Electric Charge and Electric Forces

Transcription

1 CHAPTER 19 Electricity LESSON 1 Electric Charge and Electric Forces What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. Protons and electrons have opposite electric charges. Key Concepts How do electrically charged objects interact? How can objects become electrically charged? What is an electric discharge? 2. Objects must be touching to exert a force on each other. Electric Charges Imagine a hot, summer afternoon. Dark clouds are hanging overhead. Suddenly, a bolt of lightning streaks across the sky. Seconds later, you hear thunder in the distance. The lightning released a tremendous amount of energy. Some of this energy was released as the light that flashed through the sky. Lightning also released some energy as the sound you heard as thunder. And some of the energy was released as thermal energy that heated the air. Where did the lightning s energy come from? The answer involves electric charge. Charged Particles Recall that all matter is made of particles called atoms. Also recall that atoms are made of even smaller particles protons, neutrons, and electrons. Protons and neutrons make up the nucleus of an atom. Electrons move around the nucleus. Protons and electrons have the property of electric charge, but neutrons do not. As you read further, you will learn how the interactions of charged particles affect your everyday life. Ask Questions As you read, write questions you may have in the margin. Read the lesson a second time and try to answer the questions. When you are done, ask your teacher any questions you still have. 1. Identify Which particles in atoms have the property of electric charge? Reading Essentials Electricity 347

2 2. Compare the amounts of positive and negative charge in a neutral atom. 3. Explain How can objects become electrically charged? Key Concept Check 4. Describe How do electrically charged objects interact? Positive Charge and Negative Charge There are two types of electric charge positive charge and negative charge. Protons have positive charge, and electrons have negative charge. The amount of positive charge of one proton is equal to the amount of negative charge of one electron in an atom. Oppositely charged particles attract each other. Similarly charged particles repel each other. So, a positively charged proton and a negatively charged electron attract. Two protons, or two electrons, repel, or push away. An electrically neutral atom has equal amounts of positive charge and negative charge. This is because an atom has equal numbers of protons and electrons. Electrons can move from one atom to another. When an atom gains one or more electrons, it is negatively charged. If an atom loses one or more electrons, it is positively charged. Electrically charged atoms are called ions. Any object can become electrically charged. Neutral Objects Larger electrically neutral objects have equal amounts of positive and negative charge, similar to neutral atoms. Electrically neutral objects do not attract or repel each other. Charged Objects Atoms and larger objects can gain or lose electrons. Some materials hold electrons more loosely than other materials. This makes it possible for electrons to move from one object to another. When electrons move, an unbalanced charge forms on the objects. An unbalanced negative or positive electric charge on an object is sometimes referred to as a static charge. Like an atom, an object that gains electrons has more negative charge than positive charge. It is said to be negatively charged. An object that loses electrons has more positive charge than negative charge. It is positively charged. Electric Forces The region surrounding a charged object is called an electric field. An electric field applies a force, called an electric force, to other charged objects, even when the objects are not touching. The electric force applied by an object s electric field will either attract or repel other charged objects. The figure at the top of the next page shows that objects with opposite electric charges attract each other. It also shows that objects with the same type of charge repel each other. 348 Electricity Reading Essentials

3 Charged Objects Positively and negatively charged objects attract each other. Two negatively charged objects repel each other. Two positively charged objects repel each other. Electric Force and Amount of Charge The strength of the electric force between two charged objects depends on two variables the total amount of electric charge on both objects and the distance between the objects. For example, when you brush your hair, electrons move from the brush to your hair. This causes the brush and your hair to each have a static charge. The brush is positively charged; your hair is negatively charged. Because the brush and your hair have opposite electric charges, they attract each other object. The left side of the figure below shows how the amount of charge affects electric force. Brushing your hair once or twice transfers some electrons from the brush to your hair. The force of attraction is not very strong. However, more brushing transfers more electrons from the bush to your hair. This increases the strength of the electric force of attraction between the brush and your hair. Electric Force and Distance Distance also affects the strength of the electric force between electric charges, as shown on the right side of the figure below. Brushing your hair produces an unbalanced positive charge on the brush and an unbalanced negative charge on your hair. Electric fields surround these electric charges. The fields are more intense near the charges. The strength of an electric field increases as the total amount of charge of the two objects increases. Electric Field Visual Check 5. Select Circle the diagram that illustrates objects that attract each other. Visual Check 6. State What happens to the strength of an electric field when the distance between two objects decreases? The strength of an electric field increases as the distance between the two objects decreases. Distance remains constant. Amount of charge remains constant. Reading Essentials Electricity 349

