The Characteristics of Electricity

Transcription

1 UNIT D The Characteristics of Electricity Lightning flashes around transmission lines carrying electricity to communities. 388

2 Contents 10 Static charges collect on surfaces and remain there until given a path to escape Exploring the Nature of Static Electricity 10.2 The Transfer of Static Electric Charges 10.3 Electrostatics in Our Lives DI 11 Current electricity is the continuous flow of electrons in a closed circuit Current, Potential Difference, and Resistance 11.2 Series Circuits and Parallel Circuits 11.3 Ohm s Law DI 12 We can reduce our electrical energy consumption and use renewable energy resources to produce electrical energy Renewable and Non-Renewable Energy Resources for Generating Electricity 12.2 Reducing Our Electrical Energy Consumption DI Unit Task In your Unit Task, you will evaluate methods of local electricity generation that could be used as backup sources for the regional power grid. Your investigations into the characteristics of electricity, methods of conserving electrical energy, and methods of providing electrical energy will help prepare you for your task. Essential Question How can we use local resources to generate electricity in a dependable, environmentally friendly way? 389

3 Exploring Toronto was one of many cities that were without electricity during the 2003 blackout. Some places in Ontario now celebrate Blackout Day on August 14 to remind people of how important it is to conserve energy. Electrical generating stations from Ohio to Ontario shut down, leaving 50 million people in the dark. Why? Blackout! Imagine what it would be like to live in a world without electricity. Now, count to nine. In a mere nine seconds, that scenario came true. On August 14, 2003, at 4:11 in the afternoon, 50 million people in Ontario and the northeastern United States were plunged into the largest electrical blackout in North American history. Elevators stuck between floors, subways were in blackness, traffic lights stopped working, and television screens and computer monitors went dark. Electricity is often generated far from cities and is distributed along a network that includes electrical generating stations, transmission lines, and distribution stations. This huge, interconnected system of electricity networks is called the energy grid. Ontario, New York, Michigan, and other northeastern provinces and states are part of the eastern interconnection grid. Electricity cannot be stored for long after it is generated, so all parts of the grid must maintain a balance of supply and demand. If a transmission line or generator is overloaded, that part of the 390 UNIT D The Characteristics of Electricity

4 energy grid is disconnected automatically and the electricity is sent along alternative paths. One cause of problems in an energy grid is transmission lines that touch trees so that the electricity moves through the trees into the ground instead of along the wire. Preventing Future Blackouts On August 14, a series of events, including human error, high demand, lines touching trees, automatic shutdowns, and failures of alarm systems, resulted in a huge surge of electricity in the grid. Within seconds, 256 electrical generating stations from Ohio to New York to Ontario shut down as a protective control. It took almost two full days to get all the generating stations back in operation and electricity restored to all the affected areas. This blackout raised difficult questions that could only be solved by government and electrical industry experts from both Canada and the United States working together. How did the blackout happen? What can be done to prevent such a blackout from occurring again? These are very complex questions to investigate. By working cooperatively, groups from the two countries successfully figured out the answers to these questions. Now, because of their hard work, the electrical grid is safer and better able to deal with a similar situation. Smaller, local blackouts do occur from time to time. But the knowledge learned from the mistakes of previous large blackouts helps reduce the chances of such a large-scale blackout happening again. A transmission line is automatically disconnected from the grid when it touches treetops or other objects. D1 STSE Science, Technology, Society, and the Environment Electricity Concept Map Electrical energy is often in the news. You have probably read or viewed reports about the costs and benefits of producing energy from renewable and non-renewable sources. You might be practising some ways to reduce electrical energy consumption and achieve electrical savings in your home. And you can probably describe the importance of electricity to your daily life. Now is your opportunity to get a sense of how your pieces of knowledge about electricity fit together. 1. As a class, create a concept map about electricity. Start with the word electricity in the centre of a large piece of chart paper. 2. Add categories, terms, concepts, and sketches to the map, making links between the parts that are connected. Exploring 391

5 10 Static charges collect on surfaces and remain there until given a path to escape. 392 UNIT D The Characteristics of Electricity

6 Sparks flash from the centre of a plasma ball to the point of contact where a hand touches the ball. Skills You Will Use In this chapter, you will: investigate the transfer of static electric charges by friction, contact, and induction predict the ability of different materials to hold or transfer electric charges plan and carry out inquiries to determine and compare the conductivity of various materials apply knowledge and understanding of the safe operation of electrical equipment Concepts You Will Learn In this chapter, you will: learn about the differences between electrical insulators and conductors explain how materials allow static charges to build up or to discharge analyze the design of technological devices that improve electrical efficiency or protect other devices by using or controlling static electricity Why It Is Important Static electricity is part of our daily lives. By understanding how charges build up and discharge, we can avoid problems caused by sparks and make use of static electricity to improve our lives. Before Reading Determining Importance Preview the subheadings and illustrations in Chapter 10. Which topics and illustrations are familiar? Which topics and illustrations are unfamiliar based on your background knowledge and experience? The unfamiliar topics and illustrations represent the information that is most important for you to learn. Create a list of learning goals for this chapter based on the information that represents new learning for you. Key Terms conduction conductor electrical discharge electron electron affinity friction induction insulator static electricity Static charges collect on surfaces and remain there until given a path to escape. 393

