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[CATCH CH 13: 36pp] [CATCH Ch13 opening: 2 pp] 13 KEY CONCEPTS After completing this chapter you will be able to describe the law of electromagnetic induction use Lenz’s law to predict the direction of induced current describe alternating current and its properties describe the operation of an alternating current generator describe how transformers rely on alternating current to step up or step down the voltage describe how generators, transformers, and the electrical grid work to provide electricity for the population Electromagnetic Induction Can magnetic fields create electric currents? The Back Lot Stunt Coaster at Canada’s Wonderland is a different type of roller coaster. It is missing that first big hill that you slowly get towed up and then released from. This initial build up of gravitational potential energy on typical roller coasters is necessary so that it can be converted into kinetic energy for the rest of the ride. In the Back Lot Stunt Coaster, you start horizontally and then accelerate rapidly from rest to 64 km/h in 3 s. That is faster than many sports cars. How is it done? The ride is accelerated using a linear induction motor. In chapter 12, you learned about DC motors and how they convert electrical energy into rotation. Linear induction motors use electrical energy to create a magnetic field that can cause an object to move along a linear track instead of rotating on the spot. These motors cause huge accelerations. In fact, the Kingda Ka roller coaster, at Six Flags Great Adventure in New Jersey, uses a linear induction motor to accelerate the coaster from rest to 206 km/h in 3.5 s. In this chapter, you will learn about electromagnetic induction and its many applications, from causing motion in amusement park rides to one of the most important: generating your electrical energy. STARTING POINTS Answer the following questions using your current knowledge. You will have a chance to revisit these questions later, applying concepts and skills from the chapter. 1. How can a magnetic field be used to induce an electric current? 2. Can we predict in which direction the induced current will flow? 3. What properties of an alternating current make it favourable for electricity generation? 4. What is the most efficient way to transmit electrical energy? [END PAGE 1 of 2] Chapter 13 1 [START PAGE 2 of 2] [CATCH: C13-P001-OP11USB, size CO photo of the Back Lot Stunt Coaster from Canada’s Wonderland 2/3 page] MINI INVESTIGATION ELECTRIC CURRENT FROM MOTION? Skills: Performing, Observing In this mini-investigation, you will try to produce an electric current without touching a loop of wire. Equipment and Materials: loop of wire; galvanometer; bar magnet; alligator leads (if necessary) 1. Connect the loop of wire directly to the galvanometer. 2. Move the bar magnet in various directions around or through the loop of wire without touching the wire. 3. Observe any changes on the galvanometer and record your results. A. What types of motion show that a current is flowing in the wire? Explain. [T/I] B. Is any one type of motion more effective at producing a current than others? Chapter 13 2 Explain. [T/I] [END Page 2 of 2] [END CHAPTER SUMMARY] [Start Section 13.1: 4pp.] 13.1 [CATCH C13-P002-OP11USB, size D ] Figure 1 Michael Faraday was born September 22, 1791 in London, England. At 14, he apprenticed with a local bookbinder. There he read many of the books that were being bound and developed an interest in electricity and chemistry. He was never formally trained as a scientist, yet he still published several papers in scientific journals. electromagnetic induction the production of electric current in a conductor moving through a magnetic field law of electromagnetic induction a change in the magnetic field in the region of a conductor induces a voltage in the conductor and this causes an induced electric current in the conductor. Electromagnetic Induction You know from Chapter 12 that an electric current in a conductor can produce a magnetic field. But is the opposite true as well? Can a magnetic field produce an electric current in a conductor? In 1831, Michael Faraday, an English scientist, proved just that (Figure 1). This discovery led to many of the technologies that make the electricity we use everyday. Discovery of Electromagnetic Induction In Section 12.2 you learned that a constant electric current will produce a magnetic field, so it would be logical to assume the opposite—that a constant magnetic field will produce an electric current in a conductor sitting in that constant magnetic field. It does not. Faraday discovered hat in order to produce an electric current the magnetic field needed to be continuously changing. He had discovered electromagnetic induction, the production of electric current in a conductor within a changing magnetic field. Induction means that one action causes another action to happen, often without direct contact. In his investigations, Faraday brought a permanent magnet near a conductor but not in direct contact with it, and induced a current in the conductor. The electric current flowed only while the magnet was moving in the vicinity of the conductor. We call this an “induced current” because it is not an already existing current; it is formed by the action of the magnetic field moving along the conductor. These observations led Faraday to develop what is now known as the Law of Electromagnetic Induction. [CATCH FORMATTER: set the following in a blue screen] Law of Electromagnetic Induction Any change in the magnetic field in a defined region around a conductor induces a voltage that causes an induced current in the conductor The implications of this discovery were extraordinary because for the first time in history, electricity had been generated using only a magnet. In Faraday's time, the only way of producing electrical energy was by using an electric cell or battery. Of course, batteries are extremely useful but they have disadvantages: they operate for a limited amount of time, they can be heavy and bulky, and they only produce small electric potential differences. With Faraday’s discovery all that was needed was to find a way to move the magnet continuously and the world would be able to produce electrical energy on a large scale without the Chapter 13 3 limitations of batteries. [CATCH 2-column Mini Investigation – don’t break over page] . MINI INVESTIGATION FARADAY’S RING Skills: Performing, Observing Faraday investigated electromagnetic induction using a device that he built himself. It contained two completely independent circuits. The primary circuit was connected to a source of electrical energy. The secondary circuit was only connected to a galvanometer. Equipment and Materials: 2 pieces of conducting wire; battery (or power supply); switch; soft-iron ring; galvanometer 1. Construct a Faraday’s Ring apparatus as shown in Figure 2. Coil the conducting wire tightly around the ring. Be sure to coil the conducting wire the same number of times on each side of the ring. [CATCH C13-F001-OP11USB, size C] Figure 2 Faraday's Ring 2. 3. A. B. C. Close the switch in the primary circuit and observe the galvanometer in the secondary circuit. Open the switch in the primary circuit and observe the galvanometer in the secondary circuit. What happened to the galvanometer when the primary circuit switch was closed? [T/I] What happened to the galvanometer when the primary circuit was switched off? [T/I] Was there a difference in the direction of the currents in parts 2 and 3? [T/I] Electromagnetic Induction and Faraday’s Ring The Faraday’s Ring (Figure 1) you constructed in the Mini Investigation is a demonstration of electromagnetic induction. Closing the switch in the primary circuit causes an induced voltage in the conducting wire which in turn causes a constant electric current in the conducting wire. This constant electric current produces a magnetic field in the primary coil. The soft-iron ring enhances the strength of the magnetic field and the ring itself becomes magnetized. This change of the magnetic field in the soft-iron ring (from zero to some value) induces a voltage and an electric current in the secondary circuit. However, once the magnetic field is stable and no longer changing, the electric current in the secondary circuit no longer exists. Remember that you need a changing magnetic field to induce an electric current. When the switch is opened, the magnetic field in the primary coil disappears because there is no longer an electric current. The magnetic field in the soft-iron ring collapses from maximum strength to zero. This change in the magnetic field causes an induced electric current in the opposite direction in the secondary circuit. Direct currents only produce electromagnetic induction for brief instants when the primary circuit is switched on or off. Factors Affecting Electromagnetic Induction There are several factors that determine the amount of electric current that can be produced through electromagnetic induction. Each of the following factors must be considered independently. Chapter 13 4 Coiling the Conductor In Section 12.4, you learned that by coiling a conductor you can create a magnetic field similar to that of a bar magnet. The magnetic fields from both sides of the loop interact to produce a more pronounced magnetic field in the centre of the loop. Similarly, with electromagnetic induction, a coiled conductor will have more induced electric current in it than will a straight conductor. The Number of Loops in the Coil You know from section 12.4 that increasing the number of loops in a coiled conductor or solenoid produces a stronger magnetic field for a given electric current. With electromagnetic induction, the number of loops in the coil is directly proportional to the magnitude of the electric current induced in the conductor for a given change in the magnetic field. So, the greater the number of loops in a coil, the more electric current can be induced for a given change in the magnetic field. The