Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
CLASS 23: PRACTICAL ELECTRICITY 23.1. INTRODUCTION Electricity plays an important role in everyday life. Understanding how electricity works allows you to deal with it in a safe manner. This chapter examines some of the applications of electricity. 23.2. GOALS Understand the difference between AC and DC current; Explain how a transformer works and be able to calculate the parameters involved in determining how voltage is transformed; Be able to explain how power is transmitted and why it is more efficient to transmit electricity at high voltage instead of high current; Explain how incandescent light bulbs work; Explain what an electric ground is and how the idea of ground is used to make electrical devices safer; Understand how fuses, circuit breakers and ground fault circuit interrupters work and why they are used; Understand how electromagnets works and how they differ from regular magnets; Know the basic principles of electrical safety; and Understand how a motor works. 23.3. WIRING AROUND THE HOUSE House wiring is always done in parallel, so that you don’t have to have something plugged into each outlet for anything to work. Houses have different sets of wires that carry current to different places. 23.3.1. AC. AC stands for ‘Alternating Current’. The ‘alternating’ part comes from the fact that both the current and Voltage Or the voltage change their magnitude and sign as a function of Current time. Note that, since resistance is usually constant, voltage and current oscillate. Each cycle consists of an increase in current, current going back to zero, a decrease in current, current going back to zero, as shown in Figure 23.1. This is repeated over and over. The voltage is usually 110 V or 120 V for residential Time electricity. A battery provides DC or direct current. The graph corresponding to Figure 23.1 would be a straight line for DC Figure 23.1: This plot shows the current or voltage. In DC current, the electric field moves current (or voltage as a function of through the wire. In an AC current, there is no net motion of time.) for ac power. the electrons (the drift velocity is zero.) The electric field oscillates back and forth. In the U.S., the AC coming out of a wall socket has a frequency of 60 Hz, which means that the current goes through one oscillation every 1/60th of a second. The reason that U.S. appliances don’t work in Europe is because Europe has 50-Hz wall outlet AC and the voltage is about 230 V or 240 V. 23.3.2. Household Wiring. An appliance, such as a refrigerator or hair dryer, has a fixed resistance. Houses provide a fixed amount of voltage. The power rating of an appliance tells you how much current is required for that appliance to work. Household circuits generally are fused at 15 or 20 amps. Large appliances have their own circuits because they draw a lot of current. One circuit typically provides from 1650 to 2400 watts of power. 23.4. TRANSFORMERS 23.4.1. Electromagnetic Induction. We know that a moving charge (a current) creates a magnetic field. Can a moving magnetic field create a current? The answer is yes. If a magnetic field changes near a loop of wire (or if the loop of wire is moved in a constant magnetic field), a voltage is induced in the loop of wire. (The wire must be a loop because there must be a complete path for a current to flow.) A voltage produced using a magnetic field and a current loop that move with respect to each other is called the ‘induced voltage’ (and the current is called the ‘induced current’). The process of inducing current and voltage is called electromagnetic induction. The magnitude of the induced voltage is proportional to the number of wire loops, the strength of the magnetic field and the rate at which magnetic field lines are cut by the wire. The direction of the induced voltage changes when the magnetic field changes direction. 23.4.2. The Need for Transformers. Electricity is made by converting one type of energy into another. For example, turbines can generate electricity when they are turned by water, which is how Hoover Dam supplies power. The electrical power generated must be transported to where it will be used. A machine that produces electricity is called a generator. Originally, direct current (DC) generators could only be connected to loads that required the same voltage that the generator provided because converting DC current of one voltage to another voltage was difficult. This meant that very large currents had to go through the wires. There are two problems with this. First, there are losses when power is transmitted. The power lost is given by P = I 2R (23.4.1) where R is the resistance of the cable you are using. The further you live from the power generation site, the more cable is required, the higher the resistance, and the more power is lost on its way to your house. Second, carrying large currents requires very thick cables for long distances. The sheer weight of the cables, plus the danger of high currents if power lines were downed, made this an untenable situation. In the late 1880s, Nikola Tesla and Thomas Edison became adversaries. The electrical power grid of the country