7 10.1 Exploring the Nature of Static Electricity Here is a summary of what you will learn in this section: Solid materials are charged by the transfer of electrons. When an atom gains electrons, it becomes negatively charged. When an atom loses electrons, it becomes positively charged. Electrons can be removed from objects through friction. Particles with unlike charges attract each other, and particles with like charges repel each other. Electrical insulators and conductors are materials categorized by how freely they allow electrons to move. Figure 10.1 Electric charges cause strands of hair to repel each other and be attracted to the balloon. A Shocking Experience On a cold winter day, you have probably pulled a sweater off over your head or removed your hat and felt your hair flying up. Or maybe you have reached to touch a doorknob or the door handle of a car and received an electric shock. These examples and hairraising experiences like the one in Figure 10.1 are caused by electric charges. Electric charges are charged particles that exert an electric force on each other. These charged particles are very small. In fact, there are millions of them on each standing hair in the picture above. The accumulation or gathering of even larger numbers of electric charges can lead to some impressive electrical displays. Think back to the last time you observed a lightning storm. The large, bright flashes of lightning look like the small electric sparks you may have seen when touching the doorknob or taking off your sweater. In fact, they are the same thing, just different in size. These are all examples of static electricity. 394 UNIT D The Characteristics of Electricity

8 D2 Quick Lab Characteristics of Electric Charge A characteristic is a distinguishing trait or quality of a substance or object. Purpose To observe the characteristics of electric charge Materials & Equipment confetti or gelatin powder plastic drinking straw 2 balloons Van de Graaff generator thin paper strips clear adhesive tape 3 aluminum pie plates clear plastic cup with lid polystyrene popcorn metal rod and lab stand Procedure 1. Read through the procedure steps, and make predictions about what you think will happen in each step. Record your predictions. 2. Sprinkle some confetti or gelatin powder in a small area on your desk. Push a plastic drinking straw through your hair several times, and bring it close to the confetti or gelatin powder. Record your observations. 3. Inflate two balloons, and knot the ends. Rub one side of each balloon on your hair or clothing. Hold the balloons by the knots, and bring the rubbed surfaces slowly together. Observe the results. 4. Turn one balloon so that its rubbed surface faces away from the other balloon. Again bring the balloons together. Record your observations for steps 3 and If your classroom has a Van de Graaff generator, your teacher will demonstrate the following experiments by putting the materials for each experiment in place and then turning on the generator. Record your observations for each experiment. (a) Tape one end of the thin paper strips to the Van de Graaff generator. (b) Place a stack of three aluminum pie plates on the Van de Graaff generator. (c) Place a clear plastic cup full of polystyrene popcorn on the Van de Graaff generator. Put a loose-fitting lid on top of the cup. (d) Attach a metal rod to a lab stand, and place it close to the Van de Graaff generator. 6. Return everything you used to the areas designated by your teacher. Questions 7. (a) Which objects were attracted to each other? (b) Which objects were repelled or pushed away from each other? 8. How did your observations compare with your predictions for each step? 9. What do you think caused the movements that you observed? Static charges collect on surfaces and remain there until given a path to escape. 395

9 Electrically Charged Particles You may recall from earlier studies that an element is a pure substance that cannot be broken down into simpler substances. An element is made up of tiny particles called atoms. An atom is the smallest part of an element with the element s properties. Within an atom, there are three types of smaller particles: protons, neutrons, and electrons. Protons and electrons are electrically charged particles. Protons have a positive electric charge (), and electrons have a negative electric charge (). Neutrons have no electric charge, so they are neutral. The protons and neutrons are in the nucleus at the centre of the atom. The electrons are outside the nucleus (Figure 10.2). Although they contain electrically charged particles, atoms are neutral. The number of protons in the nucleus equals the number of electrons around the nucleus, so the number of positive and negative charges is equal. This makes an atom neutral. nucleus neutron proton electron Figure 10.2 Each atom is made up of protons and neutrons inside the nucleus and electrons in the area around the nucleus. WORDS MATTER Static is from the Greek word statikos, meaning causing to stand. The word stationary, which means not moving, is based on the same Greek word. Static Charges Objects can become charged when electrons move from one object to another. The electric charge that builds up on the surface of the object is called a static charge or static electricity. The charges are static because they remain very nearly fixed in one location on the surface of the object until they are given a path to escape. An object that has more electrons than protons is negatively charged. An object that has more protons than electrons is positively charged. You can group objects according to three kinds of charge: positive, negative, and neutral. If a neutral object obtains extra electrons, the object becomes negatively charged. If a neutral object loses electrons, the object becomes positively charged. 396 UNIT D The Characteristics of Electricity