Rate of Change of the Magnetic Field There are two cases to consider here: a coiled conductor with a permanent magnet or a Faraday’s ring apparatus. In the case of a coiled conductor with a permanent magnet, the quicker you move the magnet into, or out of, the coil, the higher the rate of change you cause in the magnetic field within the coil. This will cause a larger induced electric current in the conductor compared to moving the magnet slowly. In the second case of Faraday’s ring, the quicker you increase the current in the primary circuit the higher the rate of change you cause in the magnetic field in the coiled conductor and the soft-iron ring. The magnitude of the induced electric current in the secondary circuit is proportional to the rate of change of the magnetic field in the soft-iron ring. The Strength of the Inducing Magnetic Field [CATCH C13-P003-OP11USB, size D] The stronger the inducing magnetic field, the greater the induced electric current. So, a stronger permanent magnet will induce a greater electric current in a given coil. Similarly, in Faraday’s ring, a larger electric current in the primary circuit will increase the strength of the magnetic field in the coiled conductor and soft-iron ring. This will increase the induced electric current in the secondary circuit. Applications of Electromagnetic Induction [CAPTION] Figure 3 An induction cooking surface To operate any electrical device you own, you must rely on electrical power produced and supplied through generators and transformers—both devices that rely on the law of electromagnetic induction to operate. You will learn more about the generators and transformers in Chapter 13 5 Sections 13.4 and 13.5. Now we will look two other interesting applications of electromagnetic induction: induction cooking and metal detectors. Induction in Cooking [CATCH C13-P004-OP11USB, size D photo of metal detector at an airport] Figure 4 Metal detectors like this one are used to prevent dangerous objects from being carried onto an airplane. Cooking food involves the transfer of thermal energy. In an electric stove, an electric current is directed into the element which converts the electrical energy into thermal energy. That thermal energy is transferred by conduction into a metal pot. The pot needs to increase in temperature to then transfer thermal energy into food. The efficiency of this process is low because the stove element has to get hot, the pot has to get hot, and finally the food is heated. In the process, much thermal energy is lost to the environment. Cooking using an induction cooker involves a rapidly changing magnetic field in the stove element (Figure 3). The rapidly changing magnetic field induces an electric current in the pot. The electric current heats the pot because of the electrical resistance of the pot. Iron pots work better than copper or aluminum due to their higher electrical resistance. Insulating materials, like glass, will not work on an induction cooker. The main benefit of cooking with an induction cooker is that it is more efficient because it is a more direct transfer of thermal energy to the food. Another benefit is that the induction cooking surface does not get hot and food that spills onto the cooking surface will not burn. Since the induction cooking surface is not hot, the heating of the food immediately stops once the induction cooker is turned off. Metal Detectors Electromagnetic induction is also used to detect metals. Metal detectors use a coil that generates a rapidly changing magnetic field. This magnetic field will induce a current in any metal near it. The induced electric current in the detected metal will also produce an induced magnetic field of its own. Sensitive measurements of the magnetic field near a metal detector are used to detect the induced magnetic field. Metal detectors have many uses and have become quite common. They are used for humanitarian purposes to help locate buried bombs called “land mines”. Land mines were often buried during times of war but never removed. Innocent people are injured or killed when walking through areas of countryside where no warnings of land mines exist. Metal detectors are used for security purposes at airports (Figure 4). If you have been on a flight, you will have walked through one of these detectors. There also handheld devices that security guards can use to detect any metal objects on you. Metal detectors are also used by hobbyists searching for metals that might be valuable buried in the ground. Chapter 13 6 13.1 SUMMARY • The law of electromagnetic induction states that a change in the magnetic field in the region of a conductor induces a voltage in the conductor and this causes an induced electric current in the conductor • Faraday’s ring is a device that demonstrates electromagnetic induction. A current in the primary coil creates a magnetic field in the ring. The magnetic field in the ring then induces a current in the secondary coil. • The amount of induced electric current can be increased by: coiling the conductor, increasing the number of loops, increasing the rate of change of the magnetic field, and increasing the strength of the magnetic field • Electromagnetic induction is used in many technologies including: generators, transformers, induction cooking, and metal detectors. 13.1 QUESTIONS 1. A student demonstrates electromagnetic induction using a straight wire and a permanent magnet. The wire is part of a circuit that is connected to a galvanometer. What would you expect would happen in each of the scenarios listed below? [K/U] (a) The magnet is placed on top of a stationary wire. (b) The magnet is removed from the top of the stationary wire. (c) The magnet is moved slowly over the top of the stationary wire. (d) The magnet is moved quickly over the top of the stationary wire. (e) The magnet is moved back and forth over the stationary wire quickly. (f) The wire is moved past the stationary magnet quickly. 2. You need to demonstrate electromagnetic induction and wish to maximize the amount of induced current. Describe a design to accomplish this. [T/I] [C] 3. A glass cooking pot with an iron handle is placed on the cooking surface of an induction cooker. Describe how the temperature of the glass and the handle after being on the induction cooker for some time. [K/U] 4. Could you design a non-metal detector that detects things other than metal and uses electromagnetic induction? Explain. [K/U] 5. Before going into a metal detector at an airport, you must remove your belt, empty your pockets and remove your shoes. Explain why. [C][A] [END PAGE 4 of 4] [END SECTION 12.1] Chapter 13 7 [START Section 13.2: 3pp] 13.2 Lenz’s Law In 1834, Heinrich Lenz, a Russian physicist, formulated a law to describe the direction of induced electric current. He used logic and the law of the conservation of energy to deduce the direction of an induced current. You can observe the direction of current by watching a needle on a galvanometer move as you move a magnet along a conductor. Lenz determined how to predict that direction. [CATCH 2-column Mini Investigation] MINI INVESTIGATION OBSERVING THE DIRECTION OF INDUCED CURRENT Skills: Performing, Observing In this activity, you will observe the direction of the induced current when moving a magnet into and out of a coiled conductor. Equipment and Materials: coiled conductor, permanent bar magnet, 2 alligator leads, galvanometer 1. Connect the two terminals of the coil to the galvanometer. 2. At a moderate rate, push the north pole of the magnet into the coil (Figure 1). Note the direction of the current on the galvanometer [CATCH C13-P005-OP11USB, size C, setup] [CAPTION]Figure 1 3. Pull the north pole of the magnet out of the coil and note the direction of the current on the galvanometer. 4. Repeat steps 2 and 3 with the south pole of the magnet. A. How did the direction of the current compare in steps 2 and 3? [T/I] B. How did the direction of the current compare in step 4? [T/I] C. Did using the south pole or north pole change your results? Explain. [T/I] D. Predict the magnetic pole induced at the top of the coil using your right hand rule for a coiled conductor for both steps 2 and 3. [T/I] [CATCH C13-F002-OP11USB, size D] Direction of Induced Current Figure 2 North pole of a permanent magnet pushed into the coiled conductor. When a permanent magnet is pushed into a coiled conductor, the electrons in the coil respond to the magnetic field by starting to move in a particular direction by forming an electric current. The direction of the current depends on which direction the magnetic field points. For example, if the north pole of a magnet is pushed into a coil, an induced current is produced in the coil in one direction (Figure 2). If the south pole of the magnet is pushed into a coil, the induced current will be in the opposite direction. How did Lenz deduce the direction of an induced current? He used the law of conservation of energy as a starting point. Recall from Chapter 5 that energy cannot be created or destroyed. All that can be done is to Chapter 13 8 Lenz’s law If a changing magnetic field induces a current in a coil, the electric current is in such a direction that is own magnetic field opposes the change that produced it. transform one type of energy into another. When the north pole of a magnet is moved into the coil, as in Figure 2, kinetic energy exists in the movement of the magnet. The kinetic energy is transformed into electric energy in the electric current in the coil. As a result, the electric current in the coil also produces a magnetic field that points in a particular direction. Which way does the magnetic field point? Is the right side of the coil nearest the bar magnet in Figure 2 a north or a south magnetic pole? Let us first consider the conventional induced current to go around the coil in an upwards direction at the front of the coil (the opposite of the direction shown in Figure 2). In this case, the right hand rule for a solenoid indicates that the right side of the coil is a south