Canada s requirement for electrical energy rises slightly every year.

Transcription

1 CHAPTER 13 Electromagnetic induction is used to generate most of the electrical energy used today. Learning Expectations By the end of this chapter, you will: Relating cience to Technology, ociety, and the Enironment analyze the efficiency and the enironmental impact of one type of electrical energy production, and propose ways to improe the sustainability of electrical energy production Deeloping kills of Inestigation and Communication sole problems inoling the number of turns in the primary and secondary coils of a transformer inestigate electromagnetic induction, and, using Lenz s laws, the law of conseration of energy, and the right-hand rule, explain and illustrate the direction of the electric current induced by a changing magnetic field Understanding Basic Concepts explain Faraday s law and Lenz s law describe the production and interaction of magnetic fields, using diagrams, Faraday s law, and Lenz s law explain the operation of a generator explain why alternating current is presently used in the transmission of electricity describe the components of step-up and step-down transformers, and explain the operation of these transformers describe and explain safety precautions related to higher transmission oltages Canada s requirement for electrical energy rises slightly eery year. To meet this increase in demand, the production of electrical energy rises slightly eery year. Canada is the seenth largest producer of electrical energy in the world, generating MW h annually. In addition to using conentional methods of generating electrical energy such as using moing water, burning fossil fuels, and using nuclear reactions to turn the turbines in a generator Canada is deeloping emerging technologies to generate electrical energy. ome of these technologies use the un, biomass, wind, geothermal, and tidal power to produce electrical energy. The ability to generate electrical energy on a large scale has only been around for almost 130 years. In Canada, the first steam-drien power plant was built in Ottawa in 1883 to proide electrical energy for the lamps in the Parliament Buildings. Today, about 97 percent of Canada s electrical energy is produced from water, fossil fuels, and nuclear energy. Only three percent of Canada s electrical energy is produced using emerging technologies, which include biomass, wind, geothermal, tidal, and solar energy (Figure 13.1). The reasons that such a small percentage of electrical energy is produced using alternatie methods relates to different factors including the difficulty in connecting to the power grid and the establishment of power plants that use cheaper sources of fuel. Another problem facing emerging technologies is that there is no practical way to store electrical energy. This means that reliable electrical energy generation must be able to respond to different needs of the grid. As the demand for electrical energy increases, the supply must also increase. Hydroelectric and gas-burning generating stations can more easily respond to an increase in demand than other technologies. Figure 13.1 First Light olar Park is Canada s largest solar energy generating facility. Located in tone Mills, Ontario, the plant opened in October It has photooltaic solar panels on 36 hectares, and is able to generate 9.1 MW of electricity. 420 Unit E

2 13.1 Using Magnetism to Induce an Electric Current ection ummary The law of electromagnetic induction states that a changing magnetic field in the region of a conductor can induce an electric current in a closed loop. Faraday s law of electromagnetic induction states that a changing magnetic field in the region of a closed-loop conductor will induce an electric current. Lenz s law states that an induced current and emf are in such a direction as to oppose the change that produced them. Once scientists discoered that electricity could be used to create magnetism, the push was on to determine if the reerse was true could a magnetic field induce an electric current? Many scientists experimented with magnetism, but Michael Faraday was the first to be successful in inducing current with a magnet. Faraday s discoery eentually led the way to generating the electricity that we use in our homes. The Discoery of Electromagnetic Induction Michael Faraday was a great experimental scientist. Born in 1791 to a lower-class family in England, he had almost no formal education. While working as a bookbinder s apprentice, he educated himself by reading the books that were being bound. He managed to secure a job as an apprentice to ir Humphry Day, a British chemist, and deeloped a reputation as an excellent research scientist. Faraday is most remembered for demonstrating that magnetism can produce electricity. The production of electricity by magnetism is called electromagnetic induction. It is interesting to note that in 1831 when Faraday was doing his experiments in England, Joseph Henry ( ) was simultaneously inestigating electromagnetic induction in the United tates. Figure 13.2 shows a simplified ersion of Faraday s and Henry s experiment. Faraday and Henry showed that when a magnet approaches a coil, an induced current is produced in one direction. When the magnet is moed away from the coil, the induced current is produced in the opposite direction. When the magnet is held stationary, no current is induced. When the magnet is held stationary and the coil of wire is moed back and forth, similar induced currents are produced. It does not matter if the magnet or the coil of wire moes, as long as there is relatie motion between the coil of wire and the external magnetic field. Faraday s Law of Electromagnetic Induction Faraday concluded that when a wire moes perpendicular to a magnetic field or cuts through magnetic field lines an induced current is produced. Faraday s law of electromagnetic induction states: A changing magnetic field in the region of a closed-loop conductor will induce an electric current. PHYIC IIGHT To induce a current, the circuit must be a closed loop. If the conductor is not closed, charges will build up on each end, but will not flow. galanometer Figure 13.2 When a magnet is moed toward a loop of wire that is connected to a galanometer, the galanometer needle deflects. This indicates that a current is being induced in the coil of wire. A galanometer is a ery sensitie ammeter designed to measure small currents. PHYIC OURCE uggested Actiity E12 Design a Lab Oeriew on page 425 Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 421