was beginning to be established. Edison wanted the power grid to be established using direct current (DC). Many of Edison’s patents were for devices that used direct current, and DC was the first standard used in the United States. Tesla, however, devised a system that could be used to efficiently transmit and utilize AC power. He partnered with George Westinghouse to commercialize the system. Its efficiency eventually won out over Edison’s arguments for DC. According to Equation (23.4.1), higher currents result in greater losses. Given a fixed resistance, increasing the voltage lowers the current (Ohm’s law) and decreases the loss. It is thus more efficient to transmit electricity at very high voltages. When the voltage reaches a house, however, it must be decreased again to the standard 120V. 23.4.3. How Transformers Work. Transformers are used to convert voltages. Transformers are installed at substations and used to increase the voltage from the generating plant, and then again to decrease the voltage coming into homes. Increasing the voltage reduces the current in the transmission and distribution lines and decreases the size of the cables required and power losses. Transformers made it more economical to distribute power over long distances and electricity could be brought into more and more remote regions. A transformer has two parts: A primary coil, which is connected to a source of alternating current, and a secondary coil, which must be fairly close to the first coil and output the transformed voltage. An electrical current running through the primary coil creates a magnetic field. This magnetic field interacts with the wire in the second coil and creates a voltage in the second coil via electromagnetic induction. Remember that an ac field qualifies as a moving magnetic field. The amount of wire on each coil of the transformer determines how the voltage can be changed. The magnitude of the voltage induced in the secondary coil Vs is determined by the number of turns in each coil. Vp Np = Vs Ns (23.4.2) where Vp is the voltage in the primary coil, Vs is the voltage in the secondary coil, Np is the number of loops in the primary coil and Ns is the number of loops in the secondary coil. The ratio V/N is the voltage per loop. Equation (23.4.2) tells you that the primary and secondary coils each have the same voltage per loop. The number of loops thus controls the voltage. A second relationship is that (in the ideal case that there are no power losses in the transformer), the amount of power in the primary coil is equal to the amount of power in the secondary coil. In symbols: I pV p = I sVs (23.4.3) where Vp is the voltage in the primary coil, Vs is the voltage in the secondary coil, Ip is the current in the primary coil and Is is the current in the secondary coil. A step up or step down transformer steps up or steps down the voltage of an alternating current according to the ratio of wire loops in the primary and secondary coils. In Figure 23.2, the top transformer is a step-down transformer. The voltage on the left side is the 120 V that would come from the wall outlet. If you wanted to use power from the wall outlet for something that usually runs on batteries, you have to decrease the voltage. The side with the larger number of turns always has the higher voltage. The ratio of turns is 10:1, so the voltage out will be one-tenth the voltage in. The bottom transformer is a step up transformer and would be used to operate a piece of equipment that required a very high voltage. The ratio of turns is 1:10, so the voltage on the secondary is ten times the voltage on the primary. Primary Coil Np = 10 turns 120 V Primary Coil Np = 1 turn Secondary Coil Ns = 1 turn 12 V Secondary Coil Ns = 10 turns 1,200 V 120 V Figure 23.2: A step-down (top) and step-up (bottom) EXAMPLE 23.1: We need to increase a voltage from 145 V to 261V. What is the ratio of turns on the primary to turns on the secondary necessary to do this? Draw a picture known: No figure is necessary Vp = voltage on primary = 145 V Vs = voltage on secondary = 261 V Np need to find: = ratio of turns on primary to secondary Vp Equation to use Np Np Solve for unknown Ns Np Insert numbers Ns Ns = = Vs Ns = Vp Vs 145 V 261V = 0.555555 Np Answer to part b: Ns =0.556 Any time you are stepping up the voltage (the secondary voltage is higher than the primary voltage), the ratio of turns on the primary to turns on the secondary should be less than one. Comments EXAMPLE 23.2: We would like to increase the voltage from 145 V to 261V. If there are five turns on the primary, how many turns on the secondary are necessary? Draw a picture known: Vp = voltage on primary = 145 V Vs = voltage on secondary = 261 V Np = 5 turns Vp Equation to use Np Ns = Solve for unknown Insert numbers Answer Comments N s = number of turns on secondary need to find: = Vs Ns Vs Np Vp 261 V ( 5 turns ) 145 V = 9 turns Ns = N s = 9 turns The number of turns on the secondary is greater than the number of turns on the primary, which is what we expect. 