magnetic pole. But this finding is inconsistent with the law of conservation of energy. If the right side of the coil is a south magnetic pole then it would attract the north pole of the permanent magnet into the coil without the need for an external force to push the magnet through the coil. Therefore, the right side of the coil cannot be a south magnetic pole. Let us now consider the conventional induced current to go around the coil in a downward direction at the front of the coil. Using the right hand rule for a solenoid, the right side of the coil is a north magnetic pole. The permanent magnet would then be repelled by the north magnetic pole of the coil. Therefore, you would need to apply a force to push the permanent magnet into the coil, thereby doing work. You are transforming kinetic energy into electric energy in the coil. This idea is consistent with the law of conservation of energy. With similar deductive reasoning Lenz was able to summarize his findings into a law for determining the direction of an induced current, known as Lenz’s law (Figure 3). [CATCH FORMATTER: set the following in a blue screen] Lenz’s Law If a changing magnetic field induces a current in a coil, the electric current is in such a direction that its own magnetic field opposes the change that produced it. LEARNING TIP Remembering Lenz’s Law [CATCH C13-F003a-OP11USB, C13-F003b-OP11USB, C13-F003c-OP11USB, and C13-F003dOP11USB] The coil opposes whatever the magnet is trying to do. If north is moving in, the coil repels it with a north. If north is moving out, the coil attracts it back with a south. Chapter 13 9 [CATCH C13-P006-OP11USB, size D photo of drop tower at Canada’s Wonderland] Figure 3 Lenz's Law (a) The permanent magnet is being forced into the coil. If north is being forced inward, the coil must oppose that with a north pole at the top of the coil. (b) If the top of the coil must be north, then the right hand rule for a solenoid would determine the direction of the electric current to flow towards the right side of the coil in the front] (c) The permanent magnet is being pulled out of the coil. If north is being forced upward, the coil must oppose that with a south pole at the top of the coil. (d) If the top of the coil must be south, then the right hand rule for a solenoid would determine the direction of the electric current to flow towards the left side of the coil in the front. Lenz’s Law at Canada’s Wonderland Figure 4 Drop tower rides use an electromagnetic braking system that is reliable, automatic, and has parts that do not wear out. The yellow arrow is pointing at the copper strip. Drop tower rides are popular attractions at amusement parks (Figure 4). Riders are strapped into a seat and can be raised to a height of over 70 m. They are then released to free fall towards the ground. To prevent disaster there must be an extremely reliable braking system. If a friction-based braking system were used, it would need to be triggered at just the right time and the system would need constant replacement. Drop tower rides use an ingenious system that relies on electromagnetic induction. It can be explained using Lenz’s law. Each of the carts that are raised to the top of the tower have permanent magnets underneath the seats. After approximately 45 m of free fall an electromagnetic braking system kicks in. Along the bottom third of the tower there are copper strips mounted to the tower vertically. When the carts fall and the permanent magnets move past the copper conductor, an electric current is induced in the copper. The induced current then produces a magnetic field. Applying Lenz’s law, the induced magnetic field must oppose the field that created it. The opposing repulsion force acts to create a reliable, no-friction braking system. 13.2 SUMMARY • Lenz’s law states: If a changing magnetic field induces a current in a coil, the electric current is in such a direction that its own magnetic field opposes the change that produced it. 13.2 QUESTIONS 1. For each of the diagrams below, determine the direction of the induced current in the coil and the magnetic poles on the coil. [T/I] [CATCH C13-F004a-OP11USB, C13-F004b-OP11USB, C13-F004c-OP11USB, and C13F004d-OP11USB, place 2 across, 2 down for a total width of C.] Chapter 13 10 2. How would your answers to question 1 change if the coils were moved in the opposite direction to the arrows shown above instead of the magnets? [K/U] 3. When a magnet is pushed into a coil to induce a current, the magnetic field that is created will never attract the magnet into the coil. Explain why this is the case.[K/U] 4. In a drop tower ride, would using an electromagnet in the carts, instead of the permanent magnets under the seats, work equally well? Would it be equally reliable? Explain. [A] [END PAGE 3 of 3] [END SECTION 13.2] Chapter 13 11 [START Section 13.3: 4pp] 13.3 alternating current an electric current that repeatedly and periodically reverses direction Alternating Current You know that Faraday’s law of electromagnetic induction requires that a changing magnetic field is needed to produce an electric current. If you push a permanent magnet into a coil an electric current is produced as long as the magnet is moving. Lenz’s law predicts the direction of the current; however, the current goes in one direction for only as long as you move the magnet in one direction. It will not be possible for you to move the magnet into the coil in the same direction indefinitely. At some point, you will need to pull the magnet in the opposite direction. As soon as you reverse the direction of the magnet, the electric current also reverses direction. A current that periodically reverses direction is called an alternating current. Development of Alternating Current LEARNING TIP Short forms for current Alternating current is shortened to AC while direct current is shortened to DC. WEBLINK War of the Currents To find out more about Tesla and Edison’s battle for electrical power distribution, go to NELSON SCIENCE Recall from Section 11.5 that direct current is a flow of electrons in one direction only. To cause an electric current, a potential difference is applied across the circuit by a source of electrical energy, like a battery. This causes the electrons present throughout the entire circuit to move in one direction. Charges flow from one terminal of the battery through the circuit and eventually back to the other terminal. This situation describes the current in the small circuits you studied in chapter 11. Do the principles involved in these small circuits apply to a large circuit like the circuits in your home or even the electrical power grid? Today’s electrical power grid does not rely on direct current. The transfer of electrical energy using direct current is limited to how far it can be transferred without significant energy loss in the form of thermal energy. However, direct current was the standard used in the first electrical power grids. In 1882, Thomas Edison built the Pearl Street power station in Manhattan to illuminate homes and stores. He set up an electrical power grid using direct current, but he was only effectively able to transfer electrical energy to 193 buildings. Nikola Tesla, an inventor and electrical engineer, developed a competing system that used alternating current. The alternating current system could transfer energy from a power plant more efficiently than the direct current system could. In 1896, the Edward Dean Adams Station, a U.S. hydroelectric power station, delivered the first alternating current along a power grid to the city of Buffalo and its industries. Edison and Tesla both fought to have their systems accepted and in the end alternating current became the favoured choice. In 1922, the Sir Adam Beck generating station started producing alternating current electricity in Niagara Falls, Canada. [CATCH weblink] Chapter 13 12 What is Alternating Current? [CATCH C13-F005-OP11USB, size D] [CAPTION] Figure 1 [to come] [CATCH: C13-F006-OP11USB, size D Figure 2 A distribution panel controls all the circuits in your home. You can think of this panel as the power supply for your home. Note the red, black, and white service line. Alternating current is a back and forth motion of charges. To understand alternating current, look at the graphs of current and voltage versus time shown in Figure 1. As the voltage (V ) increases the conventional current (I ) in the wire begins in the positive direction. The voltage increases until it reaches a maximum positive value. At the same instant, the current also reaches a maximum positive value. The voltage then decreases through zero until it reaches a maximum negative value. The current similarly decreases and then starts reversing and heading in the negative direction only when the potential difference passes through zero. In Figure 1, the process repeats at a frequency of 60 Hz, which is the frequency of electricity in the North American electrical power grid. This means that the current is going in the positive direction, reversing, and going in the negative direction 60 times each second. So the electrons in the wire never travel the complete length of the circuit; rather they move back and forth in more or less the same spot. Note that the graphs both go through zero twice during each cycle as the current is changing directions. This implies that the circuit is effectively off for an instant. You may ask why we do not notice this effect. In fact, lights do get dimmer and brighter during each cycle, but the cycles occur so quickly that our eyes cannot detect it. Since the electrons move back and forth in more or less the same spot, you may think the current to be ineffective at transferring energy. To understand how an alternating current can cause an energy transfer, consider a clothes washing machine. In a clothes washing machine, there is an agitator that swishes the wet clothes back and forth. The agitator still causes changes in energy and thus work is done to clean the clothes. In an alternating current circuit, electrons move back