Download 2 Energy Transfer

Name
CHAPTER 14
Class
Date
Heat and Temperature
SECTION
2 Energy Transfer
KEY IDEAS
As you read this section, keep these questions in mind:
• What are three kinds of energy transfer?
• What are conductors and insulators?
• What makes something a good conductor of heat?
How Can Energy Be Transferred?
In the morning, you might turn on the shower and wait
for the water to warm up. Outside your home, the dew on
the grass evaporates when sunlight hits it. In your freezer,
the water in ice trays becomes solid after the water cools
to 0 ºC. None of these things could happen without energy
transfer. There are three different ways that energy can be
transferred between objects: conduction, convection, and
radiation.
The figure below shows an example of a situation in
which all three kinds of energy transfer are occurring.
Heat moves through the metal wire in the girl’s hand by
conduction. Convection in the air carries heat away from
the fire. The light from the flames moves by radiation.
Energy moves
through the wire by
conduction.
READING TOOLBOX
Compare As you read this
section, make a table
comparing convection,
conduction, and radiation. In
the table, define each term
and give examples of it.
READING CHECK
1. Identify What are the
three ways that energy can
be transferred?
Heated air carries
energy away from the
fire during convection.
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2. Identify How is visible
light transferred from the
fire?
The fire gives off visible
light through radiation.
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Energy Transfer continued
ENERGY TRANSFER BY CONDUCTION
READING CHECK
3. Describe How is energy
transferred during thermal
conduction?
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4. Apply Concepts Could
conduction happen between
two objects that were not
touching? Explain your
answer.
One way that energy transfers between objects is
by conduction. Thermal conduction is the transfer of
energy through the collisions of particles. Heat can move
by conduction between two objects that are touching one
another. Conduction also happens within the particles of
a substance.
Imagine that you put a marshmallow on one end of a
metal coat hanger. Then, you hold the marshmallow over
a campfire flame. The wire absorbs heat from the fire
and warms up. Then, the end of the wire that you are
holding gets warmer. Energy moves from the wire to
your hand as heat. This energy transfer through the wire
and from the wire to your hand are examples of thermal
conduction.
Conduction happens when particles collide with
one another. For example, the wire nearest the flame
is warmer than the wire farther away. Therefore, the
particles in the wire near the flame are moving quickly.
They collide with other particles further from the flame.
Energy moves from the fast-moving particles to the
slower-moving particles. The slower particles begin to
move faster. In this way, heat moves from one end of the
wire to the other.
Before the wire is near a flame,
it is the same temperature
everywhere. Therefore, all the
particles in the wire have the
same average kinetic energy.
EHHDBG@<EHL>K
5. Infer In the bottom
image, which side of the wire
is hottest?
Heat flows from the fire into
the end of the wire. The
particles in the wire near the
fire move more quickly. They
collide with other particles in
the wire. This transfers energy
from the warmer parts of the
wire to the cooler parts.
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Energy Transfer continued
ENERGY TRANSFER BY CONVECTION
Above a campfire, hot air expands and moves upward,
carrying energy with it. This movement of energy through
the movement of matter is called convection.
Convection is possible only in fluids. Most fluids are
liquids or gases. During convection, the density of a portion of a fluid changes. In most cases, a fluid’s density
changes because its temperature changes. The denser
fluid sinks below the less dense fluid.
Convecting fluids often form convection currents. A
convection current forms when density differences
cause a fluid to flow along a circular path. Convection
currents can form in any fluid that is not the same density everywhere. An example of how convection currents
form in air is shown in the figure below.
6. Define What is
convection?
Cool
air
Cool
air
Convection
currents
READING CHECK
Warm
air
EHHDBG@<EHL>K
Convection
currents
7. Explain Why does the air
directly above the flame rise?
ENERGY TRANSFER BY RADIATION
If you stand next to a campfire, you may feel its
warmth. The fire gives off energy in the form of
electromagnetic waves. These include infrared radiation,
visible light, and ultraviolet rays. Radiation is energy
that is transferred as electromagnetic waves.