23.5. LIGHT BULBS Sir Joseph Swan of England and Thomas Edison of America (in 1878 and 1879, respectively) independently invented the light bulb. Within 25 years, millions of people around the world had electric lights in their homes. Although Edison usually gets most of the credit, he had a large group of very talented employees who made important advances. For example, Lewis Howard Latimer, an AfricanAmerican draftsman and inventor who worked for Edison, invented a method of making carbon filaments that made light bulbs more reliable. Latimer also supervised installation of electric lights in the cities of New York, Philadelphia, Montreal, and London. 23.5.1. Light and Heat. What happens to the power lost given by Equation (23.4.1)? The answer is that the energy of the current is transformed into heat. If the heat flows into the right material, the material will become hot enough to glow and produce light. This is the principle on which light bulbs, space heaters and toasters work. A material with a higher resistance will heat up more than a material with a lower resistance, and a material will heat more if more electrons are going through it Figure 23.3: The insides of an than if fewer electrons are going through it. incandescent light bulb. 23.5.2. How Light Bulbs Work. Incandescent light bulbs are fairly simple. A clear light bulb will give you a good view of the parts of the incandescent light bulb, as shown in Figure 23.3. The most important part of the bulb is the filament, which is a coiled wire in the center of the bulb. One end of the filament (shown in red in Figure 23.3) travels to the side of the bulb, connecting with the metal part of the base that screws into the socket. The other side of the filament travels to the pointy part at the very end of the bulb. The other structures are primarily there to hold the filament in place and ensure that the two sides of the filament don’t touch each other. The light bulb works because current goes through the high-resistance filament and heats it hot enough (about 2200-2500 °C, which is about 4500°F) that it glows. Light bulbs are filled with inert gas because exposure to air hastens the degradation of the filament. The inert gas is usually argon, argon/nitrogen mixed or, in more expensive bulbs, a krypton/xenon mix. The filament of a standard 60-watt light bulb is a tungsten wire over six feet long, but less than one-hundredth of an inch in diameter. The wire is coiled to make a filament just a few centimeters long so that it fits in a compact bulb. The wire is long and thin because that shape provides the most efficient conversion of electrical energy to light and heat. Almost 95% of the energy that goes into a light bulb comes out as heat, so light bulbs are very inefficient. The color emitted by a bulb depends on the temperature to which the filament is heated. The temperature of the Sun is 5800°C (10,500°F), which produces the natural sunlight with which we are familiar. Unfortunately, no material we know of can be heated to that temperature without melting, so the incandescent light bulbs we use have a different color light than natural sunlight. 23.5.3. Why do light bulbs burn out? The fundamental reason is that the The filament breaks. That’s what you hear rattling around when you shake a light Figure 23.4: insides of a three-way bulb. When a material such as tungsten is heated to very high temperatures, atoms from the surface actually leave the filament (which may cause a black light bulb. The two film on the light bulb). The process of changing from a solid to a gas is called filaments are on the sublimation. When enough atoms are ejected from the filament, it will become left and right. weak and break, which means that the light bulb is now an open circuit. Tungsten sublimes around 2500°C, but the process is relatively slow. Light bulbs are filled with inert gases because exposure to air hastens the decay of the filament. The inert gas is usually argon, a mix of argon and nitrogen or a mix of krypton and xenon in more expensive bulbs. These gases slow the sublimation of the tungsten filament. 23.5.4. How are halogen bulbs different than regular light bulbs? Halogen bulbs take this filamentrebuilding step even further. There is a chemical reaction between the tungsten atoms and the halogen gasses (which usually are bromine and/or iodine). The halogen gases react with the tungsten atoms that deposits on the glass bulb and return them to the filament. This is not a controlled process, however, and the filament changes from being uniform to being bumpy. Eventually, one spot will become weak and break, so the filament is only as strong as the weakest part. Recycling the filament allows halogen lamps to operate at higher temperatures than conventional light bulbs, so the light produced is whiter and the halogen bulb is more energy efficient. The halogen gases that are used can be toxic, which is why these types of bulbs carry warnings about not touching their insides if the bulbs are broken. 23.5.5. How do three-way bulbs work? Three-way bulbs usually have two filaments. The three levels of brightness are given by lighting filament 1, filament 2 or both filaments. Figure 23.4 shows one filament on the left and one on the right. The filaments are two different thicknesses, which produce different brightness levels. 