and forth to cause energy changes in the electrical device and thus work is done. All the relationships that you have learned about circuits and their components still apply with alternating current, for example, Ohm’s law. In the graphs in Figure 1, as the voltage increases, so does the electric current. This is in accordance with Ohm’s law, and the current depends on the resistance of the wire that is used to transmit the alternating current. Household Circuits Homes have limits to how much alternating current they can draw. Smaller homes and apartments may only require 50 to 100 A of electric current. Larger homes require 150 to 200 A of electric current. The wire that enters your home from the power grid is called the service line and the amount of current it carries will be labelled at Chapter 13 13 CAREER LINK Electrician To find out more GO TO NELSON SCIENCE [CATCH: stack C13-P007a-OP11USB, size D and C13-P007b-OP11USB, size D] Figure 3 This plug and receptacle is used for your electric stove or clothes dryer. It is designed for a maximum alternating current of 30 A at a voltage of 240 V. the point of entry. For example, you may have a 100 A service going to your home. The service enters your distribution panel which controls all the circuits in your home (Figure 2.) The AC voltage oscillates between a maximum positive voltage and a maximum negative voltage. Since the maximum voltage is reached for a brief instant twice during a cycle, we do not state that voltage. Instead, we use a value that is called the “root mean squared” or RMS value. This is an effective voltage that is available most of the time during a cycle. It also allows for the use of one voltage in calculations instead of a constantly changing voltage. Your home is designed so that an effective voltage of 240 V is applied to the electrons in the wires in your home. Only a few of your household appliances require 240 V, such as an electric stove or clothes dryer. Most appliances only require an effective voltage of 120 V. To accommodate both requirements, a three wire system is used. Two of the wires are considered “hot” and are coloured red and black. The third wire is white and is called the neutral wire. The voltage between the black and the white, and the red and the white, is 120 V. The voltage between the red and the black is 240 V. This way, an electrician can wire the home’s circuit with just two of the wires. Typically, the electrician uses only the black and the white wire when 120 V is required. If an appliance requires 240 V, then the black and red wires are used. There is a fourth wire that is also an essential part of the wiring called the ground wire. Ground wires can be bare (not-insulated) or green and are electrically connected to the ground via your electrical distribution panel. The purpose of the ground wire is prevent stray currents from reaching you when you touch a circuit and directs them safely into the ground. As a safety feature, all electrical receptacles and switches must be grounded. [CAREER LINK] The receptacles for 240 V appliances and 120 V appliances are different. A 240 V plug may have 4 prongs, one for each of the wires: red, black, white, and bare (or green)(Figure 3). Most 120 V plug require 3 prongs, one for black, white, and ground. There are also many devices, like lamps, that use 2 prong plugs and do not have a ground pin. Some electronic devices (such as laptop computers) require direct current because they cannot operate with alternating current. These electronic devices need an adaptor that converts the alternating current into direct current. The electrical requirements of some common household appliances are listed in Table 1. [CATCH Table] Table 1 Electrical requirements of some common household appliances Device Type of current Current Voltage requirements required requirements (A) (V) Electric stove AC 30 240 Chapter 13 14 Clothes dryer AC 30 240 Air conditioner AC 20 240 Iron AC 5 120 Hair dryer AC 12.5 120 Refrigerator AC 6 120 40 “ LCD television DC 18 12 Desktop computer (excluding monitor) DC Telephone charger DC 0.5 to 20 (depends on the component) 1 3.3 to 12 (depends on the component) 5 Safety Systems Many safety systems exist in your home to prevent harm to you, damage to your appliances, or electrical fires. These include fuses, circuit breakers, ground fault circuit interrupters, and arc fault circuit interrupters. Fuses Fuses are devices that are placed in series with one of the parallel branches in your home. It the current exceeds a maximum value that the fuse is rated, the wire inside of the fuse melts which opens the circuit and prevents any more current. Fuses are one use only. Fuses may also be found inside of appliances as a further measure of protection. Circuit Breakers Circuit breakers are devices that prevent too much current in a wire for an extended period of time. If too much current is in a wire, the wire will heat up and the insulation may melt and potentially cause a fire. Circuit breakers work by using a strip of metal containing two different metals fused together, called a bimetallic strip. When too much current is present, the bimetallic strip heats up and bends. This causes the breaker to "trip" which turns off the circuit. If a circuit breaker trips, you should consider why it tripped. It could be because too many loads are connected in parallel to the same circuit and are requiring too much current. Using fewer electrical devices on that circuit should prevent the tripping. Circuit breakers are reusable and can be reset. Ground Fault Circuit Interrupters (GFCI) GFCIs are installed in places like bathrooms. Their purpose is to detect any differences in the current going into a circuit compared to the current going out. Chapter 13 15 If the GFCI does detect a difference it immediately shuts off and prevents current. Suppose that you touch an electrical outlet with a wet hand and then reach for an appliance with another wet hand. It is possible that you may set up a circuit pathway for electric current to travel through your body. This could cause an electrocution. The GFCI would very quickly detect the difference in current and immediately turn off the circuit. GFCIs respond much faster than regular circuit breakers because they are designed to trip with small current fault whereas regular circuit breakers will only trip if the current exceeds the maximum rating. For example, a GFCI will trip if a current difference of 0.006 A is detected whereas a current of over 15 A would need to be detected to trip a regular breaker. Arc Fault Circuit Interrupters (AFCI) AFCIs prevent sparking or arcing, which could start a fire. Arcing occurs when electric current travels through the air and causes a spark. This may happen if the insulation around the wiring has become frayed. The bare wire can possibly move near a metal part and cause the arcing to occur. The temperature of an arc is very high and could cause a fire. Arcing may not be enough to cause a circuit breaker to trip, so the AFCI prevents current flow if an arc is detected. AFCIs also act faster than regular circuit breakers. 13.3 SUMMARY • Alternating current is an electric current that repeatedly and periodically reverses direction. • Alternating current frequency is 60 Hz in the North American power grid. • Some appliances require alternating current whereas others require direct current. • Your home requires a certain amount of current at a voltage of 120 V and 240 V. • The circuits in your home are protected by fuses, circuit breakers, ground fault circuit interrupters, and/or arc fault circuit interrupters. 13.3 QUESTIONS 1. Do electrons travel from the power plant to your home to provide electrical energy? Explain your answer.[K/U] 2. In alternating current electricity, is the voltage proportional to the current? Explain. [K/U] 3. (a) Would you notice if the frequency of the alternating current electricity was reduced to 2 Hz? Explain. [K/U] [C] (b) In the late 1950’s the frequency of alternating current was changed from 25 Hz to 60 Hz. Suggest a reason for this change. 4. How do 120 V plugs differ from 240 V plugs? [K/U] 5. A laptop computer requires 12 V DC and yet it is plugged into a home’s wall outlet. What must be involved to satisfy the requirements of the laptop? [K/U] 6. Describe the differences between fuses, circuit breakers, GFCIs, and AFCIs. [K/U] 7. Most household circuits in North America use 120 V, while in Europe 240 V are most commonly used. Research to find out why this difference exists. Is one system better then the other? [T/I] [END PAGE 4 of 4] [END SECTION 13.3] Chapter 13 16 Chapter 13 17 [Start Section 13.4: 6pp] 13.4 electric generator device that converts other forms of energy into electrical energy Electricity Generation The large-scale production of electrical energy that we have today is possible because of electromagnetic induction. The electric generator, which provides electricity for most places in the world, relies on the law of electromagnetic induction to operate. An electric generator is a device that converts other forms of energy into electrical energy. These other forms of energy can include thermal, gravitational, or kinetic energy. As you know from Chapter 5, the energy sources used to power generators can come from either renewable or non-renewable sources, each with their own benefits, disadvantages and environmental impacts that must be considered. In Section 13.3, you learned that alternating current was chosen for the transmission of electrical energy. So we will first look at the generation of alternating current. The Alternating Current Generator Electromagnetic induction requires a changing magnetic field to produce an electric current. In an AC generator there are two ways of changing the magnetic field. Either a permanent magnet can be spun inside a coil or a coiled conductor can be spun inside a magnetic field. We will examine a coiled conductor spinning inside a magnetic field in a single-loop generator. The AC generator shown in Figure 1 shows a single loop