10.1. Exploring the Nature of Static Electricity. A Shocking Experience

Transcription

1 10.1 Exploring the Nature of Static Electricity Here is a summary of what you will learn in this section: Solid materials are by the transfer of electrons. When an atom gains electrons, it becomes negatively. When an atom loses electrons, it becomes positively. 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 particles that exert an electric force on each other. These 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

2 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 loosefitting 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

3 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 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 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 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. An object that has more protons than electrons is positively. 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. If a neutral object loses electrons, the object becomes positively. 396 UNIT D The Characteristics of Electricity

4 ist9_ch10.qxd 7/21/09 3:14 PM Page 397 Home Quit Friction and the Movement of Electrons All solid materials are by the transfer of electrons. How do atoms lose or gain electrons to become electrically? 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 and the amber becomes negatively (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 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

5 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 and which material will be negatively. 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

6 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 object is brought close to a negatively 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 CharlesAugustin de Coulomb was a French physicist who worked with electric charges and made several important discoveries (Figure 10.6). He showed that when two objects are placed closer together, the attraction or repulsion increases. When the 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 CharlesAugustin de Coulomb ( ) Static charges collect on surfaces and remain there until given a path to escape. 399

7 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

8 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 allpurpose 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

9 DI Key Activity D3 Question Inquiry Activity Investigating Static Electricity What is the effect of 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 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 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 vinyl Approaching Object Predictions Observations vinyl Figure Balance the ruler on the watch glass on top of the beaker. acetate acetate ruler ruler acetate vinyl vinyl acetate 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. 6. Bring a acetate strip near one end of the ruler. Record your observations. Analyzing and Interpreting 7. Usually, vinyl is negative and acetate is positive. How does this information explain your observations? Skill Practice 8. Describe how you would modify the procedure in this activity so that you could identify the type of charge on a object. Forming Conclusions 9. Write three statements that summarize your observations. 402 UNIT D The Characteristics of Electricity

10 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 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 and which becomes more negatively 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