23.5.6. Fluorescent Light Bulbs provide light using an entirely different process. The electric current excites gas molecules that are trapped in the bulb. The gas molecules then emit ultraviolet light (which is not visible) and the ultraviolet light excites phosphors on the inside of the tube, which convert the ultraviolet light to visible light. Fluorescent lights are cooler and more energy efficient, but the spectrum of light they produce is markedly on the blue side and often makes things appear very differently than they do under natural sunlight. This is why sometime things you buy at a store appear a different color when you get them home and look at them under natural sunlight. 23.5.7. Light Bulb Characteristics. Bulbs are rated by voltage and resistance. The batteries you use must provide at least the amount of voltage stated to light the bulb. Christmas tree mini-lights, for example, are 2.5 V lights with a resistance of about 7-8 Ω. They can be lit with two D-cells, or with a 9-V battery. If you use a 9-V battery, the bulb won’t last very long because they are designed to run at 2.5 V. They probably will burn out in 30-60 minutes. You can light the bulb with one D-cell, but it won’t be very bright. 23.6. GROUNDING Why do some plugs with two prongs have one prong larger than the other? Why do some plugs have three fuse Hot wire prongs? The answer has to do with the way circuits are wired. In general, there is a ‘hot’ wire and a ground wire in two-prong circuits. Ground wire What is ground? You can think of the wire that goes to ground as a very low resistance path that all the electrons would like to take. Ground usually is somewhere where the electrons can be safely Figure 23.5: A two-prong plug. dissipated. A real ground is often a long metal pipe stuck in the ground. The Earth is so massive that it acts like a source or sink of electrons. You can deposit or withdraw many electrons and not change the overall charge of the Earth because each electron makes a miniscule contribution to the total charge. The third prong you see on electrical plugs is a ground – it is there because, if there is a need for the electrons that are being carried in the other two wires to leave those wires, they need go through a safe path and not, for example, through a person. The ground plug funnels the electrons safely away from people. If current is flowing and it has a choice of going through you or through a metal wire to ground, it will take the wire because the resistance of the wire is less than the resistance of the human body (unless, of course, the human body is wet). Plugging in a lamp makes a complete circuit. The current runs from the hot wire through the appliance cord, through the light bulb filament and back through the ground wire, as shown in Figure 23.5. Light bulbs are not particular about which way the current goes through the filament, but some appliances require that the current pass through them in a particular direction. Making the plug polarized (i.e. one prong is larger than the other so that it only fits one way into the electrical outlet) allows the manufacturer to control which way the current goes through the device. hot Electrons can flow through you if you are touching the case and there is a short to the case hot The third prong provides an easy path to ground Figure 23.6: A two-prong plug (left) compared to a three-prong plug (right). There is a safety hazard with two-prong plugs, however. If you are using something with a metal case – like a power tool – you have to be worried about a short circuit that provides a path for electrons that goes through the part of the tool you hold. If the proper path is broken for some reason (like the cord is frayed), the electrons have nowhere to go except the case. If you’re holding the case, the electrons can go through you because you are the shortest path to ground. If there is a third prong, the case can be grounded, providing a safe way for the electrons to leave the tool if there is a short, as shown in Figure 23.6. 23.7. SWITCHES AND FUSES 23.7.1. Switches. A switch allows you to create an open circuit. When the switch is open, there is no complete path for the current to take. The current sees, in effect, an infinite resistance. An item plugged into a wall switch draws current when turned on. A switch allows us to save energy by preventing it from drawing current. 23.7.2. Fuses. Although we want wires to heat up in a light bulb, we don’t want the wires to our lamps to heat up because they could cause a fire. Many pieces of electronic equipment can handle only some maximum current or they will be damaged. A fuse is a piece of wire or other metal designed to melt at a pre-set current. The material selected and fuse characteristics are determined by calculating how much heating will occur when the maximum current passes through the material. The fuse is designed so that it melts when that amount of current goes through it. The melting of the fuse causes an open circuit and the potentially damaging current doesn’t flow to any other more sensitive devices. Melting the metal strip is called ‘blowing a fuse’. The fuse is a one-time device: Once it is melted, you