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.
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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.
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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?
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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
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LESSON
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3. Suppose you rub a nylon scarf against a polyester scarf. Which will become positively