10 ist9_ch10.qxd 7/21/09 3:14 PM Page 397 Home Quit Friction and the Movement of Electrons All solid materials are charged by the transfer of electrons. How do atoms lose or gain electrons to become electrically charged? One common cause of electron transfer is friction, which occurs when objects rub against each other. Friction is the force resisting the relative motion of two surfaces in contact. When two objects rub together, the force of friction can remove electrons from one object and cause them to transfer to the other object. As one object loses electrons, the other object gains them, as shown by the amber and fur in Figure If you count the electrons in Figure 10.4, you will notice that no electrons are lost during the process of charging. They are simply transferred. The position of the positive charges does not change during the process of charging. W O R D S M AT T E R Electricity comes from the Greek word elektron, meaning amber, which is fossilized tree resin (Figure 10.3). Amber has been used for thousands of years to study static electricity. Figure 10.3 Amber is fossilized tree resin. This piece of amber contains bugs that were living on the tree and got caught in the amber. electrons neutral (a) neutral negative (b) positive (c) Figure 10.4 The amber and the fur are electrically neutral (a). If you rub the amber with the fur, electrons transfer from the fur to the amber (b). As a result, the fur becomes positively charged and the amber becomes negatively charged (c). It s important to remember that the transfer of the charges from one object to another is possible because the two objects are rubbing against each other. Both objects are neutral before they are rubbed together. They become charged as a result of the rubbing. For any charging procedure, it s important to keep in mind that new electric charges are not being created. The electrons in each object are just being rearranged within the object or transferred to another object. Static charges collect on surfaces and remain there until given a path to escape. 397

11 Suggested Activity D3 Inquiry Activity on page 402 Electron Affinity Different substances have different abilities to hold on to electrons. The tendency of a substance to hold on to the electrons is called electron affinity. Table 10.1 lists a series of selected materials in order of their electron affinity. You will notice that the higher the material is in the list, the greater the tendency for that material to lose electrons. This means that if you rub together two materials listed in the table, you can determine which material will be positively charged and which material will be negatively charged. For Table 10.1 A Triboelectric Series Tend to lose electrons Tend to gain electrons example, if you rub nylon and steel together, the nylon will become positive and the steel will become negative. The nylon will lose electrons, because it is higher in the table. The electrons from the nylon are transferred to the steel, making the steel negative. This table is referred to as a triboelectric series. The term comes from tribos, a Greek word meaning to rub. Note that there can be a slightly different order for materials such as fur or wood depending on which type of animal the fur is from and which type of tree the wood is from. () human hands (dry) glass human hair nylon cat fur silk cotton steel wood amber ebonite plastic wrap Teflon () Learning Checkpoint 1. Where are electrons in the atom? 2. What does static mean in static electricity? 3. What happens when two objects made out of different materials are rubbed together? 4. What term describes an atom s tendency to hold on to electrons? 5. In each of the following pairs, state which one is more likely to give up electrons. (a) wood or human hair (b) plastic wrap or steel (c) cotton or silk 398 UNIT D The Characteristics of Electricity

12 Laws of Attraction and Repulsion You may have heard the expression opposites attract in discussions about people. This is definitely true for electric charges (Figure 10.5). Scientists studying the interaction of objects have observed that when a positively charged object is brought close to a negatively charged object, the two objects attract each other. When two objects with the same charge are placed close together, the objects repel each other. Opposite charges attract. During Reading Visualizing and Picture Mapping Good readers use the strategy of visualizing to understand the important details of a large amount of complex information. One way to visualize is to create a picture map. Using the information about the laws of attraction and repulsion, begin drawing pictures to represent the information provided in this section. Add to your picture map as you read about electrical insulators and conductors. Like charges repel. Figure 10.5 If you increase the amount of charge on objects, the attraction or repulsion also increases. As a result of many scientific investigations, scientists have established the following laws of static electric charges. The law of attraction states that particles with opposite charges attract each other. The law of repulsion states that particles with like charges repel each other. Coulombs Charles-Augustin de Coulomb was a French physicist who worked with electric charges and made several important discoveries (Figure 10.6). He showed that when two charged objects are placed closer together, the attraction or repulsion increases. When the charged objects are moved farther apart, the attraction or repulsion decreases. In his honour, the metric unit for electric charge is named the coulomb (C). One coulomb equals electrons added to or removed from a neutral object. Figure 10.6 Charles-Augustin de Coulomb ( ) Static charges collect on surfaces and remain there until given a path to escape. 399

13 Electrical Insulators and Conductors Another way to group materials is by their conductivity. Conductivity is the ability of materials to allow electrons to move freely in them. Materials that hold onto their electrons and do not allow them to move easily are called electrical insulators. An electrical insulator is a solid, liquid, or gas that resists or blocks the movement of electrons, as shown in Figure Dry wood, glass, and plastic are all examples of electrical insulators. An insulator can hold a static charge because static charges remain nearly fixed in place (a) Insulator: The electrons () are bound tightly to the nuclei () so they resist movement. (b) Conductor: The electrons are not as tightly bound to the nuclei. They can move away from the nuclei. Figure 10.7 Electrons in an insulator cannot move freely. Electrons in a conductor can. Materials that allow electrons to change positions are called conductors (Figure 10.8). Conduction is the movement or transmission of electrons through a substance. Examples of conductors include the metals copper and aluminum. Some materials allow only some movement of electrons. This is the category of materials called fair conductors. In a fair conductor, the electrons do not move as freely as in a conductor, but they are not held almost in place as they are in an insulator. Figure 10.8 The metal wire in the electric fence allows electrons to move. The plastic insulator resists the movement of electrons. 400 UNIT D The Characteristics of Electricity

