve. 2. Answers for regulation may include: Auto companies are not likely to develop or market the cars on their own; electric cars are efficient and quiet with low upkeep costs. Answers against regulation may include: Technology should progress as people need and want it; electric cars are expensive and unpopular, and they need frequent recharging. 3. Students should state their opinions and provide reasons based on facts. 4. Students’ letters should be written in a persuasive style.
When an electric car is turned on, current flows from the battery to a controller. The controller converts the battery’s direct current into alternating current that can be used by the motor. The controller also determines how much power the motor needs. Electric cars rely indirectly on fossil fuels used at electric power plants. However, these cars don’t have engines that burn fossil fuels, so they have no harmful emissions. For this reason, the U.S. government has begun requiring vehicle manufacturers to meet development requirements for electric cars. Should car manufacturers be required to make electric cars?
The Viewpoints Car Companies Should Be Required to Make Electric Cars
Car Companies Should Not Be Required to Make Electric Cars
The issue seems clear to those in favor of electric cars. If everyone agrees that electric cars have the lowest emissions of any automobile, shouldn’t everyone use them? These people feel that the best way to encourage drivers to use electric cars is to require car manufacturers to produce them. It may take people time to get used to driving electric cars, but now is the time to start changing attitudes.
Other people argue that production of electric cars should not be encouraged by government regulations. This type of technology should progress as people need and want it.
Electric cars are efficient and quiet, and they have low upkeep costs. Today’s electric cars are expensive because they are a relatively new technology and aren’t widely used. When the cars can be mass produced, prices will drop.
Opponents of required production also point out that electric cars have some drawbacks. With gasoline-powered cars, you can fill the tank and drive hundreds of miles before refueling. Electric cars run on batteries that need frequent recharging.
Research and Decide 1. Defining the Issue Describe the major issues involved in requiring car companies to produce electric cars.
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4. Writing in Science Write a letter to the editor of a local newspaper stating your opinion.
2. Analyzing the Viewpoints What are some reasons people think the government should or should not regulate production of electric cars? 3. Forming Your Opinion Should car companies be required to produce electric cars? Explain why or why not.
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Have students further research the issues related to this topic.
At present electric cars are expensive and unpopular. Recent sales of these cars have been low, so manufacturers don’t want to keep making them.
For: More on this issue Visit: PHSchool.com Web Code: cch-2203
