chapter
5
Thermal Energy
2
section ●
Conduction, Convection, and Radiation
What You’ll Learn
the three ways heat is
transferred
■ the difference
between insulators
and conductors
■ how insulators control
the transfer of thermal
energy
■
Before You Read
There is an old joke that says a sweater is something you put
on when your mother is cold. On the lines below, write about
a time when you felt warm, but someone else felt cold.
Read to Learn
you read the section, highlight
the definition of each word that
appears in bold.
B Compare and Contrast
●
Make the Foldable shown below
to help you understand how
conduction, convection, and
radiation are similar and different.
Conduction
Convection
Radiation
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CHAPTER 5 Thermal Energy
Conduction
The three ways that thermal energy can be transferred from
place to place are conduction, convection, and radiation.
In conduction, heat is transferred by direct contact.
Conduction occurs when the particles within a substance or
between two substances bump into each other and transfer
kinetic energy. Conduction occurs because the particles in
matter are in constant motion.
How does conduction transfer thermal energy?
The figure shows how thermal energy
is transferred when one end of a metal
spoon is heated by a flame. The kinetic
energy of the particles near the flame
increases. The heated particles bump
into surrounding particles. In these
collisions, thermal energy is transferred from particles with more kinetic
energy to particles with less kinetic
energy. The collisions continue as thermal energy is transferred
to the other end of the spoon. When heat is transferred by conduction, thermal energy is transferred from place to place without transferring matter. Thermal energy is transferred by the
collisions between particles, not by movement of matter.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Identify Definitions As
What are thermal conductors?
Energy can be transferred by conduction in all materials.
But energy moves more quickly in some materials than in
others. Conduction is faster in solids and liquids than in gases.
In gases, particles are farther apart. Collisions with other
particles happen less often in gases than in solids or liquids.
The best thermal conductors are metals. Remember that an
atom has a nucleus surrounded by electrons. In metals, there
are electrons that are not bound to individual atoms. They
move easily through the metal and bump into other electrons.
This helps the electrons transfer thermal energy. Silver, copper,
and aluminum are some of the best energy conductors.
Convection
Thermal energy can also be transferred by a method called
convection. Convection is the transfer of thermal energy in a
fluid by the movement of warmer and cooler fluids from one
place to another. A fluid is a substance that can flow. It can be a
liquid or a gas. The movement of fluids from one place to
another causes currents. These currents transfer thermal energy.
●
C Compare Make the
following Foldables to compare
conductors and insulators.
Conductors:
Insulators:
1. Explain why metals are
good conductors.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How is heat transferred by convection currents?
When a pot of water is heated on a stove, heat is transferred
by convection currents. First, the water molecules at the bottom of the pot gain thermal energy from the stove. The water
becomes less dense as it is heated. The water at the top of the
pot is still cool, so it is more dense than the warm water. Since
the warm water is less dense, it rises. As it rises, it is replaced
at the bottom of the pot by the sinking, cooler water. The
cycle continues until all the water in the pot is at the same
temperature. This rising-and-sinking action is a convection
current. Also, as the warmer water rises, it heats some of the
cooler water that it comes in contact with by conduction. In a
convection current, both conduction and convection transfer
thermal energy.
2. Explain A convection
oven has a fan to move
the air inside it. Is the air
in a regular oven also
affected by convection?
Why or why not?
How do convection and conduction differ?
Both convection and conduction transfer heat. However,
convection is different from conduction. Convection transfers
thermal energy by moving particles from one place to
another. Warm particles change places with cooler particles.
In conduction, thermal energy is transferred by particles
bumping into surrounding particles. But, in conduction,
no particles actually move from place to place as they do in
convection.
Reading Essentials
79
Picture This
How does heat travel
through space? There is
Sun
almost no matter
between the Sun and
Earth. So heat does not
Outer
space
travel by conduction or
convection. Instead, heat
is transferred from the
Sun to Earth by radiation.
Radiation is the transfer of energy by electroReflected by
atmosphere
magnetic waves. They
carry energy through
Atmosphere
Reflected
solids, liquids, and gases.
Absorbed by
by surface
But they can also carry
atmosphere
energy through empty
space. Energy transferred
Absorbed by
by radiation is often
Earth
called radiant energy.
When you sit by a fireplace, you feel warm because heat is transferred by radiation
from the fire to your skin.
Radiation
3. Make a Drawing In the
first box, draw an icecovered mountain. Then
draw arrows to show how
radiation is reflected and
absorbed by an ice-covered
mountain range. In the
second box, draw trees to
represent a rain forest.
Then draw arrows to show
how radiation is reflected
and absorbed by a dark
rain forest.
Radiation
When radiation strikes a material, three things can happen.
The material absorbs some of the energy. It also reflects some
of the energy. And some of the energy may be transmitted, or
passed through, the material. The amount of energy absorbed,
reflected, and transmitted depends on the type of material.
Light-colored materials reflect more radiant energy, while
dark-colored materials absorb more radiant energy. When material absorbs radiant energy, the thermal energy of the material
increases. The figure above shows what happens to radiant
energy from the Sun as it reaches Earth.
How is radiation different in solids, liquids,
and gases?