of conducting wire set between the poles of two permanent magnets. There are two slip rings and two brushes. Each slip ring is connected to a different side of the loop. Slip ring 1 is connected to the left side of the loop while slip ring 2 is connected to the right side of the loop. The slip rings rotate with the loop. The brushes are stationary and make contact with the slip rings to allow current to be directed out to the external circuit. The loop spins on the axis of rotation. The spinning force is provided by an external source of energy. For example, recall from Chapter 5 that at a hydro-electric power plant the falling water turns the blades of a turbine that is connected to the electrical generator. [CATCH – C13-F007-OP11USB, size B Chapter 13 18 [CAPTION] Figure 1 A single-loop AC generator In Figure 1, the loop of the generator is being forced clockwise. Since the loop is moving inside the magnetic field provided by the external magnets, an electric current will be induced in the loop as described by Faraday’s law. The direction of the induced current is predicted by Lenz’s law. Remember that an induced current also creates an induced magnetic field around the loop. The induced magnetic field around the loop will oppose the field that created it. Let us examine the left part of the loop, in cross section, closest to the north pole of the external magnet (Figure 2). [CATCH C13-F008-OP11USB, size B] [CAPTION] Figure 2 [TO COME] The magnetic field around the wire will oppose the external magnetic field. This happens because the field lines at the top of the wire are pointed in the same direction as the field lines from the external magnet. Recall that magnetic field lines pointed in the same direction will result in a force of repulsion. The force on the conductor coming from an external source of energy will overcome the magnetic repulsion force between the field from the conductor and the field from the magnets. Using the right hand rule for a straight conductor, the conventional current points into the page. Let us now examine the loop, in cross section, closest to the south pole of the external magnet (Figure 3). [CATCH C13-F009-OP11USB, size B] [CAPTION] Figure 3 [TO COME] On this side of the loop, the conductor is being forced downward and the magnetic field from the loop will oppose the external magnetic field. Again the field lines from the wire will point in the same direction as the field lines from the external magnet and cause repulsion. Using the right hand rule for a straight conductor, the conventional current points out of the page. Therefore, the rotation of the loop in a clockwise Chapter 13 19 direction in the magnetic field will cause a conventional current in the loop in the direction shown in Figure 4. In the following tutorial we will investigate the direction of the current in the external circuit attached to the generator shown in Figure 4. [CATCH C13-F010-OP11USB, size B] [CAPTION] Figure 4 [TO COME] [CATCH 2-column Tutorial: but place the first 2 sentences in 1-column format (as intro) and then remaining text runs in 2-columns] TUTORIAL 1 EXPLAINING THE AC GENERATOR As the loop of an AC generator spins, a current will form in the external circuit connected to the generator. What is the direction of the current in the external circuit? We have showed that a clockwise current forms in the generator’s loop when the loop rotates in a clockwise direction. We will now turn to determining the direction of the current in the external circuit. We will use a galvanometer connected to the external circuit to determine the direction of the induced current. Let us first consider what will happen as the loop is rotated from it initial orientation. In Figure 5, the current in the loop heads towards slip ring 2, which contacts brush 2, and into the external circuit as shown. Since the conventional current goes into the galvanometer at the positive terminal, the galvanometer needle points to the positive side of the scale. [CATCH C13-F011-OP11USB, size C] [CAPTION] Figure 5 In addition the factors discussed in Section 13.1, the amount of induced current also depends on the angle of the conductor in relation to the external magnetic field. The induced current is at a maximum when the plane of the loop is parallel to the external magnetic field, As the loop rotates towards 90 of rotation, the amount of current will decrease. Once the loop reaches perpendicular to the magnets (or 90 of rotation), the current reads zero as shown in Figure 6. [CATCH C13-F012-OP11USB, size C] [CAPTION] Figure 6 As the loop rotates from 90 of rotation and approaches 180, the conventional current in the loop now goes into slip ring 1. This reverses the direction of the current in the external circuit as shown in Figure 7. Note that the conventional current goes into the galvanometer at the negative terminal. The galvanometer needle moves to the negative side. [CATCH C13-F013-OP11USB, size C] [CAPTION] Figure7 Chapter 13 20 As the loop rotates away from 180, the current will once again decrease until it reaches zero at 270 of rotation as shown in Figure 8. [CATCH C13-F014-OP11USB, size C] [CAPTION] Figure 8 At this point the current will once again reverse direction and enter the external circuit at slip ring 2. This will start the whole process over again. If the galvanometer readings are plotted on a graph, it will look like Figure 9. [CATCH C13-F015-OP11USB, size C] [CAPTION] Figure 9 The rotation rate of the loop matches the frequency of the current changing directions. In large scale generators, the frequency of the alternating current is set to 60 Hz in North America. Any generators connected to the electrical grid are synchronized to this frequency. [CATCH 2-column Mini Investigation] MINI INVESTIGATION WHAT FACTORS AFFECT ELECTRICITY GENERATION? Skills: Planning, Performing, Observing, Communicating Generators use coils and magnets to generate electricity. In this investigation, you will determine what factors affect the amount of current produced. Equipment and Materials: 2 different coils, galvanometer, 2 alligator leads, 2 bar magnets 1. Connect the coil with fewer windings to the galvanometer using the alligator leads. 2. Plan a procedure to test the factors that affect the amount of current produced. 3. Record your observations in a series of statements that are framed as: changing the __________, produced a maximum reading on the galvanometer of ______. A. How did moving the magnet into the coil faster affect the amount of current? [T/I] B. How did reversing the pole of the magnet put into the coil, while keeping the speed of the magnet going into the coil constant, affect the amount of current? [T/I] C. How did using two magnets affect the amount of current? [T/I] D. How did changing the number of windings in the coil affect the current? [T/I] Factors Affecting Generator Output The single-loop AC generator discussed in Tutorial 1 is useful for demonstration purposes. To increase the amount of current of the generator, we could use a coiled conductor wrapped around a soft-iron armature. This increases the strength of the induced magnetic field. We could also rotate the armature faster or use stronger external magnets. In Tutorial 2 we will look at using a coil in a generator. [CATCH 2-column Tutorial] TUTORIAL 2 USING A COIL IN AN AC GENERATOR A coil-type generator includes a coil wrapped around a soft-iron armature as shown in Figure 10. This armature is rotated by an external source of energy. How does this design affect the direction of current in the external circuit? [CATCH C13-F016-OP11USB, size C] Chapter 13 21 [CAPTION] Figure 10 The rotation of the generator shown in Figure 10 is clockwise. As the shaded side of the armature moves away from the north pole of the external magnet, Lenz’s law determines the left side of the armature to be a south magnetic pole. The armature is being forced away from north. Thus the induced magnetic field must oppose being pulled away by attracting with a south pole. Using the right hand rule for a coil, the direction of the conventional current is as shown in Figure 11. [CATCH C13-F017-OP11USB, size C] [CAPTION] Figure 11 As the shaded side of the armature spins away from the north pole of the external field magnet, the amount of current increases to a maximum until the shaded side of the armature starts approaching the south pole of the external field magnet. The current increase can be explained by referring back to Tutorial 1. When the single loop was oriented so that it was perpendicular to the external magnetic field, the induced current was zero. When the single loop was oriented so that is was parallel to the external magnetic field, the induced current was at a maximum. The same applies whether you are using one loop or several loops as in the design in this tutorial. Using Lenz’s law, the shaded side of the armature will resist going towards south by repelling. This would make the shaded side of the armature a south pole. Using the right hand rule, the direction of the conventional current is as shown in Figure 12. [CATCH C13-F018-OP11USB, size C] [CATCH C13-F021-OP11USB, size D] [CAPTION] Figure 12 Now the shaded side of the armature will spin away from the south pole of the external field magnet. This time Lenz’s law predicts the shaded side of the armature to be north. This is because the shaded side of the armature is moving away from the south pole of the external field magnet. The shaded side of the armature will have to resist moving away by attracting to a north pole. The current now reverses direction as shown in Figure 13. [CAPTION] Figure 15 A DC generator [CATCH C13-F019-OP11USB, size C] [CAPTION] Figure 13 As the shaded side of the armature moves away from the south pole of the external field magnet, the current increases to a maximum value until the shaded side of the armature starts approaching the north pole of the external field magnet. The shaded side of the armature remains north on the side approaching north, causing repulsion. The current will go in