14 Table 10.2 gives some examples of conductors, fair conductors, and insulators. There are variations within each category, as some materials are better or poorer conductors than others. Table 10.2 Conductivity of Selected Materials Conductors Fair Conductors Insulators copper water with dissolved minerals rubber aluminum moist air wood iron human body plastic mercury carbon pure water other metals soil metal oxides, such as rust Water as a Conductor Notice in Table 10.2 that water is an insulator only if it is pure. However, most water has dissolved minerals in it, so its conductive properties change and it becomes a fair conductor. This is why you do not want to be in a lake during a thunderstorm. If lightning hits the water, the electric charges from the lightning will spread out through the water and cause you serious or fatal injury. This is also why you should not use water to try to put out an electrical fire (Figure 10.9). You also need to take care not to operate electrical appliances near water or with wet hands. Figure 10.9 Use an all-purpose fire extinguisher for an electrical fire. Learning Checkpoint 1. (a) What does the law of attraction state? (b) What does the law of repulsion state? 2. What is a coulomb? 3. Define electrical insulator. 4. What does conduction mean? 5. (a) Name two examples of good conductors. (b) Name two examples of fair conductors. (c) Name two examples of insulators. Take It Further A Faraday cage is an enclosure made of conducting material that protects its contents from electric charges. Find out how airplanes, cars, and even some specially designed clothes can act as Faraday cages. Start your research at ScienceSource. Static charges collect on surfaces and remain there until given a path to escape. 401

15 DI Key Activity D3 Question Inquiry Activity Investigating Static Electricity What is the effect of charged objects on each other and on neutral objects? Materials & Equipment 2 vinyl strips clear adhesive tape ring stand paper towel 2 acetate strips beaker watch glass wooden ruler or metre stick Skills Reference 2 SKILLS YOU WILL USE Adapting or extending procedures Drawing conclusions 4. Bring one of the charged vinyl strips close to the suspended acetate strip. Make sure the two strips do not touch each other. Record your observations. 5. Place the beaker upside down on the desk or table. Place the watch glass on top of the beaker as shown in Figure Balance the ruler so it is lying flat and centred on the watch glass. Bring a charged vinyl strip near, but not touching, one end of the ruler. Record your observations. Procedure 1. Copy the following table into your notebook to record your findings. Give your table a title. Hanging Object charged vinyl charged acetate charged acetate ruler ruler Approaching Object Predictions Observations charged vinyl charged acetate charged vinyl charged vinyl charged acetate Figure Balance the ruler on the watch glass on top of the beaker. 6. Bring a charged acetate strip near one end of the ruler. Record your observations. Analyzing and Interpreting 7. Usually, charged vinyl is negative and charged acetate is positive. How does this information explain your observations? Skill Practice 2. Tape one end of a vinyl strip to the ring stand so the strip hangs down. Rub the hanging vinyl strip with the paper towel to charge it. Then, rub the other vinyl strip with the paper towel, and bring that vinyl strip close to the suspended strip. Record your observations in your table. 3. Repeat step 2, using the two acetate strips and the paper towel. Record your observations. 8. Describe how you would modify the procedure in this activity so that you could identify the type of charge on a charged object. Forming Conclusions 9. Write three statements that summarize your observations. 402 UNIT D The Characteristics of Electricity

16 10.1 CHECK and REFLECT Key Concept Review 1. (a) Draw a diagram of an atom that has four protons, five neutrons, and four electrons. (b) Label each particle with its name and whether it is positive (), negative (), or neutral. 2. (a) What is friction? (b) Explain how friction can be used to transfer electrons. Use two substances from the triboelectric series in Table 10.1 on page 398 in your answer. 3. Explain why this statement is false: A neutral object contains no charge. 4. State the two laws of static electric charges. 5. Where are static charges held? 6. Why might a plastic rod that contains a large number of electrons not have a static charge? 7. (a) What is the difference between a conductor and an insulator? (b) What is an example of a conductor? (c) What is an example of an insulator? 8. (a) What is the difference between a conductor and a fair conductor? (b) What is an example of a fair conductor? 9. Why can you not use water to put out an electrical fire? Connect Your Understanding 10. Do two identical objects become statically charged when you rub them together? Explain why they do or do not. 11. Copy this chart into your notebook. For each pair, predict which substance becomes more positively charged and which becomes more negatively charged when the two substances are rubbed together. Use Table 10.1, A Triboelectric Series on page 398, to help you make predictions. Charged Pairs Pairs cotton, steel cotton, silk human hair, human hands (dry) Teflon, wood glass, plastic wrap 12. Make a list of five different ways in which you experience static electricity in your own life. 13. (a) While fishing in an aluminum boat in the middle of a lake, you notice storm clouds forming nearby. Why is it a good idea to get to shore as fast as possible? (b) Would your answer change if the lake somehow became filled with distilled water with no ions present in it? Explain why or why not. Reflection Becomes More Positively Charged Becomes More Negatively Charged 14. What are two questions about static electricity that you would like to explore further? For more questions, go to ScienceSource. Static charges collect on surfaces and remain there until given a path to escape. 403

