Electric Charges
Electricity and Magnetism Electric Charges and Electric Forces Key Concepts 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
• How do electrically charged objects differ? • How do objects become electrically charged? • How do electrically charged objects interact?
1. Electrically charged objects always attract each
other. 2. Electric fields apply magnetic forces on other
electric fields.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Electric Charges Have you ever walked across a carpeted floor, reached for a metal doorknob, and received a small shock? The shock comes from electric charges jumping between your fingers and the doorknob. What are electric charges? Where do they come from? Why do they jump from one object to another? In this lesson, you will learn the answers to these questions. Atoms are the tiny particles that make up all the matter around you. An atom has a nucleus made up of two kinds of smaller particles. These particles are protons and neutrons.
ACADEMIC VOCABULARY nucleus (noun) basic or essential part; core
Atom
An atom also is made up of electrons. Electrons move around the atom’s nucleus, as shown in the figure to the right. Protons and electrons have a property called electric charge. Neutrons do not have electric charge.
Neutron
Visual Check
1. Identify Circle the electrons.
Proton Reading Essentials
Building Vocabulary As you read, underline the words and phrases that you do not understand. When you finish reading, discuss these words and phrases with another student or your teacher.
Electron Electricity and Magnetism
241
Positive and Negative Charge There are two types of electric charge—positive and negative. Here, positive and negative do not mean more and less. The terms are simply names scientists use to talk about the two types of electric charge. Protons have positive charge. Electrons have negative charge. The amount of positive charge of a proton equals the amount of negative charge of an electron.
Reading Check
2. Explain Why are atoms electrically neutral?
Reading Check
Atoms have equal numbers of positive protons and negative electrons. A particle with equal amounts of positive charge and negative charge is electrically neutral. Electrically neutral atoms make up all objects. Therefore, objects are normally electrically neutral, too. However, electrons sometimes transfer between objects. How does transferring electrons affect objects? When electrons move from one electrically neutral object to another, both objects become electrically charged. An object is electrically charged when it has an unbalanced amount of positive charge or negative charge. Objects can be either positively charged or negatively charged.
Positively Charged An object that has lost one or more
when electrons move from one electrically neutral object to another?
electrons as it comes in contact with another object has an unbalanced electric charge. An object that has lost electrons has more protons than electrons. Thus, the object has more positive charge than negative charge. The object is positively charged.
Negatively Charged An object that gains one or more
Key Concept Check
4. Differentiate How do electrically charged objects differ?
electrons also has an unbalanced electric charge. An object that has gained electrons has more electrons than protons. Thus, the object has more negative charge than positive charge. The object is negatively charged.
Materials and Electric Charge How do electrically neutral objects become electrically charged? For example, how can a balloon and a stuffed toy become electrically charged? At first, the balloon and the toy are both electrically neutral. You rub the balloon against a wool-covered toy. Wool does not hold electrons as tightly as rubber. Electrons transfer from the toy to the balloon when they are in contact. Whether an object becomes positively charged or negatively charged depends on the material it contacts. Some materials hold electrons more tightly than others.
242
Electricity and Magnetism
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
3. State What happens
Electron Movement in Common Items Becomes Positive Glass Nylon Wool Silk Aluminum Paper Cotton
Interpreting Tables 5. Interpret Is it easier for electrons to move from aluminum or paper?
Wood Rubber Copper Polyester Polystyrene Polyvinyl chloride Becomes Negative
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
The table above lists common materials in order of how tightly they hold electrons. Notice glass is above wool in the table. This means that if you rub a glass cup against a wool toy, electrons transfer from the glass onto the wool. The glass becomes positively charged, and the wool becomes negatively charged.
Key Concept Check
6. Explain How do the balloon and the stuffed toy become charged?
Electric Discharge You read that objects can become electrically charged. However, an electrically charged object tends to lose its unbalanced charge after a period of time. The loss of an unbalanced electric charge is an electric discharge. Some electric discharges happen slowly. For example, electrons on negatively charged objects discharge, or move, from the object onto water molecules in the air. You may have noticed that the static cling of electrically charged clothing lasts longer on dry days than on humid days when there is more water vapor in the air.