the direction as shown in Figure 14. [CATCH C13-F020-OP11USB, size C] [CAPTION] Figure 14 DC Generators Chapter 13 22 A DC generator, as shown in Figure 15, has the same design as a DC motor. It has a split ring commutator instead of slip rings. The split ring commutator serves to prevent the current from changing direction in the external circuit as it does in the AC generator. However, the induced current in the coil in the armature is still the same as has been shown in the tutorials. In the case of the DC motor, electrical energy is transferred into the motor to cause rotation or kinetic energy. In the case of a DC generator, rotation or kinetic energy (for example, from falling water, wind, or high pressure steam) is used to turn the coil to generate electrical energy. Therefore, a generator may be considered a motor in reverse. RESEARCH THIS WIND TURBINES Skills: Researching, Identifying Alternatives, Communicating Wind is a renewable resource that many are tapping into to generate electricity using an AC generator. Many different designs of wind turbine are being engineered. Some are large scale and able to deliver enough electrical energy to power 5000 homes. Others are small scale and can power only one home. As you learned in this section, in a generator, the rotation rate of the loop is directly related to the frequency of the alternating current. Our electrical grid requires 60 Hz electricity. Different design philosophies are put in place to achieve this. 1. Choose a wind turbine technology. You can consider large-scale land-based turbines, off-shore turbines, or some of the novel small-scale turbines for residential applications. 2. Research how the technology works. Highlight the type of turbine used, the generator used, the type of technology used to control the electricity to feed it into the grid, and where the electricity is being used or developed. A. At what rotation rate does the wind turbine spin and how is the electricity controlled? [T/I] [A] B. What are the maintenance considerations? [K/U] [T/I] C. Prepare a tri-fold pamphlet highlighting your research. [T/I][C][A] [CATCH WEBLINK ICON] 13.4 SUMMARY • An electric generator is a device that converts other forms of energy into electrical energy. • Alternating current generators are designed with loops of a conductor that spins in a magnetic field. The end of the loops are connected to two different slip rings which allow it to produce alternating current. • Coiling the conductor around an armature increases the strength of the induced magnetic field making the generator produce more current. • Spinning the armature faster and/or using a stronger external magnetic field will also increase the current produced by the generator. • DC generators are designed like a DC motor except energy is put into spinning the coil to generate electricity rather than putting electrical energy into the motor to cause it to spin. 13.4 QUESTIONS 1.Sketch the generator shown in Figure 1, but change the rotation to counter clockwise. Answer the following questions based on your diagram. Consider the starting angle to be at 0°. [T/I] [C] (a) At what angle(s) relative to your starting point would you expect maximum current in the loop? (b) At what angle(s) relative to your starting point would you expect a zero current? (c) Sketch a graph of the induced current in the external circuit. (d) What effect would reversing the polarity of the external magnets have on the current? 2. How does the rotation rate of the loop in the generator in Figure 1 compare to the frequency of the alternating current? [K/U] Chapter 13 23 3. How many times per second does a generator armature rotate in North America? [K/U] 4. The generator shown in Figure 16 is rotated counter-clockwise. Determine the polarity of the magnetic field on the armature, the direction of induced current in the coil, and the direction of current in the external circuit. [T/I] [CATCH C13-F022-OP11USB, size C] [CAPTION] Figure 16 5. What effect would each of the following have on the generator shown in Figure 16? [K/U] (a) increasing the number of loops in the coil (b) decreasing the rotation rate of the armature (c) removing the soft iron from the armature (d) removing the brushes (e) using electromagnets for external field magnets 6. Is Figure 17 a DC or AC generator? Explain your answer. [K/U] [CATCH C13-F023-OP11USB, size C ] [CAPTION] Figure 17 [END PAGE 6 of 6] [END SECTION 13.4] Chapter 13 24 [START Section 13.5: 4pp] 13.5 transformer electromagnetic device that can raise or lower voltage [CATCH: C13-P008-OP11USB, size D Figure 1 Photo of a transformer. For example, the transformer for a computer. step-down transformer a transformer with fewer secondary windings than primary windings. step-up transformer a transformer with more secondary windings than primary windings. Transformers The electrical devices you use everyday all have different electrical energy requirements. An electric stove require lots of electrical energy, while an LED requires very little. Some devices require different currents and voltages. For example, a computer may require only 12 V to operate, so the voltage in your home needs to be lowered from 120 V to 12 V. Devices that are capable of raising or lowering AC voltage are called transformers. Transformers are used in many electronic devices so that the AC voltage is lowered or raised to a value that the device is designed for (Figure 1). Adapters, like cell phone chargers have transformers as part of their circuitry. Adapters also contain a circuit that converts AC voltage to DC voltage. How transformers work To understand how a transformer works, recall Faraday’s ring from Section 13.1. The ring had a primary circuit and a secondary circuit. These circuits are not in physical contact, but a current in the primary circuit induces a current in the secondary circuit. According to the law of electromagnetic induction, a changing magnetic field is required to induce a current. A changing magnetic field can be produced by using alternating current. An alternating current in the primary coil is the most critical part to producing an alternating current in the secondary coil of Faraday’s ring. Suppose that we change the number of windings in the coils on either the primary or secondary circuit of Faraday’s ring. Would the same AC voltage be measured across both the primary circuit and secondary circuit? Transformers have different numbers of windings on the primary circuit compared to the secondary circuit and a soft-iron core. If there are fewer windings in the secondary circuit than the primary circuit, then the voltage on the secondary side will be less than the voltage on the primary side. Transformers that have fewer windings on the secondary circuit than the primary circuit are called step-down transformers (Figure 2(a)). They are called step-down transformers because they lower the AC voltage by a specific amount. If the situation is reversed and there are more windings on the secondary circuit, then the voltage will be higher on the secondary side (Figure 2(b)).Transformers that have more windings on the secondary circuit than the primary circuit are called step-up transformers. They are called step-up transformers because they increase the AC voltage by a specific amount. So, we can lower or raise the voltage in the Chapter 13 25 secondary circuit just by changing the number of windings. [CATCH – C13-F024a-OP11USB and C13-F024b-OP11USB, total width B] Figure 2 (a) A step-down transformer has fewer windings on the secondary coil than the primary coil. (b) A step-up transformer has more windings on the secondary coil than the primary coil. [CATCH 2-column Mini Investigation] MINI INVESTIGATION OBSERVING TRANSFORMERS AT WORK Skills: Performing, Observing In this investigation, you will observe how a transformer works with direct current and alternating current. Equipment and Materials: variable AC/DC power supply, 2 AC/DC multimeters with probes, 2 alligator leads, transformer with different number of windings on primary and secondary coils. 1. Set up a circuit with the variable DC power supply connected to the transformer using the leads as shown in Figure 3. Make sure that the power supply is off. [Catch C13-P009-OP11USB, size C, setup] [CAPTION] Figure 3 2. With the power supply off, set to the voltage specified by your teacher. 3. Set your multimeter to measure DC voltage and connect one multimeter to the primary coil and the other multimeter to the secondary coil. 4. While watching the display on your multimeters, turn on the DC power supply. Record your observations. Turn off the power supply and record your observations. 