17 10.2 The Transfer of Static Electric Charges Here is a summary of what you will learn in this section: Electroscopes are instruments that detect static charge. In charging by contact, an orginally neutral substance gains the same charge as the charged object that touched it. In charging by induction, an originally neutral substance gains the opposite charge to the charged object. Neutral objects are attracted to charged objects. Grounding an object transfers electrons between the object and the ground, making the object neutral. An electrical discharge occurs when charges are transferred quickly. Figure The bits of paper are attracted to the statically charged comb. Charged Objects What does dust on a computer screen have in common with paper on a comb (Figure 10.11)? In both examples, there is attraction between objects with unlike charges. You may have noticed a similar effect when you unpack a box containing polystyrene packing foam and the little pieces of foam stick to your skin and clothes. Polystyrene is very low on the triboelectric series and becomes charged very easily. How do you know when an object is charged? Rather than testing whether the object sticks to something else, you can use an electroscope, which is an instrument that can detect static charge. The electroscope was first invented in 1748 by a French clergyman and physicist named Jean Nollet. A metal-leaf electroscope has two very thin metal pieces, called leaves, suspended from a metal rod (Figure on the next page). The metal rod is attached to a top plate or metal knob. When a charge is transferred to the plate or knob, the charges spread out over the whole structure, including the leaves. The greater the charge, the greater the separation between the leaves. An electroscope is one of the devices that can be used to study static electricity. The study of static electric charges is called electrostatics. 404 UNIT D The Characteristics of Electricity

18 D4 Quick Lab Using an Electroscope Purpose To determine what happens to an electroscope when different charged objects are brought near it Materials & Equipment plastic comb or straw or ebonite rod metal-leaf and/or pith-ball electroscope glass, acrylic, or acetate rod wool sweater silk fabric Procedure Part 1 Metal-Leaf Electroscope 1. Charge the comb or straw by running it through your hair, or rub an ebonite rod on a wool sweater. 2. Bring the charged object near, but not touching, the top of the electroscope (Figure 10.12). Observe the motion of the metal leaves. Then, move the object away and observe the leaves again. Record your observations. 3. This time, touch the charged object to the top of the electroscope. You can rub the object along the top of the electroscope if necessary. Observe the motion of the metal leaves. Then, move the object away and observe the leaves again. Record your observations. 4. Charge the glass, acrylic, or acetate rod by rubbing it with the silk fabric. Repeat steps 2 and 3 using this charged rod. Part 2 Pith-Ball Electroscope 5. Charge the comb or straw by running it through your hair, or rub an ebonite rod on a wool sweater. 6. Bring the charged object near the pith ball but do not touch it (Figure 10.13). Record your observations. 7. This time, touch the pith ball with the charged object. Then, touch it again. Record your observations. 8. Charge the glass, acrylic, or acetate rod by rubbing it with the silk fabric. Repeat steps 6 and 7 using this charged rod. Questions 9. What role did friction play in this activity? 10. With your group, explain what happened in Part 1, using your knowledge about charges. Assume your object had a negative charge placed on it. 11. With your group, explain what happened in Part 2, using your knowledge about charges. Assume your object had a negative charge placed on it. Figure Metal-leaf electroscope Figure Pith-ball electroscope Static charges collect on surfaces and remain there until given a path to escape. 405

19 Detecting Static Charge In order to predict what charge is transferred to an electroscope, you can use a standard set of charged objects, such as ebonite and glass. Ebonite is a hard rubber material that is low on the triboelectric series and readily accepts electrons. When ebonite is rubbed with fur, it becomes negative (Figure 10.14). Glass is high on the triboelectric series and tends to give away electrons. When glass is rubbed with silk or plastic, it becomes positive, as shown in Figure Figure To test unknown charges, you can use the known charges on an ebonite rod (a) and a glass rod (b). Suggested Activity D5 Quick Lab on page 412 When a negatively charged rod is brought near a neutral electroscope, the electrons in the electroscope are repelled by the rod. The electrons move down into the leaves of the electroscope. The leaves are now both negatively charged, so they repel each other and move apart (Figure 10.15). When the negatively charged rod is taken away, the negative charges in the electroscope are no longer repelled, so they move throughout the leaves, stem, and knob. The leaves drop down, and the electroscope is neutral again. (a) (b) Figure The leaves are not separated in the neutral electroscope (a). The leaves repel each other when they are charged negatively or positively (b). 406 UNIT D The Characteristics of Electricity