3 Figure 13.3 A segment of a closed loop of wire moes through a magnetic field. The magnetic field ( B ), elocity of the wire ( ), and wire must be perpendicular to each other to induce an emf and a current. PHYIC IIGHT The term electromotie force is inaccurate. The term was introduced by Alessandro Volta to describe the force that was causing the charge separation in his battery. Howeer, emf is not a force. It is an electrical potential difference measured in olts (V). Electromotie Force (emf) When a wire moes through a magnetic field not only is a current induced, but an electric potential difference is also induced. This induced electric potential difference is often called the electromotie force (emf). An emf and current can only be induced in a wire if the wire moes perpendicular to the magnetic field (Figure 13.3). Factors Affecting the Electromotie Force and Induced Current Faraday discoered that three factors influence the magnitude of the emf and induced current in the wire. These factors are: the elocity of the wire as the elocity increases, the emf and induced current increase the strength of the magnetic field as the strength of the magnetic field increases, the emf and induced current increase the length of the wire in the external magnetic field as the length of the wire increases, the emf and induced current increase Concept Check 1. What is Faraday s law of electromagnetic induction? 2. What is the emf and why is the term misleading? 3. What factors affect the emf and induced current? PHYIC OURCE Explore More What roles do elocity, magnetic field strength, and length of wire play in electromagnetic induction? PHYIC OURCE uggested Actiity E13 Quick Lab Oeriew on page 425 The Direction of Induced Current Although Faraday and Henry discoered induced currents in 1831, they were not able to explain why an induced current was produced first in one direction and then in the opposite direction. It took the work of Russian physicist Heinrich Lenz ( ) to understand the factors that determine the direction of an induced current. Lenz s Law In 1834, Lenz recognized that the moement of a wire in an external magnetic field creates a current that, in turn, produces a force to oppose its motion. His description of the interaction between the induced current and the resulting force on a wire is known as Lenz s law, which states: An induced current and emf are in such a direction as to oppose the change that produced them. In other words, the induced current will create a magnetic force that acts on the wire. The direction of this force is opposite to the direction of the wire s elocity. 422 Unit E Electricity and Magnetism

4 Right-Hand Rule for Induction in traight Wire Conductors We can use the right-hand rule for induction to determine the direction of the induced conentional current, emf, and magnetic force in a wire (Figure 13.4). Your thumb indicates the direction of elocity ( ). The magnetic force ( F m ) is in the opposite direction. Your extended fingers point in the direction of the magnetic field ( B ). Your palm faces the direction of both the induced current and induced emf. I I B Right-Hand Rule for Induction in olenoids If a magnet moes toward a solenoid, the motion of its field lines across the wires will induce a current. Figure 13.5(a) shows the north pole of a permanent magnet approaching a solenoid. According to Lenz s law, the direction of the induced current in the wire will create a magnetic field to oppose the motion of the magnet. The induced current will flow in such a way as to create a north pole nearest the magnet, repelling it. We can use the right-hand rule for solenoids to predict the direction of the induced current (Figure 13.5(b)). Your thumb points in the direction of the north pole of the solenoid. Your fingers curl in the direction of the induced current. F m Figure 13.4 The right-hand rule for induction (a) (b) ammeter Figure 13.5 (a) A magnet moes toward a solenoid. (b) The induced current creates a north pole nearest the magnet to repel it. If the north pole of the magnet is moed away from the solenoid, according to Lenz s law the induced current will create a south pole nearest the magnet to oppose that motion (Figure 13.6). The induced current always opposes the motion of the wire or magnetic field. Figure 13.6 If the motion of the permanent magnet is reersed, the induced current creates a magnetic field to oppose this motion. Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 423

5 Example 13.1 Practice Problems 1. Determine the direction of the induced current in the solenoid shown in Figure Figure Determine the motion of the bar magnet necessary to produce the current shown in Figure Figure Answers 1. The induced current in the solenoid flows through the ammeter right to left. 2. The bar magnet is moing away from the solenoid. I Determine the direction of the induced current in the solenoid in Figure Figure 13.7 Gien direction of magnet: pole toward solenoid Required direction of the induced current in the solenoid Analysis and olution According to Lenz s law, the induced current in the solenoid creates a magnetic field to oppose the motion of the bar magnet. The solenoid must hae a south pole facing the bar magnet to oppose its motion. We can use the right-hand rule for solenoids to determine the direction of the current. The thumb points in the direction of the north pole of the solenoid and the fingers curl in the direction of the current (Figure 13.8). Figure 13.8 Paraphrase The induced current in the solenoid flows through the ammeter from right to left. PHYIC OURCE Take It Further Explore the design of a metal detector. Find out how the detector uses Lenz s law in its operation. Lenz s Law and the Law of Conseration of Energy The law of conseration of energy states that in an isolated system, energy cannot be created or destroyed. Energy can only be transferred from one form to another. Imagine if the wire in Figure 13.4 on page 423 produced a force in the same direction as its moement. In this case, the wire would increase its elocity because of this force. As the elocity of the wire increased, the force would also increase. In effect, this situation would create energy and iolate the law of conseration of energy. Lenz s law the direction of an induced current opposes the change that produced it is simply the law of conseration of energy applied to electromagnetic induction. 424 Unit E Electricity and Magnetism