have to replace it. You can generally tell if a fuse is good by looking to see whether it is continuous. A fuse was one of the essential features of Edison's electrical power distribution system. An early fuse was positive charges N positive charges said to have successfully protected an Edison installation from tampering by a rival from a gaslighting company that wanted to sabotage his work. 23.8. ELECTROMAGNETS A current creates a magnetic field and the strength of the magnetic field is proportional to the amount of current passing through the wire. If you want S tmagnetic field from a wire, one option is to put more current through the wire; however, this creates problems. The current generates heat, which can melt wire, plus large currents are costly to produce. A Figure 23.7: The magnetic field produced by a loop of wire. second approach is to make a solenoid. As Figure 23.7 shows, a single wire loop has all of the fields pointing the same way (up) inside the loop and all of the fields pointing the same way outside the loop (down). A loop thus produces a field. If we stack a bunch of loops, all with the current going the same Figure 23.9: The magnetic field produced by a coil of Figure 23.8: A magnetic material strengthens wire is strengthened when a magnetic material is the magnetic field compared to if just the air introduced. This is the same principle as wrapping were present. wire around a nail to make an electromagnet. way, the fields from each loop add, creating a much larger field while still using the same amount of current. All of the field lines are concentrated in the center of the solenoid, creating a strong magnetic field. This is called an electromagnet. The advantage of an electromagnet over a regular bar magnet is that an electromagnet can be turned on and off. Electromagnets are used, for example, in junkyards to pick up and move cars. Turning off the electromagnet allows the car to be released. The magnetic field can be made even stronger by using a magnetic material to further focus the magnetic field lines. If the gap between two poles is filled with air (Figure 23.8), the magnetic field lines make their usual pattern (top picture). If, however, you place a piece of iron between the poles, the iron will be magnetized by the field produced by the current. The magnetic field lines are drawn into the iron, thus strengthening the magnetic field. This can be used to make a stronger electromagnet, as shown in Figure 23.9, but one that still can be turned on and off. The number of turns in the coil determines how large the field is. A coil with 50 turns produces a field 50 times greater than a coil with one turn. 23.9. CIRCUIT BREAKERS A circuit breaker serves the same function as a fuse; however, a circuit breaker is designed to be able to be reset. 23.9.1. Simple Circuit Breaker. A circuit breaker does the same thing as a fuse – it prevents current from flowing if the current becomes too large. A simple circuit breaker is illustrated in Figure 23.10. The left side shows the closed configuration, in which current is allowed to flow. The current from the hot wire wraps around a magnet, forming an electromagnets. The wire, number or coils, etc. are chosen so that the electromagnet is turned off if the current is less than the maximum allowable current. If the current exceeds this value, the electromagnet becomes strong enough to pull apart the contact and break the path for the current, as shown in the right side of Figure 23.10. S N Pivot Current out S N Pivot Spring No Current out – open circuit Current from live wire Spring Current from live wire is large enough to activate the electromagnet N N Current from live wire Neutral wire N Current from live wire Neutral wire S S Pivot N Pivot S S Figure 23.10: A simple circuit breaker in the usual position (left) and when tripped by too large a current (right) When the circuit breaker trips, there is no current running through the circuit and the electromagnet turns off. The contact can be pushed back together (that’s what you’re doing when you flip the switch of a circuit breaker that has been tripped) and will stay together unless the current exceeds the maximum value again. 23.9.2. Ground Fault Circuit Interrupter (GFCI). A GFCI is a more sophisticated type of fuse, with the goal of protecting people from electric shocks rather than protecting wiring. A GFCI works compares the current going into and the current coming out of a device. If they aren’t equal, it means that current must be going somewhere not in the planned circuit and the GFCI shuts off the current. Figure 23.11 (left) shows that the current from the hot and the neutral wires both pass around electromagnets. The gold bar in the middle is a magnetic rocker. The strength of the magnetic field is the same in both coils because they both have the same current. If the circuit is functioning correctly, both ends of the rocker are equally attracted by the electromagnets; however, if all the current that goes into the circuit doesn’t come out, the magnetic field on one side of the GFCI decreases, which opens the switch, as shown in the right-hand side of Figure 23.11. Figure 23.11: A simple circuit