20 Charging by Contact As you learned in section 10.1, electrons can be transferred through friction. Electrons can also be transferred through contact and conduction. You can charge a neutral object by contact when you touch it with a charged object. Charging by contact occurs when electrons transfer from the charged object to the neutral object that it touches. The neutral object gains the same type of charge as the object that touched it because the electrons move from one object to the other (Figure 10.16). During Reading Understanding Terms and Concepts A Frayer quadrant can help you understand a term or the concept it represents. Divide a rectangle into four sections, and put the term or concept as the rectangle s title above it (e.g., Charging by Contact). In the top left section, write a definition of the term using your own words and words from the text. In the top right section, write facts related to the term. In the lower left section, write examples of the term from the textbook. In the lower right section, write non-examples of the term. (a) Figure (a) When a negatively charged object touches a neutral object, electrons move to the neutral object, making it negative. (b) (b) When a positively charged object touches a neutral object, electrons move from the neutral object to the positive object and make the neutral object positive. Suggested Activities D6 Inquiry Activity on page 413 D7 Inquiry Activity on page 414 Induction Induction is the movement of electrons within a substance caused by a nearby charged object, without direct contact between the substance and the object. If you rub a rubber balloon on your hair, electrons will transfer from your hair to the balloon, making the balloon negative. The charges stay in a nearly fixed, or static, position on the balloon because rubber is an insulator. When you bring the negatively charged balloon near a neutral wall, the negatively charged electrons on the balloon repel the negative charges on the wall, making that part of the wall a positive surface. The balloon is said to induce a charge on the wall because it charges the wall without contacting it (Figure 10.17). Figure The negatively charged balloon has induced a positive charge on the wall s surface without touching the wall. Static charges collect on surfaces and remain there until given a path to escape. 407

21 Charging by Induction When you charge an object by induction, you use a charged object to induce a charge in a neutral object and then ground the charged object so it retains the charge. This newly charged object has the opposite charge to the charge on the charging object. Grounding is the process of connecting a charged object to Earth s surface. When you connect a charged object to the ground, you provide a path for charges to travel to or from the ground. Figures and show the process of charging by induction. Grounding occurs in diagram (b). electrons Figure (a) When a negatively charged object comes near a neutral electroscope, it repels the electrons in the neutral electroscope. (b) When you ground the neutral electroscope, you provide its electrons with a path away from the repulsive influence. Some electrons leave the electroscope. (c) When you remove the ground and the charged object, the electroscope is left with a positive charge because it has lost some electrons. electrons Figure (a) When a positively charged object comes near a neutral electroscope, it attracts electrons in the neutral electroscope. (b) When you ground the neutral electroscope, you provide a path for electrons to go toward the positive influence. (c) When you remove the ground and the charged object, the electroscope is left with a negative charge because extra electrons are trapped on it. 408 UNIT D The Characteristics of Electricity

22 Learning Checkpoint 1. What does an electroscope detect? 2. In the contact method of charging, what charge does a neutral substance gain compared to the object that touched it? 3. In induction, what charge does a neutral substance gain compared to the object brought near it? 4. What is the difference between charging by contact and charging by induction in terms of electron transfer? 5. What is grounding? Electrical Discharge Once an object is charged, the charges are trapped on it until they are given a path to escape. When electric charges are transferred very quickly, the process is called an electrical discharge. Sparks are an example of electrical discharge (Figure 10.20). Have you walked across a carpet and reached for a doorknob only to be shocked when you created a spark (Figure 10.21)? When you shuffle your feet in slippers or socks on a carpet, electrons are transferred through friction and you build up a static charge. When your hand reaches toward the neutral doorknob, the excess electrons transfer due to induction. Figure When a spark occurs, the air becomes a passage for the electrons to travel. Collisions between moving electrons and air particles release light and can also make a crackling sound. Transfer of charge from girl to door 9G10.42 Transfer of charge from carpet to girl Figure When electrons jump between your hand and a doorknob, you can receive a surprising shock. Static charges collect on surfaces and remain there until given a path to escape. 409

23 Lightning Lightning is an example of a very large electrical discharge caused by induction. In a thunderstorm, a charged area, usually negative, builds at the base of the cloud (Figure (a)). The negative charge at the base of the cloud creates a temporary positive area on the ground through the induction process (Figure (b)). When enough charge has built up, a path of charged particles forms (Figure (c)). The cloud then discharges its excess electrons along the temporary path to the ground, creating a huge spark lightning (Figure (d)). This discharge creates a rapid expansion of the air around it, causing the sound of thunder. electrons electrons (a) (b) (c) (d) Figure Lightning is an atmospheric discharge of electricity. It is interesting to note that air is normally an insulator. If it were not, lightning would occur every time that clouds formed. For lighting to occur, charges in the clouds must build up to the point where the air cannot keep the charges separated from the ground. At this point, the air stops being an insulator and becomes a fair conductor, resulting in a lightning strike. Earth is a donator or receiver of charge and is so large that overall it is not affected by the electron transfer of huge lightning strikes. As a result, the ground is always considered neutral. 410 UNIT D The Characteristics of Electricity