6 REQUIRED KILL E12 Design a Lab PHYIC OURCE Designing an experimental procedure Drawing conclusions Faraday s Induced Current Experiment Question How can you induce a current in the secondary coil of Faraday s iron ring? Actiity Oeriew In this actiity, you will conduct your own inestigation to answer the question aboe. You will write a hypothesis and identify all of ariables and how to control the appropriate ones. You will then carry out your inestigation. Your teacher will gie you a copy of the full actiity. Prelab Questions Consider the questions below before beginning this actiity. 1. What conditions are necessary to induce a current in a wire? iron bolt primary coil secondary coil battery switch galanometer Figure A modified ersion of Faraday s experiment 2. What difficulties might you encounter if a battery is connected to the primary coil and is used to induce a current in the secondary coil? E13 Quick Lab PHYIC OURCE The Direction of Induced Emf and Current Purpose To obsere the direction of the induced current in a solenoid Actiity Oeriew In this actiity, you will attach a galanometer to a solenoid and record the direction the needle moes when you bring a magnet toward the solenoid. You will compare what happens when the north pole of a magnet approaches the solenoid with what happens when the south pole of a magnet approaches the solenoid. Your teacher will gie you a copy of the full actiity. Prelab Questions Consider the following questions before beginning this actiity. galanometer Figure Actiity setup 1. What prediction does Lenz s law make about the direction of the induced current through a solenoid? 2. How will the direction of the magnet s motion affect the direction of the induced current? Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 425

7 13.1 Check and Reflect Key Concept Reiew 1. What generalization can you make to relate the elocity of a wire moing perpendicularly through an external magnetic field and the induced current in the wire? 2. Explain why a solenoid contains many loops of thin wire instead of one loop of thick wire. 3. Write a general statement that describes Lenz s law in your own words. Be sure to include the words motion, wire, force, induced current, and magnetic field. 4. Lenz s law is often considered to be the law of conseration of energy for electromagnetic induction. Explain why this is the case. 5. Will a current be induced in the wire segment shown in the following diagram? Explain your answer. 9. (a) Determine the direction of the force on the wire segment shown in the following diagram. (b) Determine the direction of the induced current and emf in the wire. Question What pole (north or south) of a bar magnet is approaching the right side of the solenoid shown in the following diagram? Question 10 ammeter Question 5 Connect Your Understanding 6. A student accidently drops a bar magnet through the top of a solenoid that is standing upright on a desk. Explain what the student might obsere with respect to the acceleration of the magnet. 7. If a loop of wire moes through a magnetic field as shown below, explain what a student would see in an ammeter attached to the loop. 11. A student wants to increase the induced current in a solenoid. The student places a bar magnet on either side of the solenoid and moes the solenoid to the left as shown in the following diagram. Will this procedure work? Explain your answer. Question 11 ammeter 12. A solenoid is moed away from a bar magnet as shown in the following figure. Determine the direction of the induced current through the ammeter. Question 7 8. A student holds the south pole of a permanent magnet near one end of a solenoid. Explain how the student could create a magnetic field in the solenoid so that its south pole is nearest the magnet. Reflection ammeter Question What concept did you find most difficult to grasp in this section? Why? For more questions, go to PHYIC OURCE 426 Unit E Electricity and Magnetism

8 13.2 The Generator and Electrical Energy Generation ection ummary A generator conerts mechanical energy to electrical energy. Methods of generating electrical energy inole using flowing water, burning fuels, or using nuclear, wind, tidal, solar, or geothermal energy. Each method of generating electrical energy has an enironmental impact. Hand-cranked radios and flashlights are the perfect solution for people who use these deices whether on camping trips or at the cottage but hae limited access to batteries (Figure 13.13). To charge the deice, the hand crank is turned, which charges a rechargeable battery contained within the deice. The hand crank is connected to a small generator in the radio that charges the battery. The Generator After Faraday discoered how to induce an electric current in a wire, he attached two copper wires to a copper disc that was placed between the poles of a horseshoe magnet. By rotating the disc, he produced a continuous current. He had built the first generator (Figure 13.14). A generator is a deice that conerts mechanical energy into electrical energy. A generator has a design similar to an electric motor. It has an armature, stator, a type of commutator, and brushes just as in an electric motor. Howeer, instead of being connected to a battery, a generator is connected to an electrical load, such as a resistor or light bulb. Usually a turbine (set of fan blades) is connected to the shaft of the armature. When the turbine rotates drien by steam, water, or wind the shaft rotates and the armature moes in relation to the magnetic field, producing current through the load. The DC Generator Figure shows a DC generator. ote that the generator has a split ring commutator. The direction of the current in the DC generator remains the same through the light bulb at all times because of the split ring commutator. When the turbine is shaft made to spin clockwise, segment AB moes up through the external magnetic field and I B segment CD moes down. In this case, the elocity of AB is up and the elocity of CD is down. Using the right-hand rule for electromagnetic induction, we know that the induced current moes from A to B to C to D. It also flows from right to left through the light bulb. split ring commutator light bulb A D I brush armature Figure The rechargeable battery in a radio can be recharged using a generator that is turned by a hand crank. Figure A generator based on Faraday s design. turbine C PHYIC OURCE uggested Actiity E14 Quick Lab Oeriew on page 431 Figure A DC generator. A turbine is connected to a shaft. When the turbine spins, the armature turns. Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 427