5. Disconnect the transformer from the DC power supply and connect the alligator leads to the AC connection. Set your multimeters to measure AC voltages and repeat Step 4. A. How effectively did the transformer work with direct current? [T/I] [C] B. How did the AC voltage on the primary coil compare to the AC voltage on the secondary coil? [T/I] C. Is your transformer a step-up or step-down transformer? [T/I] Conservation of Energy in Transformers Transformers must obey the law of conservation of energy. Therefore, the energy going into the primary coil must equal the energy coming out of the secondary coil if there are no energy losses. Recall, from Section 5.5, that the change in energy is expressed as E Pt . Power in an electrical circuit is expressed as the product of voltage and current, or P VI . Using the energy and power equations, we can express the law of the conservation of energy as shown below. (The subscript p represents primary and the subscript s represents secondary.) Chapter 13 26 Ep Es Pp t Ps t Pp Ps substitute P VI VpIp VsIs In the above expression, you can see that both sides of the equation have the same terms. Whatever amount of energy goes into the primary coil must come out of the secondary coil. If the number of windings is the same on both sides, then the voltages and currents will be the same. In a step-down transformer, the voltage on the secondary coil, Vs, is lower than the voltage on the primary coil, Vp. So, from the equation above and the law of the conservation of energy, we can deduce that the current on the secondary side Is must be greater than the current on the primary side, Ip. In a step-up transformer, the voltage on the secondary coil, Vs, is higher than the voltage on the primary coil, Vp. To comply with the law of the conservation of energy, the current in the secondary coil Is must be less than the current on the primary side Ip. Therefore, the voltage and current are inversely proportional. For example, if voltage is doubled, then current is halved, and vice versa. Transformer Equations From the law of conservation of energy we derived the following equation: LEARNING TIP Transformer Equations You can just remember one equation: Vp Vs Is Np I p Ns VpIp VsIs Grouping I and V together gives Vp Vs Is Ip (equation 1) The voltage in the coil is directly proportional to the number of windings and therefore Vp Vs Np Ns (equation 2) where NP is the number of windings on the primary coil and NS is the number of windings on the secondary coil. We can also express current in a transformer with respect to the number of windings by combining equations 1 and 2. Chapter 13 27 Vp Vs Is Ip Vp and Vs Np Ns therefore Is Np I p Ns So the current is inversely proportional to the number of windings. You will use these equations in the following tutorials. LEARNING TIP Significant digits and windings [CATCH 2-column Tutorial] The number of winding is an exact number and does not limit the number of significant digits in a calculation. TUTORIAL 1 VOLTAGE IN A TRANSFORMER We will use the equation Vp Vs Np Ns to solve a problem involving voltage in a step-down transformer. Sample Problem 1 A step-down transformer used in an adaptor for a laptop has a primary voltage of 120 V. There are 250 windings in the primary coil and 25 windings in the secondary coil. Calculate the voltage in the secondary coil. Given: primary voltage, Vp = 120 V number of windings in the primary coil, Np = 250 number of windings in the secondary coil, Ns = 25 Required: voltage in the secondary coil, Vs Analysis: Vp Vs Np Ns rearrange the equation so that all the variables are as shown because it allows for easier subsequent rearranging VpNs VsNp solve for Vs Vs Solution: VpNs Np (120V)(25) 250 Vs 12V Vs Statement: The voltage in the secondary coil is 12 V. Practice 1. A step-down transformer has a primary voltage of 240 V. The number of windings in the primary coil is 550 and the number of windings in the secondary coil is 110. Determine the voltage of the secondary coil. [T/I] 2. A step-up transformer has a primary voltage of 31.0 V. The number of windings in the primary coil is 211 and the number of windings in the secondary coil is 844. Determine the voltage of the secondary coil. [T/I] [CATCH 2-column Tutorial] TUTORIAL 2 CURRENT IN A TRANSFORMER Vp I We will use the equation s to solve a problem involving current in a step-down transformer. Ip Vs Sample Problem 1 A step-down transformer used in the adaptor for a cell phone charger has a primary voltage of 120 V and a secondary voltage of 5.0 V. The current in the primary coil is 1.0 A. Calculate the current in the secondary coil. Given: primary voltage, Vp = 120 V secondary voltage, Vs = 5.0 V Chapter 13 28 primary current, Ip = 1.0 A Required: secondary current, Is Vp I Analysis: s Ip Vs Is VpIp solve for Is Solution: Vs (120V)(1.0A) 5.0V Is 24A Is Statement: The current in the secondary coil is 24 A. Practice 1. A step-down transformer has a primary voltage of 240 V and a secondary voltage of 12 V. The primary current is 0.15 A. Determine the current in the secondary coil. [T/I] 2. A step-up transformer has a primary voltage of 620 V and a secondary voltage of 12000 V. The current in the secondary coil is 1.3 A. Determine the current in the primary coil. [T/I] Transformer Efficiency The law of conservation of energy states that no energy is lost, but in practice some energy will be converted into unusable energy. In a transformer, some of the energy is transformed into unusable thermal energy in the coils, as well as sound energy. Some transformers make a noticeable hum because the transformer core is vibrating. Typically, transformers are better than 90% efficient. To maximize efficiency, the coils are made from conductors with low resistance like copper and the core is rectangular in shape to ensure that the magnetic field lines go through both coils effectively. 13.5 SUMMARY • A transformer raises or lowers AC voltage. It consists of a primary coil, secondary coil, and soft-iron core. • Step-up transformers have more secondary windings than primary windings and increase the voltage in the secondary coil • Step-down transformers have fewer secondary windings than primary windings and decrease the voltage in the secondary coil • The voltage is directly proportional to the number of windings • The current is inversely proportional to the number of windings Vp • The equations related to transformers are: Vs Is Np I p Ns . Np Ns , Is Vp I p Vs 13.5 QUESTIONS 1. Why do transformers need an alternating current to operate continuously? [K/U] 2. How can you tell the difference between a step-up or step-down transformer? [K/U] 3. A student is discussing transformers and states that "the voltage and current both increase in a Chapter 13 29 step-up transformer." Explain why this is not possible. [K/U] 4. Suppose that you increase the number of windings on the secondary coil compared to the primary coil. What would you expect the effect on voltage and current would be? [K/U] 5. Would a device that has the same number of windings on both the primary coil and secondary coil be classified as a transformer? Explain. [K/U] 6. Are transformers 100% efficient? Explain. [K/U] 7. Copy Table 1 into your notebook and find the missing values. [T/I] Table 1 Vp 12 V 30 V Vs Np 120 V 100 110 V 600 120 Ns Ip Is step up or step down 1.2 A 100 150 mA 0.28 A 1.68A 8. The number of windings on the secondary coil of a transformer is 1.5 times less than on the primary coil. If the primary coil has a current of 3.0 A and a voltage of 12.0 V, what will be the voltage and current on the secondary coil? [T/I] [END PAGE 4 of 4] [END SECTION 13.5] Chapter 13 30 [START Section 13.6: 4 pp] 13.6 Power Plants and the Electrical Grid In Section 13.3, the historical competition between the use of AC and DC electricity was discussed. Efficiency was one of the main reasons why AC electricity won over DC. But why is AC more efficient? To answer that question, we must first examine the transmission of electrical energy along a conductor. Transmission Efficiency and Current A single generator at one of today’s large-scale power plants can produce over 300 MW of power. If the voltage is 10 kV and P = 300 MW, then using the power equation P VI we can calculate for current: [FORMATTER: please stack the following equation; only 1 equal sign per line] I P / V 300 MW/ 10 kV 30 kA A current of 30 kA will generate a significant amount of thermal energy in a wire. To determine how much power is lost in a transmission wire, again use the power equation: P VI Now we substitute for V using Ohm’s law V IR into the previous equation to derive a new power equation P (IR)(I ) P I 2R This equation can be used to calculate the amount of power lost in the wire due to thermal energy losses. Assume that a generator produces 300 MW (3 x 108 W) of power at a current of 30 kA, which travels through a transmission wire with a resistance of 0.1 . Using the new power equation PI R 2 (30 kA) (0.1 ) 2 (30000 A) (0.1 ) 2 9 107 W P 90 MW So 90 MW of power is converted to unusable thermal energy. This represents a loss of 30%. Note that the lost power is proportional to the current squared, so if we could lower the current going through the wire, there would be much less power lost. Fortunately, we have a technology that can lower the current, increase the voltage, and keep the same power: a transformer. By using a transformer at a power plant we can step up the voltage. Suppose that we step up the voltage to 100 kV. This would lower the current to 3 kA. Repeating the calculation, we find Chapter 13 31 PI R 2 (3 kA) (0.1 ) 2 (3000 A) (0.1 ) 2 9 105 W P 0.9 MW This represents a loss of 0.3%, which is a significant improvement. This is the main reason why we generate AC electricity at power plants. Transformers will only work with AC electricity. Without the transformer, the amount of power lost in transmission would be impractical. The Electrical Power Grid The electrical power grid is a giant circuit composed of many parallel circuits. There are many sources of energy in a variety of different power plants feeding electrical energy into the grid. The grid transmits AC power using transformers which step up and step down the voltage where necessary. Figure 1 shows a representation of how transformers are used. [CATCH C13-F025-OP11USB, size B] [CAPTION] Figure 1 [TO COME] In Figure 1, the generator produces 20 kV AC which is immediately stepped up to 230 kV or higher to minimize energy loss. The electricity is then sent along power transmission lines suspended high above the ground supported by towers (Figure 2). If the voltages were higher, then the electricity could discharge through the air and into the ground—there is a limit to how much the voltage can be stepped up. Very specialized training and equipment is needed to maintain or repair the equipment used in these high-voltage transmission lines. The electricity is gradually stepped down in voltage at a district transformer station, a local transformer station, a substation (see Figure 3), and then a pole or ground transformer in your neighbourhood (see Figure 4). [CATCH run Figures 2, 3, and 4 across the page, size A total] [CATCH C13-P010-OP11USB] Figure 2 High-voltage electricity from power plants is transmitted across the province as needed using these towers to support the wires. [CATCH C13-P011-OP11USB] Figure 3 Transformers at this substation step down the