24 Electrostatic Generators Scientists use several devices in the laboratory to study how static charges create lightning and other phenomena, such as the static that affects clothes coming out of the dryer. Early electrostatic generators were called friction machines because they used direct contact between different surfaces to create charged areas. A glass sphere or cylinder was rubbed mechanically by a pad to charge it up. More recent machines, such as the Van de Graaff generator, create charge through friction between the roller and belt and then transfer the charge to a large metal sphere, as shown in Figure Take It Further Sometimes, lightning strikes start from the ground and go to a cloud. There are also cloud-tocloud lightning strikes. Find out more about different types of lightning. Create a visual display of your findings. Use ScienceSource as a starting point. charge collector metal sphere Teflon roller rubber belt insulating support nylon roller motor-driven pulley Figure (b) The static charge on a Van de Graaff generator has a hair-raising effect on these students. comb Figure (a) This Van de Graaff generator is set up so its dome is negatively charged. A Van de Graaff generator can also be charged positively by using different roller materials. A Wimshurst machine creates charges on two slowly rotating disks with metal strips placed around the disks (Figure 10.24). The charge is built up using induction between the front and back plates as the disks turn in opposite directions. The excess charge is collected by metal combs with points near the turning disks. Figure The Wimshurst machine uses induction to build up charge and create sparks. Static charges collect on surfaces and remain there until given a path to escape. 411

25 D5 Quick Lab Charge Sorter Materials that tend to lose electrons are higher on a triboelectric series. Materials that tend to gain electrons are lower on a triboelectric series. Purpose To sort materials based on their ability to hold on to static charge Materials & Equipment materials such as fur, silk, aluminum, paper towel, leather, wood, amber, hard rubber, Styrofoam, plastic wrap, vinyl (PVC) and Teflon metal-leaf electroscope known charged object, such as an ebonite rod rubbed on fur to create a negative charge CAUTION: Some people are allergic to fur. Procedure 1. Make a table like the one below to list your materials, predictions, and results. Give your chart a title. Record your predictions. Materials Prediction of Charge Actual Charge A B A B A B 4. Use a charged ebonite rod to test the charge on the electroscope by bringing it near the knob. Do not touch the rod to the electroscope (Figure 10.25). Observe the motion of the leaves. 5. Record the charge of material A. 6. Ground the electroscope by touching it with your hand. Then, charge the electroscope using material B. 7. Use a charged ebonite rod to test the charge on the electroscope by bringing it near the knob. Do not touch the rod to the electroscope. Observe the motion of the leaves. 8. Record the charge of material B. 9. Repeat steps 38 for each pair of materials. Questions 10. Which materials were good electron receivers and would appear lower on a triboelectric series? 11. Which materials were good electron donors and would appear higher on a triboelectric series? 12. Create a triboelectric series by listing the materials you used in order, according to their electron affinity. 1. fur silk 2. fur aluminum 3. silk aluminum 4. silk paper 2. Record your predictions for what charge each material in each pair will have when the materials are rubbed together. 3. Rub together the first pair of materials, A and B. Then, touch material A to the knob of the electroscope to charge the electroscope. Figure To test the charge on the electroscope, bring the charged ebonite rod near it. Do not touch it. 412 UNIT D The Characteristics of Electricity

26 D6 Inquiry Activity Charging by Contact Skills Reference 2 SKILLS YOU WILL USE Making predictions Observing, and recording observations Question What charge does the electroscope gain compared to the charging rod? Materials & Equipment ebonite rod fur metal-leaf electroscope CAUTION: Some people are allergic to fur. Procedure 1. Make a table like the following to record your predictions and observations. Give your table a title. Record your predictions. Trial glass rod silk Motion of Leaves Predictions Observations 4. Rub the ebonite rod with the fur again. Bring it near, but not touching, the top of the electroscope. Record your observations using a labelled diagram. 5. Charge the glass rod by rubbing it with silk. Bring the glass rod near, but not touching, the top of the electroscope. Record your observations using a labelled diagram. 6. Touch the top of the electroscope with your hand. Trial B 7. Repeat steps 24 using a glass rod charged with silk. Use a charged ebonite rod in steps 5. Repeat step Return all materials to the areas designated by your teacher. Analyzing and Interpreting 9. (a) Explain why the leaves moved when the ebonite rod touched the electroscope in step 3. (b) What charge was left on the electroscope? Trial A Trial B Trial A ebonite rod touching ebonite rod near glass rod near glass rod touching glass rod near ebonite rod near 10. (a) Explain why the leaves moved when the glass rod touched the electroscope in step 5. (b) What charge was left on the electroscope? 11. How do your predictions compare with your observations? 12. In terms of charge movement, explain in words and diagrams the effect of: (a) an identically charged rod near the electroscope (b) an oppositely charged rod near the electroscope Skill Practice 13. Explain how you would find the charge of an unknown material. 2. Charge the ebonite rod by rubbing it with the fur. 3. Brush the ebonite rod against the top of the electroscope. Record your observations of the electroscope leaves using a labelled diagram. Forming Conclusions 14. Write a summary statement about the charge the electroscope gains and the charge of the influencing rod. Static charges collect on surfaces and remain there until given a path to escape. 413