9 As the turbine spins, the armature changes position. Figure shows the position of the armature after it has made one-quarter turn. At this position, no current or emf is induced in the armature because the elocity of the armature is parallel to the magnetic field. This means that in traelling one-quarter of a turn, the armature has gone from producing a maximum current and emf to producing no current and emf. Figure shows a graph of the induced current and emf as the armature makes one full turn. B Induced DC and EMF s. Turn of Armature A D C Induced Current and emf Turn of Armature Figure At this position, the elocity of AB and CD is parallel to the external magnetic field. Figure A graph of the induced current and emf produced by a DC generator. The AC Generator An AC generator produces alternating current. It differs from a DC generator in one important aspect: it uses two solid ring commutators as shown in Figure The induced current and emf in the armature are able to alternate direction eery half turn because there is no split ring commutator to keep the current in the same direction as in a DC generator. An AC generator creates an induced current and emf as shown in Figure This alternating current is used in electrical energy generation and transmission. It is the same type of current used in your home. Figure The AC generator has two separate commutators, which make it possible for the current to change direction eery half-turn of the armature. commutators light bulb A shaft I B D I armature turbine C Induced Current and emf Induced AC and EMF s. Turn of Armature Turn of Armature Figure A graph of the induced current and emf produced by an AC generator. 1 Concept Check PHYIC OURCE Explore More How does the induced current depend on the position of the armature in a generator? 1. Predict the effect that a stronger magnetic field would hae on the induced current and emf of a generator. 2. Predict the effect that the elocity of the armature in a generator has on the induced current and emf. 3. Explain the differences between a DC generator and an AC generator. 428 Unit E Electricity and Magnetism

10 Generating Electrical Energy Most of the electrical energy that we use is produced by large generators. These generators use water, steam, or wind to turn a turbine, which turns coils of wire inside large magnets inside the generator. Almost 60 percent of Canada s electrical energy is generated by harnessing the power of flowing water. The rest is generated by burning fuels to produce steam, or using nuclear, wind, tidal, solar, or geothermal energy. PHYIC OURCE uggested Actiity E15 Decision-Making Analysis Oeriew on page 431 Hydroelectric tations In a hydroelectric generating station, water flows from a waterfall or through a dam past a turbine, which causes the turbine to turn (Figure 13.20). The turbine is connected to a generator that conerts the mechanical energy from the turning motion of the turbine into electrical energy. Ontario generates just oer 24 percent of its electrical energy in hydroelectric stations. Table 13.1 lists some adantages and disadantages of hydroelectric stations. Table 13.1 Adantages and Disadantages of Hydroelectricity Adantages Disadantages no pollutants produced uses a renewable resource (water) Fossil Fuel Generating tations can only be built near waterways large financial and enironmental costs associated with building a dam Coal, oil, and natural gas are fossil fuels that are burned in generating stations to heat water and produce steam (Figure 13.21). The steam spins a turbine to produce electrical energy. Ontario generates about 22 percent of its electrical energy using fossil fuels. Table 13.2 lists some adantages and disadantages of burning fossil fuels to generate electrical energy. Figure The Caribou Falls hydroelectric station near Kenora, Ontario Table 13.2 Adantages and Disadantages of Fossil Fuels Adantages relatiely low cost associated with building plants cost of electrical energy produced is ery low Disadantages uses non-renewable resources releases pollutants into the atmosphere olar-thermal Energy A solar-thermal farm has arrays of mirrors that focus sunlight to heat water and produce steam to drie turbines. Table 13.3 lists some adantages and disadantages of using solar-thermal energy to generate electrical energy. Table 13.3 Adantages and Disadantages of olar Energy Adantages renewable no pollutants produced Disadantage must be located in sunny location high cost of installation Figure The Lambton Generating tation is a coal-fueled station in southwestern Ontario. PHYIC OURCE Take It Further Determine the cost of electrical energy produced by different methods. Rank in order of highest to lowest cost. Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 429

11 uclear Power Plant Fifty-three percent of the electrical energy in Ontario is produced using the energy from nuclear fission reactions. The energy produced when uranium atom s split through fission is used to heat water and conert it to steam, which passes through the turbines of a generator (Figure 13.22). Table 13.4 lists some adantages and disadantages of nuclear power plants. Table 13.4 Adantages and Disadantages of uclear Power Plants Adantages Disadantages Figure The Darlington uclear Generating tation proides about 20 percent of Ontario s electrical energy needs. produces enormous amounts of energy no pollutants emitted into the atmosphere Wind Energy are costly to build and maintain produces dangerous radioactie waste materials that must be carefully stored uses non-renewable resources Wind turns large turbines attached to generators (Figure 13.23). The amount of electrical energy generated depends on the speed of the wind. Currently wind energy proides less than one percent of Ontario s electrical energy. Table 13.5 lists some adantages and disadantages of using wind energy to generate electrical energy. Table 13.5 Adantages and Disadantages of Wind Energy Adantages no pollutants produced turbines do not interfere with land use Disadantages wind is unpredictable bird migration may be affected Figure A series of wind turbines on the shore of Lake Erie near Port Bruce, Ontario. Other Energy ources There are other energy sources solar photooltaic, biomass, tidal, and geothermal that can be used to produce electrical energy. Table 13.6 summarizes these energy sources, and the adantages and disadantages associated with each source. Table 13.6 Other ources of Energy Energy ource How It Works Adantages and Disadantages olar photooltaic Biomass Tidal Geothermal a solar photooltaic cell can directly turn sunlight into electrical energy organic waste products decompose to produce methane gas, which is burned to heat water and produce steam the graitational force of the Moon produces powerful tides that turn turbines geothermal areas, such as hot springs, produce hot water and steam to drie turbines and produce electrical energy solar energy is non-polluting and renewable solar cell efficiency is low so many cells are needed electrical energy produced is ery expensie waste matter is readily aailable releases pollutants into the atmosphere tidal energy is dependable and renewable a tidal station can impact marine life a tidal station can only be built in specific areas geothermal energy is non-polluting and renewable in Canada, geothermal areas are confined to certain areas in British Columbia 430 Unit E Electricity and Magnetism