voltage so that it is low enough to be transmitted to neighbourhoods. Chapter 13 32 CATCH C13-P012-OP11USB] Figure 4 This residential transformer steps down the voltage to 240 V for use in your home. CAREER LINK Power technicians construct and maintain generation, transmission and distribution stations. To learn more about becoming a power technician, GO TO NELSON SCIENCE The electrical power grid is monitored, and energy is fed into the grid on demand. If more energy is needed, and there is the capacity, then more is fed in. Power plants only generate the amount of electricity that is needed because electrical energy cannot easily be stored. If more is generated than needed, it is sold to other electrical grids, farther away, that need it. If we need more electrical energy and we do not have the capacity, then we purchase it from other grids at a significantly elevated cost. On some summer days in Ontario, when many air conditioners are running, we may use more electrical energy than the power plants are capable of generating. So we purchase electricity from the United States in order to make up the shortfall. The electrical grid needs maintenance from time to time. This is a very time consuming job due to the large number of wires needed. As the grid ages, continued repair and replacement is needed. The costs for this are passed along to the consumer. If you look at your electricity bill, you will find that there are costs listed for the amount of electricity used and its delivery. The delivery fee is collected to maintain the grid. [Career Link] Commercial AC Generators The generators used in power plants contain multiple coils and armatures. The field magnets are not permanent magnets because it is difficult to make a strong enough magnet. Also, permanent magnets would lose their magnetism over time because of the strong magnetic fields in the coils. So, instead, electromagnets are used. To increase the strength of an electromagnet, you increase the current. Where does the electrical energy come from to power the electromagnets? In some cases, it comes from the AC generator itself. In other cases, the AC generator has a DC generator that uses permanent magnets. The DC generator uses a source of energy (such as falling water) to generate electrical energy which is then used to power the electromagnets of the AC generator. The AC generator will use a source of energy (such as falling water) along with the DC generator to generate AC electricity. Figure 5 shows a cross section of a large scale generator and a hydro-electric power plant. [CATCH C13-F026-OP11USB and C13-F027-OP11USB,place side-by-side for a total width of B] Chapter 13 33 Figure 5 (a) Cross section of a large generator, and (b) cross section of a hydro-electric power plant. The rotation speed of a generator must be managed to maintain the desired frequency of 60 Hz. As electricity demand increases, more current is drawn into the grid and it becomes more difficult to turn the generator. You can increase the turning force of the turbine rotating the generator shaft. If this cannot be done, you can decrease the strength of the electromagnets inside the generator. This lowers the voltage. It is for this reason that the voltages continually fluctuate a small amount throughout the day. The voltages are required to be relatively constant however and can only fluctuate within a regulated amount. In Section 13.3, you learned about alternating current. The type of alternating current that was discussed in section 13.3 was single phase. Generators in power plants produce three phases of AC. You can think of the three phases as three independent alternating currents. To transmit three phases, only three wires are needed. The three alternating currents all have the same frequency but are out of phase from one another. The peak currents are offset from one another. The net result of using three-phase AC is that you can transmit more power with only a little extra conductor. 13.6 SUMMARY • Transmission of AC electricity requires the use of transformers to minimize losses. • Step-up transformers at the power plant are used to increase the voltage and decrease the current for transmission. • Step-down transformers are used throughout the grid to bring voltages down to levels that can be used in homes. • Commercial generators have multiple armatures and coils using electromagnets to generate AC electricity. 13.6 QUESTIONS 1. Describe the main reason why AC power generation was chosen over DC power generation. [K/U] [C] 2. Determine the power loss in each of the following. Express your answer as a percent. [T/I] (a) A 200 MW power plant delivers a current of 2 kA in a 10 wire. (b) A 200 MW power plant delivers a current of 200 A in a 10 wire. (c) A 10 MW wind turbine delivers a current of 3000 A in a 0.50 wire. 3. Why is electrical energy generated on demand? [K/U] [ [C] 4. What is the difference between the electrical generators you learned about in Section 13.5 and the commercial generators discussed in this section? [C] [A] 5. What is the benefit of generating three-phase alternating current? [C] [A] [END PAGE 4 of 4] [END SECTION 13.6] Chapter 13 34 Chapter 13 35 [START INVESTIGATION: 1p] INVESTIGATION 13.2.1 OBSERVATIONAL STUDY Investigating Electromagnetic Induction The generation of electrical energy uses the principle of electromagnetic induction. What factors affect the strength of the induced current and make electricity generation more effective? Purpose To change the magnetic field and observe the direction of induced current Equipment and Materials • 2 alligator leads • 2 different coils of wire (one with more windings than the other) • galvanometer (or ammeter) • 2 bar magnets Procedure 1. Connect the 2 alligator leads to the terminals of one coil of wire and the galvanometer terminals. Make note of which direction the coil is wound. 2. Gently move the north pole of one magnet into the coil and observe the direction of the deflection of the galvanometer needle. Using the conventional current convention, describe the direction of current in the coil. 3. Move the magnet into the coil quickly, and again observe the galvanometer. Using the conventional current convention, describe the direction of current in the coil. 4. Repeat steps 2 and 3 but this time pull the magnet out of the coil. 5. Repeat steps 2 and 3 but this time move the south pole of the magnet into the coil. 6. Repeat steps 2 and 3 using two magnets (with poles aligned to increase the strength of the magnetic field) 7. Repeat steps 2 and 3 by moving the coil in the direction of the magnet. SKILLS MENU Questioning Hypothesizing Predicting Planning Controlling Variables Performing Observing Analyzing Evaluating Communicating The magnet should be aligned with the core of the coil. 8. Repeat steps 2 and 3 using the other coil (with a different amount of windings). Analyze and Evaluate (a) Identify the manipulated and responding variables when investigating speed of the moving magnet versus current. [T/I] (b) Make a statement about the direction of conventional current when moving the magnet into the coil compared with moving it out of the coil. [T/I] [C] (c) Make a statement about the magnitude of the conventional current when the speed of the magnet changes. [T/I] [C] (d) Increasing the speed of the coil increases its kinetic energy. How does the current in the coil ensure that the law of the conservation of energy is obeyed? [T/I] (e) How does changing the magnetic pole affect the direction of conventional current? [T/I] (f) Does moving the coil versus moving the magnet change the results? Explain. [T/I] (g) How did using the second coil affect your results? Explain. [T/I] (h) From your results in step 2, use your right hand rule for a coil to determine the magnetic pole at the entrance point of the magnet. Are the results consistent with Lenz’s law? Describe how the results are or are not consistent. [T/I][C] Chapter 13 36 Apply and Extend (a) What would happen if you rotate a magnet inside the coil? [A] (b) Does the orientation of the magnet change the amount of current? Explain. [T/I] (c) The coil uses an insulated conductor for the windings. What effect would using bare conductor for the windings have? [A] [END PAGE 1 of 1] [END INVESTIGATIONS] Chapter 13 37 [START Chapter Summary: 1 page] Ch13 Summary Summary Questions [FORMATTER: set the following questions in two columns] 1. Create a study guide based on the points listed in the margin on page XXX. For each point, create three or four sub-points that provide further information, relevant examples, explanatory diagrams, or general equations. 2. Look back at the Starting Points questions on page XXX. Answer these questions using what you have learned in this chapter. Compare your latest answers with those that you wrote at the beginning of the chapter. How has your understanding changed during the study of this Chapter? Note how your answers have changed. Vocabulary law of electromagnetic induction (p. XXX) Lenz's Law (p. XXX) alternating current (p. XXX) electric generator (p. XXX) transformer (p. XXX) step-down transformer (p.XXX) step-up transformer (p.XXX) [CATCH Career Pathways feature] Grade 11 Physics can lead to a wide range of careers. Some require a college diploma or a B.Sc. degree. Others require specialized or post-graduate degrees. This graphic organizer shows a few pathways to careers mentioned in this chapter. 1. Select an interesting career that relates to Electromagnetic Induction. Research the educational pathway you would need to follow to pursue this career. 2. What is involved in becoming a [XXXX]? Research at least two pathways that could lead to this career, and prepare a brief report of your findings. [CATCH C13-F028-OP11USB Size B. Career Pathways Graphic organizer] [END Career Pathways] [END Chapter Summary] Chapter 13 Electromagnetic Induction 38 [START Chapter 13 Self-Quiz: 1 page] To come [END Chapter 13 Self-Quiz] [START Chapter 13 review: 6 pages] To come [END Chapter 13 review] Chapter 13 Electromagnetic Induction 39