Reading Check
7. Define What is an electric discharge?
Some electric discharges happen quickly. For example, lightning is the sudden loss of unbalanced electric charges that build up in thunderstorm clouds. Another example is the continual electric discharge through a fluorescent light that causes a powder inside the tube to glow brightly.
Reading Essentials
Electricity and Magnetism
243
Electric Insulators and Conductors Make a horizontal three-tab book with an extended tab. Use it to organize your notes on the relationships between electric forces. (–) (–)
(+) (+)
(+) (–)
Electric Forces
8. Apply Name one other material that might be a good electric insulator.
Objects of different materials become electrically charged as they come in contact. The charge can remain where objects touch, or the charge can spread over the entire object. Think of a balloon rubbing against a sweater. Charges from the sweater stay in the area of the balloon that touched the sweater. However, as you walk across a carpet, charges from the carpet spread over your entire body. Your hand receives an electric shock as you reach for a metal doorknob. Electric charges do not spread over the balloon because electrons cannot easily move in rubber. A material in which electric charges cannot easily move is an electric insulator. Plastic, wood, and glass also are electric insulators. Materials in which electric charges easily move are electric conductors. Some of the best electric conductors are metals such as iron, copper, gold, and aluminum.
Electric Fields and Electric Forces
Electric fields surround charged objects. To open a door, your hand must touch the door to apply a force to it. However, an electrically charged object does not have to touch another charged object to apply an electric force to it. For example, the two charged balloons in the example above repel each other even though they do not touch.
Reading Check
9. Explain What is an electric field?
244
Electricity and Magnetism
How do charged objects apply electric forces to each other without touching? The answer is a bit of a mystery. However, scientists know there is a region around a charged object that applies an electric force to other charged objects. This invisible region around any charged object where an electric force is applied is an electric field.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Suppose you rub two balloons on a wool sweater. Electrons transfer from the sweater to the balloons. Both balloons become negatively charged. The sweater becomes positively charged. You notice that the sweater attracts, or applies a pulling force on, the balloons. However, the balloons repel, or apply a pushing force to, each other. The force that two electrically charged objects apply to each other is an electric force.
Charged Objects
– + Electrically neutral, no force
Opposite charges attract.
–
–
+
Like negative charges repel.
Electric force depends on the types of charge. You read that an electric force can be a push or a pull. Whether the force is a push or a pull depends on the types of charge on the objects, as shown in the figure above. If both objects are positively charged or if both objects are negatively charged, the two objects push each other away. In other words, objects with the same type of charge repel each other. If one object is positively charged and the other object is negatively charged, the two objects pull each other together. In other words, objects with opposite types of electric charge attract each other.
+
Like positive charges repel.
Visual Check
10. State why the negative and positive balloons attract each other.
Key Concept Check
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
11. Summarize How do electrically charged objects interact?
Reading Essentials
Electricity and Magnetism
245
Mini Glossary electrically charged: when an object has an unbalanced
electric field: the invisible region around any charged object
amount of positive charge or negative charge
where an electric force is applied
electrically neutral: when a particle has equal amounts of
electric force: the force that two electrically charged objects
positive charge and negative charge
apply to each other
electric conductor: a material in which electric charges
electric insulator: a material in which electric charges
easily move
cannot easily move
electric discharge: the loss of an unbalanced electric charge 1. Review the terms and their definitions in the Mini Glossary. Write a sentence explaining the difference between an electric conductor and an electric insulator.
2. On the first line in the ovals, identify the three particles that make up an atom. On the second line, indicate whether the particle has a positive, negative, or neutral electric charge.
Particles of an atom
charged? Which will become negatively charged?
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?
246
Electricity and Magnetism
Connect ED
Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.