12 E14 Quick Lab PHYIC OURCE Turning a Motor into a Generator Purpose To turn a DC motor into a generator ammeter oltmeter Actiity Oeriew In this actiity, you will conert a DC motor into a generator. Your teacher will gie you a copy of the full actiity. Prelab Questions Consider the questions below before beginning this actiity. 1. What similarities exist between a DC motor and a DC generator? motor/generator light bulb Figure etup for turning a motor into a generator. 2. What changes can be made to a DC motor to turn it into a DC generator? DI Key Actiity E15 Decision-Making Analysis A ustainable Choice Issue There are enironmental impacts associated with the different ways of generating electrical energy. How can we improe the sustainability of a particular method of electrical energy production? Figure Panels of solar photooltaic cells installed on a roof. PHYIC OURCE REQUIRED KILL Organizing information ummarizing information Actiity Oeriew In this actiity, you will read about and research one method of electrical energy generation. You should look at the efficiency and the enironmental impact of that particular method. You should also propose ways to improe the sustainability of the energy source used. For example, you could research the efficiency and enironmental impacts of a coal-burning plant, and learn about the technologies being deeloped to improe the efficiency of coal. You will use information found in your textbook and at Physicsource to learn more about the different generation methods and technologies. You will present your findings in the form of a Wiki, a presentation, a ideo, or podcast. Your teacher will gie you a copy of the full actiity. Prelab Questions Consider the questions below before beginning this actiity. 1. What are the enironmental impacts of different electrical energy generation technologies? 2. Which methods of generating electrical energy use renewable resources? Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 431

13 13.2 Check and Reflect Key Concept Reiew 1. Compare the purpose of a generator with the purpose of an electric motor. 2. Identify the difference in design and power output of a DC and AC generator. 3. ketch the graphs of the induced current and emf of a DC generator and an AC generator. Explain the graphs. 4. Identify two common design features of fossil fuel, nuclear, and solar-thermal generating plants. Connect Your Understanding 5. Explain how Lenz s law might manifest itself in the operation of a generator. 6. Predict what the power generation graph would look like for an AC generator if it were designed to use two armatures as shown in the following diagram. Question 6 7. The design of the generator inoles a loop of wire rotating in a static magnetic field. Discuss the feasibility of a generator design where a magnet rotates in a static coil of wire. 8. Which power generation technology does not require a generator? Discuss whether this is an adantage or not. 9. Identify a reason why wind power may not be a practical way of generating electrical energy on a large scale. 10. In what ways are wind, tidal, and hydroelectric technologies different from nuclear, fossil fuel, biomass, solar-thermal, and geothermal technologies? 11. Create a concept map for the different electrical energy generation technologies. 12. askatchewan has hot summers and cold winters with many clear days. The land is generally flat in the south, where most of the population lies. It has coal, oil, and natural gas resources. Discuss a type of electrical energy generation technology that would be well-suited for askatchewan. Explain your reasoning. 13. Proide two reasons why Ontario obtains 53 percent of its electrical energy needs from nuclear power plants. 14. Explain the difference between solar thermal and solar photooltaic power generation technologies. 15. oa cotia is a maritime proince. It has natural gas, coal, wind, and hydropower resources. Imagine that you are part of a committee that has the task of determining the type of electrical energy generation facilities that oa cotia will use in the future. Choose a method of electrical energy generation that you think would be suitable for this region. Justify your choice. 16. A teacher suggests that two identical motors can be connected to each other so that with just an initial rotation of the shaft of one of the motors, the second motor would start to run. After the second motor begins to run, it would power the first and the process would continue foreer. Discuss the logic of this scenario. Reflection 17. Explain any difficulty you may hae had understanding the operation of a generator, and how you oercame this difficulty. For more questions, go to PHYIC OURCE 432 Unit E Electricity and Magnetism

14 13.3 The Transmission of Electrical Energy ection ummary A transformer can step up (increase) or step down (decrease) oltage. There are precautions related to keeping the public safe from the hazards of high oltage. The transmission and distribution of electrical energy connects generating plants to consumers. All across Canada, a network of generating plants and transmission lines transmit electrical energy from the source (generating plant) to homes, businesses, and industries where the electrical energy is used. The Power Grid Power plants such as hydroelectric and fossil fuel generating stations, and nuclear power plants make up one part of the power grid. The power grid is a network of generating plants, substations, and transmission lines that moes electrical energy from the power plant to the end user of the electrical energy (Figure 13.26). transmission lines and towers substation Figure The power grid transfers electrical energy from the power plant to the user. The whole grid is a complete circuit. transformer power plant transmission substation homes businesses industries The Power Plant and Transmission ubstation Electrical energy has to trael long distances from the power plant to urban centres. The generators at a power plant produce a large amount of electrical energy at a relatiely low oltage typically only 11 to 22 kv. This oltage is not high enough to be efficiently transmitted long distances because too much electrical energy would be lost from resistance in the transmission lines. A transmission substation located near the power plant is used to increase the oltage up to 500 kv (Figure 13.27). The electrical energy is then transmitted oer large distances along high-oltage transmission lines. Power ubstation A power substation reduces the high oltage and redirects it in multiple directions, often to different areas of a city where the oltage is again reduced. By the time the electrical energy reaches your home it has been reduced to 240 V. Electrical outlets in a house proide either 240 or 120 V. The oltage is either increased or decreased using transformers. Figure A transmission substation increases the oltage. Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 433