breaker in the usual position (left) and when tripped by too large a current (right) The GFCI breaks the circuit as soon as there is any change between current in the two arms of the device, whereas the circuit breaker waits until some preset level has been reached. The GFCI reacts much more quickly than a conventional breaker. A GFCI can sense leaks of as little as 0.005 A and can break the circuit in 0.025 s. 23.10. ELECTRICAL SAFETY The human heart depends on electrical signals to tell it when to beat. If current from an external source travels across your heart, it can cause the heart to go into fibrillation. The paddles used to get a heart beating (defibrillators) supply a current, but it’s a specific (small) current that is meant to get your heart beating correctly again. You will often see electricians hold one hand behind their back (usually their left one) when working so that they are less likely to form a complete circuit. Most people can feel a current as small at 0.0005 A. A current of 0.005 A is painful and one of 0.01 A can cause muscle spasms that might result in your not being able to let go of the source of the current. Current greater than 0.18 A prevents breathing. The most important rule of electrical is thus: Don’t make yourself part of a circuit. This may not be as easy as you think because you can make yourself part of a circuit without knowing it fairly easily. The human body’s resistance is about 1MΩ (although this depends on the person’s size, amount of body fat, etc.) If you’re wet, your resistance can decrease to 1000 Ω. A potential difference of 120 V will yield a current of about (Ohm’s Law isn’t exactly valid in this case, but it is close enough to give us an estimate). V R 120 V = 1000 Ω = 0.12 A I= This is a large enough current to be harmful. If you are not grounded, you are essentially an infinite resistance; however, if you are standing on (or very close to) something metallic, or if your skin is wet (which significantly decreases your resistance), you present a more attractive path for the current to reach ground. This is why bathrooms and kitchens are more dangerous in terms of electrical hazards than other parts of the house. 23.11. MOTORS In a motor, electrical energy is converted to mechanical energy. The motor has two working parts. A stationary electromagnet called a field N brushes N S commutator S Armature Load Figure 23.12: A simple motor. coil commutator commutator armature commutator axle armature axle Figure 23.13: Different views of the commutator, armature and axle. axle magnet and a cylindrical, movable electromagnet called an armature. The armature is on an axle and rotates in the magnetic field of the field magnet. The field magnet can be a permanent magnet or an electromagnet. A simple motor is shown in Figure 23.12. The armature is an electromagnet, so it can be turned on and off when desired. When the current is on, the unlike poles of the two magnets attract and the armature rotates. If the current is DC, the armature would stop when it aligns with the opposite pole. The commutator, however, has insulated segments (see Figure 23.13) so when it turns halfway, the commutator segments switch brushes and the current goes through the armature in the opposite direction. This switches in the current changes the poles in the armature and the armature rotates further. When it had rotated another half turn, the current again switches direction and the armature rotates another half turn. There are many different configurations, but they all work on the same principle. The wires from the winding of the armature are attached to the commutator – one to each half. Figure 23.13 shows the armature/commutator assembly. Since the commutator rotates with the axle, small stationary brushes maintain contact with the commutator and serve as electrical contacts. A DC motor applet can be found at: http://home.a-city.de/walter.fendt/phe/electricmotor.htm 23.12. SUMMARIZE 23.12.1. Definitions: Define the following in your own words. Write the symbol used to represent the quantity where appropriate. 1. Electromagnetic induction 2. Induced voltage 3. Generator 4. Primary coil and secondary coil (for a transformer) 23.12.2. Equations: For each question: a) Write the equation that relates to the quantity b) Define each variable by stating what the variable stands for and the units in which it should be expressed, and c) State whether there are any limitations on using the equation. 1. The relationship between the current running through a power transmission line and the power loss due to the resistance of the wire. 2. The relationship between the voltages that can be produced in a transformer and the number of turns on each coil. 23.12.3. Concepts: Answer the following briefly in your own words. 1. What does a transformer do and why are transformers necessary? 2. How does a transformer work? 3. Why are circuit breakers rated in terms of current and not voltage? 4. What is the difference between a step up and a step down transformer? 5. On what factors does the magnitude of the induced voltage depend? 6. What is an electrical ground? 7. Explain why electrical plugs have three wires. 