27 D7 Inquiry Activity Charging by Induction Skills Reference 2 SKILLS YOU WILL USE Gathering, organizing, and recording relevant data from inquiries Interpreting data/information to identify patterns or relationships Question What charge does the electroscope get compared to the charging rod? Materials & Equipment ebonite rod glass rod silk CAUTION: Some people are allergic to fur. Procedure 1. Make a table like the following. Give your table a title. Record your predictions. Trial Trial A Trial B Trial A ebonite rod away ebonite rod near glass rod near glass rod away glass rod near ebonite rod near fur metal-leaf electroscope Motion of Leaves Predictions Observations 2. Charge the ebonite rod by rubbing it against the fur. 4. Remove your hand from the electroscope, and then move the ebonite rod away. Observe what happens to the leaves of the electroscope. Record your observations. 5. Bring a charged ebonite rod near the electroscope. Record what happens to the electroscope leaves. 6. Bring a charged glass rod near the electroscope. Record what happens to the electroscope leaves. Trial B 7. Repeat steps 25 except start by charging a glass rod against silk in step 2. Use a charged ebonite rod for step 6. Analyzing and Interpreting 8. (a) Compared to the original rod that was brought near the electroscope, what charge did the electroscope end up with? (b) How do you know? 9. Explain what happens to the electrons in the electroscope when your hand touches the electroscope. 10. (a) Why did you have to remove your hand first before you moved the rod away? (b) What would have happened if you had moved the rod away and then your hand? Skill Practice 11. How else could you ground the electroscope? Forming Conclusions 12. Summarize the method of charging by induction by using diagrams labelled with the motions of charges. 3. Bring the ebonite rod near the electroscope. Be careful not to touch the rod to the electroscope. While you hold the rod there, touch the top of the electroscope with your hand. 414 UNIT D The Characteristics of Electricity

28 10.2 CHECK and REFLECT Key Concept Review 1. How are lightning and a spark similar? 2. (a) How do objects become negatively charged using the contact method? (b) How do objects become positively charged using the contact method? 8. (a) Why do the leaves of the charged electroscope shown below move farther apart if a rod with the same charge is brought near? (b) Why would the leaves move closer together if the rod had the opposite charge to the electroscope? 3. Explain how a substance becomes temporarily charged by induction when a charged object is brought near. 4. Explain how to charge an object permanently using induction. 5. Using a sequence of labelled diagrams, explain how a positive balloon will stick to a neutral wall. Under each diagram, describe the motion of the charges. Connect Your Understanding 6. (a) How does the process of grounding occur when you receive a spark from touching a metal shopping cart? (b) How does the process of grounding occur during a lightning strike? 7. What would change about the way an electroscope worked if its metal knob were replaced with a plastic knob? metal knob Question 8 9. A person walks across a carpet, touches a metal doorknob, and receives a shock. If the same person were carrying a metal rod, she would not experience a shock when touching the doorknob. Why? 10. Suppose a five-year-old child asks you to explain why there is lightning. Write a simple explanation that you could share with the child. You may wish to include a diagram. Reflection 11. What are two things about static electricity that you know now but you did not know before you started this chapter? Question 7 For more questions, go to ScienceSource. Static charges collect on surfaces and remain there until given a path to escape. 415

29 10.3 Electrostatics in Our Lives Here is a summary of what you will learn in this section: Lightning rods are used to prevent damage to buildings. Grounding static charges can help prevent sparks near flammable fuels. Paint sprayers work better if the object and the paint have different charges. Photocopiers use electrostatic principles in their operation. Grounding wires prevent damage to electrical equipment. Electrostatic precipitators work by creating charged waste particles and using electrostatic attraction to remove the particles. Figure Lightning can strike tall buildings repeatedly during a storm. The CN Tower (extreme right of photograph) is struck by lightning more than 70 times a year. Lightning Storm Awareness On a hot and humid summer night, lightning strikes a building in Toronto (Figure 10.26). Along with the lightning, there would have been loud claps of thunder. You may have noticed that as a storm moves closer, the time between lightning and thunder decreases. This occurs because lightning travels very fast, at the speed of light. Thunder travels much more slowly, at the speed of sound. If you see lightning and hear thunder at the same time, the storm is right above you. Summer storms are common in Ontario and across Canada, but many people do not know what to do in these extreme weather conditions. Lightning storm safety begins by watching for towering cloud formations that signal developing storms. Lightning can strike up to 15 km from where it is raining. As a guideline, if you can hear thunder, you are in striking distance and should look for shelter. Safe shelter includes a large building because it will be properly grounded if there is a strike. Cars, school buses, and other vehicles are also safe places, provided that the windows are rolled up and you do not touch metal parts of the vehicle. 416 UNIT D The Characteristics of Electricity