15 Transformers Figure A transformer transforms oltage from one leel to another using induction. AC power supply primary coil light bulb secondary coil Figure A typical transformer consists of two coils of wire wrapped around an iron core. To minimize electrical energy losses in the transmission system, electrical energy is transmitted at high oltage and low current. At its destination, howeer, a low oltage is required by the consumer. A transformer is a deice that transfers electrical energy from one circuit to another. In other words, a transformer can increase (step up) or decrease (step down) the oltage. Transformers come in different sizes (Figure 13.28). The transformers at substations can be bigger than a car, but transformers can also be so small that they can fit on circuit boards in deices such as TVs and radios. They are found in appliances where the supplied oltage is different from that required by the deice. For example, adapters that plug into a wall outlet are transformers designed to reduce the house oltage of 120 V to a few olts. Transformer Design A transformer consists of two separate coils of wire wrapped around an iron core. The two coils are insulated so that no electricity can pass from one coil to the other through the iron core (Figure 13.29). Each coil is connected to its own circuit. The coil that is connected to an AC power supply is called the primary coil. The secondary coil is connected to a separate circuit that has an electrical load. When an AC current is applied to the primary coil, it creates an expanding and collapsing magnetic field. As the current increases in the primary coil, the magnetic field it creates moes through the iron core across the secondary coil, inducing a current in the secondary coil (Figure 13.30(a)). This is the effect we saw when the north pole of a magnet moed toward the coils of a solenoid (Figure 13.30(b)). When the current decreases in the primary coil, the magnetic field collapses and recedes, which induces a current in the opposite direction in the secondary coil (Figure 13.30(c)). This is the effect we saw when the magnet moed away from a solenoid (Figure 13.30(d)). (a) (b) (c) (d) AC power supply light bulb AC power supply light bulb primary coil secondary coil primary coil secondary coil Figure (a) A current is induced in the secondary coil when an expanding magnetic field moes through it. (b) The same effect is created when a magnet moes toward a solenoid. (c) The current in the secondary coil of the transformer changes direction as the magnetic field collapses and moes away from it. (d) The same effect is created when a magnet moes away from a solenoid. Calculating the Induced Current in a Transformer A transformer has no moing parts and is ery efficient. For an ideal transformer, we assume there is no loss of energy between the primary and secondary coils, and we assume that the power in the secondary coil (P s ) is equal to the power in the primary coil (P p ). Mathematically this can be expressed as: 434 Unit E Electricity and Magnetism

16 P p P s V p I p V s I s V p _ V s _ Is I p where V p is the oltage in the primary coil in olts (V), V s is the oltage in the secondary coil, I p is the current in the primary coil in amperes (A), and I s is the current in the secondary coil. Experiments with transformers in the 1800s found that the number of turns of wire in the primary coil affects the oltage in the secondary coil. The relationship between the oltage and number of turns can be expressed as: PHYIC OURCE uggested Actiity E16 Quick Lab Oeriew on page 437 V p _ V s _ p s where represents the number of turns of wire. It is a number without a unit. The relationship between the oltage, induced current, and the number of turns in a transformer can be expressed as: V p _ V s _ p _ I s s I p tep-up Transformers A step-up transformer is used to increase the oltage from the primary to the secondary coil. It has more turns of wire in the secondary coil than in the primary coil. Figure shows a step-up transformer. AC power supply light bulb tep-down Transformers A step-down transformer is used to decrease the oltage from the primary to the secondary coil. It has a lower oltage in the secondary coil than in the primary coil. It has fewer turns in the secondary coil than in the primary coil. Figure shows a step-down transformer. The Importance of Using Transformers A step-up transformer increases the oltage so that there is less electrical energy lost during transmission. If the oltage were not increased by a transformer, the electrical energy lost because of resistance within the transmission lines would be great. To see why this is the case, we will look at two identical power plants that generate 4.0 MW of power transmitted oer 950 km. One of the power plants increases the oltage to 500 kv while the other does not. If we assume that each generator produces a oltage of V, we can calculate the current at each plant: P VI I P_ V W V 182 A A high-oltage transmission line typically has a resistance of /km. For the plant that does not step up the oltage, we can determine the power loss through the lines: P I 2 R (182 A) 2 (0.054 /km 950 km) W primary coil AC power supply primary coil secondary coil Figure A step-up transformer light bulb secondary coil Figure A step-down transformer Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 435