8. Explain how a fuse works and how that differs from a circuit breaker. 9. How does an electromagnet differ from a bar magnet? 10. An electromagnet a) uses an electric current to produce a magnetic field; b) uses a magnetic field to produce a current; c) operates only on ac power; d) has to have a magnetic core to work. 11. A transformer can change a) the voltage of ac current; b) the power of ac current; c) ac power to dc power; d) dc power to ac power 12. Why are you more likely to experience a life-threatening shock when you are wet than you are when you are dry? 23.12.4. Your Understanding 1. What are the three most important points in this chapter? 2. Write three questions you have about the material in this chapter. 23.12.5. Questions to Think About 1. Explain how a motor works. 2. What happens to current when it passes through a light bulb? 23.12.6. Problems 1. A transformer is going to be used to step down voltage from 120 V to 45.0 V. (Assume 3 s.f. for the 120 V figure.) If there are 8 turns of wire on the primary, how many turns of wire must there be on the secondary? 2. A transformer has a current of 12.0 A in the primary and 30.0 A in the secondary. What is the ratio of turns in the primary to turns in the secondary? 3. A transformer has 600 turns on the primary and 200 turns on the secondary. If the secondary voltage is 80.0 V, what is the primary voltage? If the transformer delivers 300 W, what is the primary current? PHYS 261 Spring 2007 HW 24 HW Covers Class 23 and is due March 5th, 2007 1. 2. 3. Explain in your own words how a light bulb works, including what has happened when a light bulb ‘burns out’. A transformer has 600 turns on the primary and 200 turns on the secondary. If the secondary voltage is 80.0 V, what is the primary voltage? If the transformer delivers 3.00×102 W, what is the primary current? Explain how a fuse works. What are the differences between fuses and circuit breakers? 23.13. RESOURCES Moving magnet and wire coil connected to ammeter. Current flow applet: AC vs. DC http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/currentflow.htm Electrical generator applet: http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm 23.14. ELECTROMAGNETIC SWITCHES Electromagnetic switches can be used to control larger (often high voltage) pieces of equipment. The switch itself can be relatively low voltage. These are often called ‘relays’. If you put two metals, which each expand at a different rate when the temperature changes back to back and form them into a coil, the result is a coil that winds or unwinds when there is a change in termperature. A glass tube filled with mercury is attached to the coil, and attached to the glass tube are two pieces of wire, as shown below. When the coil is at its ‘off’ position, the circuit is not complete. When the coil expands or contracts, the glass vial tips and the liquid mercury (which is a conductor) completes the circuit. No circuit Off position Complete circuit Now attach to this a circuit such as the one shown at right (from your text). The electromagnet is a coil of wire with an iron core (a solenoid). When the circuit with the mercury switch is completed, the electromagnet is charged and pulls the spring-loaded piece of iron toward it, thus making a complete circuit. On position 23.15. GENERATORS In a motor, you provide a current through the wire and the interaction between the magnetic field and the current-carrying wire produces motion. This is a conversion of electrical energy to mechanical energy. A generator provides mechanical energy and produces electrical energy. The set up is similar to that of a motor: there is an armature (here shown as a wire loop) that rotates in a magnetic field. The rotation was originally provided by things waterwheels or turbines. As the wire moves through the magnetic field, a current is generated. The current generated may be alternating current (AC) or Direct Current (DC). There are two configurations for generators. In a DC generator, all of the current (or voltage) is positive, although it does vary in magnitude. An electronic circuit called a rectifier can be used to average out the current or voltage and produce a direct current (one that doesn’t change sign). The figure below is from your book and shows your author’s diagram for a DC motor. The graph below the picture shows the output voltage from the DC generator. Contrary to what you might expect, the voltage out is not constant; however, the voltage does not change signs at any point. (There are electronic ways of averaging out the bumps in the voltage to provide what is essentially a constant voltage.) Alternating current is useful for many things, but in some cases, DC current is required. In a DC generator, all of current is positive, although it does vary in magnitude. An electronic circuit called a rectifier can be used to average out the current or voltage and produce a direct current (one that doesn’t change sign). Figure 22.4 shows a DC motor. The graph below the picture shows the output voltage from the DC generator. Contrary to what you might expect, the current/voltage out is not constant; however, the current/voltage does not change sign. The difference