END OF LESSON
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
3. Suppose you rub a nylon scarf against a polyester scarf. Which will become positively
Statement
After
• How do electrically charged objects differ? • How do objects become electrically charged? • How do electrically charged objects interact?
1. Electrically charged objects always attract each
other. 2. Electric fields apply magnetic forces on other
electric fields.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Electric Charges Have you ever walked across a carpeted floor, reached for a metal doorknob, and received a small shock? The shock comes from electric charges jumping between your fingers and the doorknob. What are electric charges? Where do they come from? Why do they jump from one object to another? In this lesson, you will learn the answers to these questions. Atoms are the tiny particles that make up all the matter around you. An atom has a nucleus made up of two kinds of smaller particles. These particles are protons and neutrons.
ACADEMIC VOCABULARY nucleus (noun) basic or essential part; core
Atom
An atom also is made up of electrons. Electrons move around the atom’s nucleus, as shown in the figure to the right. Protons and electrons have a property called electric charge. Neutrons do not have electric charge.
Neutron
Visual Check
1. Identify Circle the electrons.
Proton Reading Essentials
Building Vocabulary As you read, underline the words and phrases that you do not understand. When you finish reading, discuss these words and phrases with another student or your teacher.
Electron Electricity and Magnetism
241
Positive and Negative Charge There are two types of electric charge—positive and negative. Here, positive and negative do not mean more and less. The terms are simply names scientists use to talk about the two types of electric charge. Protons have positive charge. Electrons have negative charge. The amount of positive charge of a proton equals the amount of negative charge of an electron.
Reading Check
2. Explain Why are atoms electrically neutral?
Reading Check
Atoms have equal numbers of positive protons and negative electrons. A particle with equal amounts of positive charge and negative charge is electrically neutral. Electrically neutral atoms make up all objects. Therefore, objects are normally electrically neutral, too. However, electrons sometimes transfer between objects. How does transferring electrons affect objects? When electrons move from one electrically neutral object to another, both objects become electrically charged. An object is electrically charged when it has an unbalanced amount of positive charge or negative charge. Objects can be either positively charged or negatively charged.
Positively Charged An object that has lost one or more
when electrons move from one electrically neutral object to another?
electrons as it comes in contact with another object has an unbalanced electric charge. An object that has lost electrons has more protons than electrons. Thus, the object has more positive charge than negative charge. The object is positively charged.
Negatively Charged An object that gains one or more
Key Concept Check
4. Differentiate How do electrically charged objects differ?
electrons also has an unbalanced electric charge. An object that has gained electrons has more electrons than protons. Thus, the object has more negative charge than positive charge. The object is negatively charged.
Materials and Electric Charge How do electrically neutral objects become electrically charged? For example, how can a balloon and a stuffed toy become electrically charged? At first, the balloon and the toy are both electrically neutral. You rub the balloon against a wool-covered toy. Wool does not hold electrons as tightly as rubber. Electrons transfer from the toy to the balloon when they are in contact. Whether an object becomes positively charged or negatively charged depends on the material it contacts. Some materials hold electrons more tightly than others.
242
Electricity and Magnetism
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
3. State What happens
Electron Movement in Common Items Becomes Positive Glass Nylon Wool Silk Aluminum Paper Cotton
Interpreting Tables 5. Interpret Is it easier for electrons to move from aluminum or paper?
Wood Rubber Copper Polyester Polystyrene Polyvinyl chloride Becomes Negative
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
The table above lists common materials in order of how tightly they hold electrons. Notice glass is above wool in the table. This means that if you rub a glass cup against a wool toy, electrons transfer from the glass onto the wool. The glass becomes positively charged, and the wool becomes negatively charged.
Key Concept Check
6. Explain How do the balloon and the stuffed toy become charged?
Electric Discharge You read that objects can become electrically charged. However, an electrically charged object tends to lose its unbalanced charge after a period of time. The loss of an unbalanced electric charge is an electric discharge. Some electric discharges happen slowly. For example, electrons on negatively charged objects discharge, or move, from the object onto water molecules in the air. You may have noticed that the static cling of electrically charged clothing lasts longer on dry days than on humid days when there is more water vapor in the air.