17 Howeer, at the second plant where the oltage is increased to 500 kv, we can calculate the secondary current and determine the power loss: V p _ V s _ Is I p I s _ V pi p V s ( V)(182 A) V 8.01 A P I 2_ s R (8.01 A) 2 (0.054 /km 950 km) W The power lost through the line from the first power plant is about 1.7 MW almost half of the power generated by the plant. The power lost from the plant that increased the oltage is only 3.29 kw, which is less than 0.1 percent. Concept Check 1. Is alternating current necessary for the proper functioning of a transformer or can a steady direct current be used? Explain your answer. 2. Does the power output of a step-up transformer remain the same as the power input as the transformer increases the oltage? Explain your answer. 3. Proide a possible explanation why alternating current is used in the transmission of electricity oer the power grid instead of direct current. Example 13.2 Practice Problems 1. A transformer has a primary oltage of 45.0 V and a primary current of 6.00 A. The current in the secondary coil is 8.00 A. Determine the oltage in the secondary coil and explain whether this is a step-up or step-down transformer. 2. A transformer at the Hooer Dam has a primary oltage of 18.0 kv and a primary current of A. The current in the secondary coil is 68.0 A. Determine the oltage in the secondary coil and explain whether this is a step-up or step-down transformer. Answers 1. V s 33.8 V; step-down transformer 2. V s V; step-up transformer Determine the input power and primary oltage of a transformer that has 300 turns of wire in the secondary coil and 450 turns in the primary coil. The secondary oltage is 66.0 V and the secondary current is 2.50 A. Gien V s 66.0 V p 450 I s 2.50 A s 300 Required oltage in the primary coil (V p ) power in the primary coil (P p ) Analysis and olution ince the input power is equal to the output power, we can determine the input power using V s and I s. _ V p _ p V s s V p P p P s V s I s _ pv s (66.0 V)(2.50 A) s 165 W (450)(66.0 V) V Paraphrase The primary oltage of this transformer is 99.0 V and the power in the primary coil is 165 W. 436 Unit E Electricity and Magnetism

18 Careful: High Voltage Approximately km of high-oltage transmission lines crisscross Canada enough to cross the entire country 27 times. These lines are interconnected with each other so that if one line should fail, another line can take its place. Transmission lines can be underground or oerhead. ince these lines carry a ery high oltage, precautions must be taken to ensure the public is safe from the hazards of high oltage. Precautions Taken by the Power Company High-oltage transmission wires are used to carry electricity oer long distances. The electrical potential of the line can be as high as 500 kv and, in some areas, lines een carry oer 700 kv. Unless the line is adequately insulated from the tower, the potential difference between the line and the tower could cause the electricity to jump from the wire to ground. To preent this possibility, transmission towers hae glass or ceramic insulators to separate the line from the tower (Figure 13.33). ubstations are fenced off to preent people from getting near the high-oltage lines. In some neighbourhoods, power is deliered to homes through oerhead power lines. These lines are susceptible to seere weather such as ice, lightning strikes, and heay snowfall. If these lines come down, they present a serious hazard to the public. Many of the inherent dangers posed by power lines can be oercome by burying the lines underground. Howeer, burying power lines is not completely safe. A homeowner who digs in the yard to plant a tree, to put in pilings for a deck, or to simply landscape must be aware of the location of underground power lines. Power companies hae a call before you dig hotline that any homeowner can use. The power company will send out a technician to locate and mark the underground lines on the property. PHYIC OURCE Explore More How would you design a power grid for the safe and efficient deliery of electricity? PHYIC OURCE Take It Further ome power companies are switching to high-oltage DC (HVDC) power transmission instead of high-oltage AC (HVAC). Research HVDC and discuss its possible benefits and drawbacks. Figure The wires of a high-oltage transmission line can carry V. They are insulated from the tower by glass or ceramic insulators. E16 Quick Lab PHYIC OURCE Demonstration of Building a Transformer Purpose To design and build a transformer primary coil secondary coil nut nut Actiity Oeriew In this actiity, you will design and build a transformer. You will obsere the relationship between the number of coils in primary and secondary coils and the current. Your teacher will gie you a copy of the full actiity. AC ammeter AC ammeter Prelab Questions Consider the questions below before beginning this actiity. 1. What type of current is required to create a continually expanding and collapsing magnetic field? AC generator Figure Actiity setup for building a transformer 2. What effect does the number of turns in the primary and secondary coils hae on the output current and oltage? Chapter 13 Electromagnetic induction is used to generate most of the electrical energy used today. 437