between the DC and the AC generator is the connection that is used. A single, split ring called a commutator is used for a DC generator. (Your book’s Figure 22.6b does not show the difference very clearly.) The commutator is split, so that every half turn, there is a break in the circuit (this is when the voltage/current out goes to zero). When it starts up again, it is essentially as if the two connections switched sides, so instead of the voltage/current going negative, you get another positive loop. We can also look at an applet that shows us a DC generator: http://home.acity.de/walter.fendt/phe/generator_e.htm http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm 23.15.1. DC Generators In general, multiple loops of wire are run with different positions relative to the field. The sum of all these different voltages can be made to approximate a constant voltage, as shown in Voltage time Voltage time Figure 23.1: Combining multiple loops that start at different points to get something approximating a constant voltage 23.15.2. AC AC stands for ‘Alternating Current’. The ‘alternating’ part comes from the fact that both the current and the voltage change their magnitude and sign as a function of time. Each cycle consists of an increase in current, current going back to zero, a decrease in current, current going back to zero. This is repeated over and over. Voltage Or Current Time In the US, the AC coming out of your wall has a frequency of 60 Hz. This means that the current goes through this oscillation every 1/60th of a second. The time it takes for the current to go through one oscillation is called the period and is represented by T. The period and frequency are related: f = 1 T (23.15.1) where f is in s-1 or Hz and T is in s. Note that, since resistance is usually constant, voltage and current oscillate. The difference between and AC and a DC generator is in how the current is extracted from the generator. When a loop of wire is physically rotated, the long sides of the wires cut across the lines of force and a current is induced. The greater the rate at which the lines are cut, the greater the amount of current induced (i.e. the faster the loop spins, the more current.) Figure 20.1 illustrates this process. The lower graph is a plot of the voltage/current out as a function of the position of the wire with respect to the magnetic field. The current is zero when the wire loop is perpendicular to the magnetic field lines, becomes increasingly positive, maximizes when the loop is parallel to the field lines, then decreases, and reaches zero again when the loop is perpendicular. When the loop crosses over to the second half of its rotation, the forces are in opposite directions and the current is negative. Focus on a single charge in the wire: When the armature moves, the charge feels a force due to the field that pushes it along the wire – this is a current. The same thing happens on both sides of the loop, creating a current that goes all the way around the wire, and thus creating a potential difference between the two rings that are used for connectors. One end of the loop is connected to one of the rings and the other end to the other ring. In an AC generator, the commutator is replaced by two continuous rings. This produces an alternating voltage that changes sign. Figure 23.2: An AC generator (from Tillery’s Physical Science text) The same applet can be used to see how the AC generator works. http://home.a-city.de/walter.fendt/phe/generator_e.htm 23.16. ALTERNATORS Figure 23.3: A DC generator (from Tillery’s Physical Science text) Alternators are used in cars. The basic set up is that a rotor (armature) is driven by the alternator pulley. The current to the armature (which creates a magnetic field) is provided by the battery. A stator, which is another set of coils, is fixed in position outside the rotor. When the rotor turns, it induces a voltage in the stator coils. There are three sets of stators, located 120 degrees apart around the rotor. This means that each coil has an induced voltage that varies in time, but the voltages are out of phase so that at any given time, one of the coils is near its maximum voltage. This is AC current, and a car works on DC, so the output is run through diodes (devices that allow the current to only flow in one direction), to produce something that looks more like dc current (shown in top of figure below). Since there are three voltages, the net result is something that looks enough like DC to run the car. The alternator plays an additional role because it is connected through a feedback loop to sense the how fully your battery is charged. When you run your wipers, play your radio, use your heater, etc., it drains the battery. When the battery voltage decreases, the alternator senses this and ramps up the field current, increasing the voltage output of the alternator. Conversely, if the battery voltage goes up, then the alternator decreases the current in the field and doesn’t put out as much voltage. The output of the alternator is used to charge the battery: this is why if you idle for a long time, you can wear down your battery, and why your battery re-charges when you drive.