Reading Check
7. Define What is an electric discharge?
Some electric discharges happen quickly. For example, lightning is the sudden loss of unbalanced electric charges that build up in thunderstorm clouds. Another example is the continual electric discharge through a fluorescent light that causes a powder inside the tube to glow brightly.
Reading Essentials
Electricity and Magnetism
243
Electric Insulators and Conductors Make a horizontal three-tab book with an extended tab. Use it to organize your notes on the relationships between electric forces. (–) (–)
(+) (+)
(+) (–)
Electric Forces
8. Apply Name one other material that might be a good electric insulator.
Objects of different materials become electrically charged as they come in contact. The charge can remain where objects touch, or the charge can spread over the entire object. Think of a balloon rubbing against a sweater. Charges from the sweater stay in the area of the balloon that touched the sweater. However, as you walk across a carpet, charges from the carpet spread over your entire body. Your hand receives an electric shock as you reach for a metal doorknob. Electric charges do not spread over the balloon because electrons cannot easily move in rubber. A material in which electric charges cannot easily move is an electric insulator. Plastic, wood, and glass also are electric insulators. Materials in which electric charges easily move are electric conductors. Some of the best electric conductors are metals such as iron, copper, gold, and aluminum.
Electric Fields and Electric Forces
Electric fields surround charged objects. To open a door, your hand must touch the door to apply a force to it. However, an electrically charged object does not have to touch another charged object to apply an electric force to it. For example, the two charged balloons in the example above repel each other even though they do not touch.
Reading Check
9. Explain What is an electric field?
244
Electricity and Magnetism
How do charged objects apply electric forces to each other without touching? The answer is a bit of a mystery. However, scientists know there is a region around a charged object that applies an electric force to other charged objects. This invisible region around any charged object where an electric force is applied is an electric field.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Suppose you rub two balloons on a wool sweater. Electrons transfer from the sweater to the balloons. Both balloons become negatively charged. The sweater becomes positively charged. You notice that the sweater attracts, or applies a pulling force on, the balloons. However, the balloons repel, or apply a pushing force to, each other. The force that two electrically charged objects apply to each other is an electric force.
Charged Objects
– + Electrically neutral, no force
Opposite charges attract.
–
–
+
Like negative charges repel.
Electric force depends on the types of charge. You read that an electric force can be a push or a pull. Whether the force is a push or a pull depends on the types of charge on the objects, as shown in the figure above. If both objects are positively charged or if both objects are negatively charged, the two objects push each other away. In other words, objects with the same type of charge repel each other. If one object is positively charged and the other object is negatively charged, the two objects pull each other together. In other words, objects with opposite types of electric charge attract each other.
+
Like positive charges repel.
Visual Check
10. State why the negative and positive balloons attract each other.
Key Concept Check
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
11. Summarize How do electrically charged objects interact?
Reading Essentials
Electricity and Magnetism
245
Mini Glossary electrically charged: when an object has an unbalanced
electric field: the invisible region around any charged object
amount of positive charge or negative charge
where an electric force is applied
electrically neutral: when a particle has equal amounts of
electric force: the force that two electrically charged objects
positive charge and negative charge
apply to each other
electric conductor: a material in which electric charges
electric insulator: a material in which electric charges
easily move
cannot easily move
electric discharge: the loss of an unbalanced electric charge 1. Review the terms and their definitions in the Mini Glossary. Write a sentence explaining the difference between an electric conductor and an electric insulator.
2. On the first line in the ovals, identify the three particles that make up an atom. On the second line, indicate whether the particle has a positive, negative, or neutral electric charge.
Particles of an atom
charged? Which will become negatively charged?
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?
246
Electricity and Magnetism
Connect ED
Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.
END OF LESSON
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
3. Suppose you rub a nylon scarf against a polyester scarf. Which will become positively