4 Make a vertical three-tab book to organize your notes about electrically charged objects. Positively Charged Objects Neutral Objects Negatively Charged Objects 7. Identify What materials are good conductors? 8. Differentiate How do electric insulators and electric conductors differ? Distance Between Charged Objects A more intense electric field applies a stronger electric force to other objects. Therefore, when the brush is close to your hair, the force of attraction is very noticeable. The electric force is weaker when the charged objects are far from each other. As the distance between two charged objects increases, the electric force between them decreases. Transferring Electrons You read that a hairbrush and strands of your hair become electrically charged when electrons transfer from the brush to your hair. How do electrons move from one object to another object? Insulators and Conductors To understand how electrons move from one object to another, you need to know about two basic types of materials electric insulators and electric conductors. A material in which electrons cannot easily move is an electric insulator. Glass, rubber, wood, and air are good electric insulators. A material in which electrons can easily move is an electric conductor. Most metals, including copper and aluminum, are good electric conductors. Electric insulators and electric conductors are all around you. Electric conductors and electric insulators are used in electrical power cords. For example, copper wire is the conductor of electrons in an extension cord. Electrons easily move through the copper wire. Plastic and rubber are used as protective electric insulators around the metal wire. Electrons cannot easily move in the plastic and rubber. Electrons transfer between objects by contact, induction, or conduction. Electric insulators and conductors play an important role in these processes. Transferring Charge by Contact Have you ever rubbed an inflated balloon on a wool sweater to make the balloon stick to the sweater? Why does rubbing the balloon make it stick to the sweater? Transferring Charge Recall that some materials hold their electrons more loosely than other materials. When objects made of different materials touch, electrons tend to collect on the object that holds electrons more tightly. This is called transferring charge by contact. 350 Electricity Reading Essentials

5 Example A wool sweater holds electrons more loosely than a rubber balloon. When the balloon comes in contact with the sweater, electrons from the surface of the sweater transfer to the surface of the balloon. This creates a static charge on both objects. Because the balloon gained electrons, it has an unbalanced negative charge. Because the sweater lost electrons, it has an unbalanced positive charge. Opposite charges attract, and the balloon and sweater stick to each other. Both insulators and conductors can be changed by contact. Transferring Charge by Induction Transferring charge by induction is a process by which one object causes two other objects that are conductors to become charged without touching them. The figure below illustrates this process. Part 1 of the figure shows a negatively charged balloon repelling electrons in a metal soda can. Because the can is a conductor, electrons in the can easily move toward the far end of the can. The can is not charged because it has not gained or lost any electrons. The can is polarized. When electrons concentrate at one end of an object, the object is polarized. In part 2, when a charged balloon is brought near two cans that are touching, the balloon polarizes the cans as if they a re one object. Electrons in both cans move toward the far end of the can on the right. Then, the two cans separate. As shown in part 3, the cans that were originally polarized as a group are now individually charged. The can on the right has an unbalanced negative electric charge, and the can on the left has an unbalanced positive electric charge Conductors 9. Explain Why does the balloon have an unbalanced negative charge? 10. State What kind of objects can be charged by induction? Visual Check 11. Analyze Why do negative charges in the aluminum cans tend to move away from the balloon? Reading Essentials Electricity 351