19 13.3 Check and Reflect Key Concept Reiew 1. Outline the key steps in the generation and transmission of electrical energy. 2. What is the primary purpose of a transformer? 3. (a) What is an ideal transformer? (b) What equation is used for an ideal transformer? 4. Explain why alternating current is essential for the operation of a transformer. Connect Your Understanding 5. A step-up transformer increases the oltage. What effect does it hae on the current? 6. Identify the type of transformer shown in the following diagram. Determine the following quantities: (a) I p (b) V s (c) P p (d) P s AC power supply V 48.0 V Question 6 light bulb I A 7. A W transformer has a current in the secondary coil of 2.40 A. Determine the secondary oltage. 8. A step-up transformer has fie times the number of turns in the secondary coil as in the primary coil. If the 600-W transformer has a secondary current of 3.80 A, determine: (a) I p (b) V s (c) V p 9. A step-down transformer for a small electric appliance has a primary to secondary oltage ratio of 8:1. The primary current is 1.2 A and the primary oltage is 120 V. (a) Determine the secondary current. (b) Determine the secondary oltage. (c) Determine the power in the primary and secondary coils. 10. A 300-W step-down transformer has 3200 turns in the primary coil and a primary oltage of 100 V. Determine the number of turns in the secondary coil if the secondary current is 12.0 A. 11. A W transformer has 2800 turns in the primary coil and 750 turns in the secondary coil. A current of 15.0 A is generated in the secondary coil. (a) Determine the oltage in the secondary circuit. (b) Determine the current in the primary circuit. (c) Determine the oltage in the primary circuit. 12. Two power lines hae a current of 7.5 A and 30.0 A, respectiely. Both are 425 km long and hae a resistance of /km. Determine the power lost by each line. 13. An adapter for a small stereo plugs into a V wall outlet. The adapter is a step-down transformer that deliers 12.5 V and 1.20 A to the stereo. Determine the primary current and the power consumed by the system. Reflection 14. Partner with another student in the class and discuss the concepts that were difficult to understand in this section. For more questions, go to PHYIC OURCE 438 Unit E Electricity and Magnetism

20 Great CAADIA in Physics Harold Johns Dr. Harold Johns ( ) reolutionized the way that cancer patients were treated in Canada and around the world. He was also one of the founders of the field of medical physics. In the early 20th century, there were only two methods to treat cancer: surgery and radiation. Unfortunately, the radiation used low energy X-rays that gae high does of radiation to the skin. Eerything changed with Johns inention of the cobalt-60 therapy unit. Johns belieed that cobalt 60 a radioactie isotope could be used to treat tumours deep within patients bodies. In 1949, he worked on a team constructing a cobalt-60 unit. In 1951, the first cobalt-60 treatment occurred. Called the cobalt bomb, the cobalt-60 unit had an immediate impact on the cancer surial rate because it dropped the bomb on cancer. The cobalt-60 therapy unit has been used for oer 50 years in the treatment of cancer. Today, most cobalt units hae been replaced by high-energy electron accelerators. Johns receied many awards and honours including Officer of the Order of Canada (1977) and induction into the Canadian Medical Hall of Fame (1998). Figure Harold Johns was a respected Canadian medical physicist. Physics CAREER Medical Physicist Medical physicists are health-care professionals who hae specialized training in the medical applications of physics. They are scientists with graduate training in physics (M.c. or Ph.D.). Medical physicists work with radioactie isotopes, X-rays, ultrasound, and magnetic and electric fields in diagnosis and therapy. Most medical physicists work in hospitals in cancer treatment (radiation therapy) or diagnostic imaging. Radiation therapy inoles the use of high-energy radiation in the treatment of cancer. In this area, medical physicists are responsible for radiation therapy machine design, testing, and calibration. They are also inoled in treatment planning, which is the custom design of indiidual patient treatments. Medical physicists are also responsible for radiation safety making sure that staff and public are exposed to as little radiation as possible. In diagnostic imaging, medical physicists are inoled in the use of X-rays, ultrasound, magnetic resonance, and nuclear medicine for imaging patients. The medical physicist is responsible for machine purchasing and installation, testing, quality control, and operation. Medical physicists may also be inoled in research and teaching. They play an important role in the deelopment of new imaging and treatment techniques. Medical physicists hae played a key role in improing the diagnosis and treatment of cancer and other diseases. Figure A medical physicist takes radiation dose measurements from a linear accelerator X-ray beam. he is using a radiation detector in a large tank of water. To find out more, isit PHYIC OURCE Chapter 13 Electromagnetic induction is used to create most of the electrical energy used today. 439

21 CHAPTER 13 CHAPTER REVIEW Key Concept Reiew 1. A student is gien a solenoid and a permanent magnet. Explain two ways in which the student could induce a current in the solenoid. k 2. What effect would you see if a wire was moed parallel to magnetic field lines? k 3. Explain the relationship that the elocity, magnetic field, and length of wire hae on the induced current and emf. k 4. Determine the direction of the induced current and emf in the wire segment shown in the following diagram. t Question 4 A 5. Explain why the force affecting a wire is opposite to its elocity when it moes perpendicularly through an external magnetic field. k 6. Explain how to determine the direction of the induced current when a magnet is moed toward the core of a solenoid. k 7. Determine the direction of motion of the magnet that induced the current shown in the following diagram. t B 12. Explain why a transformer must operate with alternating current instead of direct current. k Connect Your Understanding 13. Explain how moing a magnet in the direction shown in the following diagram would induce a current in the wire. t Question Use the right-hand rule to determine the direction of the force on the wire if the wire is moing in the direction shown in the following diagram. t Question Determine the directions of the magnetic fields gien the induced current and the direction of wire moement in the following diagrams. t (a) I A B (b) Question 7 8. Under what circumstances could a wire moe through an external magnetic field and not create an induced current? k 9. In terms of design and function, compare an electric motor with a generator. k 10. How does an AC generator differ from a DC generator? k 11. Why are AC generators used for electrical energy transmission? k I I Question A student decides to build a generator by wrapping wire around a toilet paper roll and connecting the free ends to a light bulb. Then she attaches a permanent magnet to a shaft and rotates it near one end of the roll as shown in the following diagram. Will this design make the light bulb light up? a Question Unit E Electricity and Magnetism