Radiant energy can travel through the space between particles
in a solid, liquid, or gas. Particles can absorb the radiation and
emit some of the energy they absorbed. This energy then travels
through the space between particles. It is absorbed and emitted
by other particles. The transfer of energy by radiation works
best with gases. The particles in gases are much farther apart
than in solids or liquids. This extra space allows the radiant
energy to travel more easily through gases.
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CHAPTER 5 Thermal Energy
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How do radiant energy and matter interact?
Controlling Heat Flow
You probably do things every day to control the flow of heat.
For example, you put on a jacket when you leave your house on
a cool morning. And you might wear an oven mitt on your
hand when you reach into the oven to pull out a hot pan. In
both cases, you use different materials to help control the flow of
heat. Your jacket reduces the flow of heat from your body to the
surrounding air. The oven mitt reduces the flow of heat from
the hot dish to your hand.
Animal Adaptations Almost all living things have special
features that help them control the flow of heat. The antarctic
fur seal has a coat that is about 10 cm thick. This coat helps
keep the seal from losing heat. The emperor penguin has a
thick layer of blubber and thick, closely spaced feathers. This
helps the penguin reduce the loss of body heat. These adaptations help the animals survive in a climate where the temperature is often below freezing. The scaly skin of the desert spiny
lizard has the opposite effect. The lizard’s skin reflects the Sun’s
rays and keeps the lizard from becoming too hot. The leathery
skin also prevents water loss. An animal’s color also can also
help it keep warm or cool. The black feathers on a penguin’s
back allow it to absorb radiant energy.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Thermal Insulators
When you cook, you want the pan to conduct heat easily
from the stove to your food. But you don’t want the heat to
move easily to the handle of the pan. Most pans have handles
that are made from insulators. A thermal insulator is a material
in which heat flows slowly. Materials that are insulators include
wood, some plastics, fiberglass, and air. An insulator is the opposite of a conductor. Remember, conductors are materials
through which heat flows quickly. Metals like silver, copper, and
aluminum are excellent conductors. Materials that are good
insulators are poor conductors of heat. That is why many pan
handles are made of wood or plastic.
Air as an Insulator Gases, such as air, are usually better
insulators than solids or liquids. Some types of insulators
contain many pockets of trapped air. These air pockets conduct heat poorly and keep convection currents from forming.
When you wear a fleece jacket, the fibers in the fleece trap air
between the jacket and your body. This air slows down the
flow of your body heat to the colder air outside the jacket.
Your body heat warms the air trapped by the fleece and you
are wrapped in a blanket of warm air.
4. Describe How do animals’
colors help them control
their temperature?
5. Draw Conclusions Some of
the best cooking pans are
made of copper. Why do
you think this is?
Reading Essentials
81
How are buildings insulated?
6. Identify What does
building material insulation contain?
Materials that are insulators help keep warm air from flowing out of buildings in cold weather. They also keep warm air
from flowing into buildings in warm weather. These materials
are called insulation. Building insulation is usually made of
materials, such as fiberglass or foam, which contain pockets
of trapped air. Insulation is placed between a building’s outer
walls and inner walls and between the ceiling and the attic.
It reduces the flow of heat between the building and the
surrounding air.
Insulation also helps furnaces and air conditioners work
more effectively. Since the heat doesn’t escape or enter the
building as easily, the furnaces and air conditioners do not
have to work as much. This can save a great deal of energy.
In the United States, about 50 percent of the energy used in
homes is used for heating and cooling.
Picture This
7. Explain How does the shiny
surface of a thermos bottle
prevent heat transfer?
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CHAPTER 5 Thermal Energy
Have you used a thermos
bottle like the one in the figure?
A thermos bottle lets you keep
soup hot or iced tea cold. It does
this by reducing the flow of
Outer
energy into and out of the liquid
case
Reflective
in the bottle. The temperature of
surface
the liquid hardly changes over a Vacuum
number of hours. A thermos
bottle has two glass walls. The
air between the two walls is
removed so there is a vacuum between the glass layers. The
vacuum contains little matter. This vacuum prevents heat
transfer by conduction or convection between the liquid and
the air outside the thermos bottle.
The inside and outside glass surface of a thermos bottle is
coated with aluminum. The coating makes each surface
highly reflective. Electromagnetic waves are reflected at each
surface. The inner reflective surface prevents radiation from
transferring heat out of the liquid. The outer reflective surface
prevents radiation from transferring heat into the liquid.
Think about the things you do to stay warm or cool. Sitting
in the shade reduces the heat transferred to you by radiation.
Opening or closing windows increases or reduces heat transferred by convection. Putting on a jacket reduces the heat
transferred from your body by conduction. In what other
ways do you control the flow of heat?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How does a thermos work?
After You Read
Mini Glossary
conduction: the transfer of thermal energy by collisions
between particles in matter
convection: when thermal energy is transferred in a fluid by the
movement of warmer and cooler fluid from place to place
radiation: the transfer of energy by electromagnetic waves
thermal insulator: a material in which thermal energy
flows slowly
1. Review the terms and their definitions in the Mini Glossary. Write a sentence on the lines
below that shows your understanding of the term thermal insulator.
2. Complete the table to organize information about how heat is transferred.
How Heat Is Transferred
Definition
Example
Conduction
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Convection
Radiation
3.
Think about what you have learned. How did identifying definitions help
you as you read the section?
Chapter 6
End of
Section
Reading Essentials
83