6 Conduction 1 One container is full of water. The other is empty. How can you move water to the empty container? 2 Connect the containers together with the pipe. The pipe becomes a conductor of water. Water flows into the empty container. One can is electrically charged. The other is electrically neutral. How can you move electric charge to the neutral can? Touch the metal cans together with the metal wire. The metal wire becomes a conductor of electric charge. Electric charge flows into the electrically neutral can. 3 The water stops flowing when the water is the same depth in both containers. 4 Disconnect the pipe. Both containers now hold the same amount of water. Water was moved by conduction. The electric charge stops flowing when the electric charge is spread evenly over both cans. Disconnect the metal wire. Both cans now hold the same amount of charge. Electric charge was transferred by conduction. Visual Check 12. Apply What object is used in the diagram as the conductor of electric charge? Key Concept Check 13. Describe What are three ways by which objects can become electrically charged? Transferring Charge by Conduction Another way that electrons transfer between two conductors is called transferring charge by conduction. As shown in the figure above, when conducting objects with unequal charges, such as aluminum soda cans, touch, electrons flow from the object with a greater concentration of negative charge to the object with a lower concentration of negative charge. This is similar to water flowing from a container with a higher water level to a container with a lower water level. The flow of electrons continues until the concentration of charge on both objects is equal. Electric Discharge When you brush your hair, electrons transfer from the brush to your hair. What happens to these excess electrons in your hair? They transfer to objects that come in contact with your hair, such as a hat or your pillow. Electrons even transfer to the air. Gradually, your hair loses the excess electrons and its charge. The process of an unbalanced electric charge becoming balanced is an electric discharge. 352 Electricity Reading Essentials

7 Lightning Rods and Grounding An electric discharge can occur slowly, such as when your hair loses its negative charge and is no longer attracted to a brush. Or, an electric discharge can occur as quickly as a flash. For example, lightning is a powerful electric discharge that occurs in an instant. A lightning strike can severely damage a building or injure people. Lightning rods help protect people against these dangers. Buildings often have metal lightning rods attached to the roof. Thick wires connect the lightning rods to the ground. The wire from the lightning rod provides a path for the electrons released in a lightning strike to travel into the ground. Providing a path for electric charges to flow safely into the ground is called grounding. The table below provides tips on how to protect yourself from a lightning strike. Lightning Safety Tips If the time between a lightning flash and thunder is less than 30 seconds, the storm is dangerously close. Protect yourself in the following ways: Seek shelter in an enclosed building or a car with a metal top. Never stand under a tree. Do not touch metal surfaces. Get away from water if swimming, boating, or bathing. Wait 30 minutes after the last flash of lightning before leaving the shelter, even if the Sun comes out. 14. Identify What occurs when electrons move from a cloud to the ground? Interpreting Tables 15. Recognize How can you tell that a storm is close? What causes lightning? Scientists do not fully understand the causes of lightning. However, lightning appears to be related to the electric charges that separate within storm clouds. The large amounts of ice, hail, and partially frozen water droplets that thunderstorms create seem to play a role, too. Forecasting exactly where and when lightning will strike might never be possible. Accumulation of Charge Within a storm cloud, warm air rises past falling cold air. The cold air is filled with hail, ice, and partially frozen water droplets that pick up electrons from the rising air. This causes the bottom of the cloud to become negatively charged. Key Concept Check 16. Explain What is an electric discharge? Reading Essentials Electricity 353

8 17. State How does the negatively charged cloud affect Earth s surface before an electric discharge? Electric Discharge The negatively charged cloud polarizes Earth s surface by repelling negative charges in the ground. Thus, the surface of the ground becomes positively charged. When the bottom of the cloud accumulates enough negative charge, the attraction of the positively charged ground causes electrons in the cloud to begin moving toward the ground. Finally, when the electrons get close to the ground, positive charges quickly flow upward, making an electric connection between the cloud and the ground. You see this electric discharge as lightning. Accumulation of charge does not occur only between clouds and the ground. Charges also separate within or between storm clouds, causing cloud-to-cloud lightning. 354 Electricity Reading Essentials

9 Mini Glossary electric c onductor: a material through which electrons can easily move electric discharge: the process of an unbalanced electric charge becoming balanced grounding: providing a path for electric charges to flow safely into the ground electric insulator: a material through which electrons cannot easily move polarized: when electrons concentrate at one end of an object static charge: an unbalanced negative or positive electric charge on an object 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes the importance of grounding. 2. Circle the word next to each pair of charged items that describes how they will react. a. attract repel b. - attract repel c. - - attract repel 3. Describe the process of transferring charge by induction. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials Electricity 355