Radiation is different from conduction and convection
because it can transfer energy through a vacuum such as
outer space. Almost all of the energy that we get from the
sun is transferred by radiation.
READING CHECK
8. Compare How is
radiation different from
convection or conduction?
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Energy Transfer continued
What Are Conductors and Insulators?
READING CHECK
9. Describe Why does metal
get hot quickly when it is
placed near a heat source?
If you toast a marshmallow using a metal wire, the
wire gets hot quickly. If you used a wooden stick instead,
it would not heat up as fast. This is because metal is a
good conductor of heat, but wood is not. A conductor is
a type of material through which heat can move easily.
Most cooking pans are made of metal because most metals are good conductors.
Heat does not move through the wooden stick very
easily because wood is an insulator. An insulator is a
material that transfers energy poorly. Insulation is often
used in attics and walls of houses to keep heat from leaving the house. Insulation can also be part of clothing.
CHARACTERISTICS OF CONDUCTORS
READING CHECK
10. Identify What is the
main way that energy moves
through a substance?
Energy moves through a substance mainly by conduction. For energy to move through conduction, the
particles in the substance must collide with each other.
The closer the particles are to each other, the more likely
they are to collide. Therefore, materials with particles
that are close together are generally better conductors
than materials with particles that are far apart.
Remember that density is a measure of how closely
packed the particles in a substance are. The particles
in a dense material are more closely packed than those
in a lower-density material, as shown below. Metals tend
to be good conductors because most metals have high
densities.
Most gases are poor conductors. This is because the
particles in a gas are much farther apart than the
particles in a solid or liquid. The particles in a gas do
not collide with each other very often.
Dense material:
particles close together
Less-dense material:
particles far apart
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11. Infer Which of the two
materials in the figure is
most likely a gas? Explain
your answer.
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Energy Transfer continued
What Is Specific Heat?
If you place a metal spoon into a cup of hot tea, the
spoon will become warm. The tea will become cooler,
because the spoon absorbs heat from the tea. How much
will the temperatures of the tea and the spoon change?
That depends on three things:
• the mass of the tea and of the spoon
• the temperature of the tea and of the spoon
• the specific heat of the tea and of the spoon
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12. Apply Concepts If the
spoon was the same
temperature as the tea,
would the spoon heat up
when it was placed in the
tea? Explain your answer.
Specific heat (c) describes how much heat a substance must absorb in order for its temperature to change
by a certain amount. Specifically, it is the amount of heat
required to increase the temperature of 1 kg of the substance by 1 K. The higher a substance’s specific heat, the
more energy it must absorb before its temperature will
change.
The specific heat of a substance does not depend on
how much of the substance is present. For example,
100 kg of water has the same specific heat as 1 kg of
water: 4,186 J/kg•K. The tables below give the specific
heats of different substances at 25 °C.
Substance
Specific heat at
25 °C (J/kg•K)
Substance
Specific heat at
25 °C (J/kg•K)
Liquid water
4,186
Solid copper
385
Liquid
ethanol
2,440
Solid iron
449
Gaseous
ammonia
2,060
Solid silver
234
Gaseous
water vapor
1,870
Liquid
mercury
140
Solid
aluminum
897
Solid gold
129
Solid carbon
(graphite)
709
Solid lead
129
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13. Identify Which
substance would require
the most heat to produce
a temperature change:
liquid water or gaseous
water vapor?
Different substances have different specific heats. The
particles in some substances need to absorb only a small
amount of energy in order to move faster. The particles in
other substances must absorb much more energy before
they can move that fast. A substance with a high specific
heat must absorb more heat than a substance with a lower
specific heat before its particles begin to move faster.
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Energy Transfer continued
USING SPECIFIC HEAT
It takes 4,186 J of energy to raise the temperature of
1 kg of water by 1 K. If you had 2 kg of water, it would
take twice as much energy to raise the temperature by
1 K. It would also take twice as much energy to raise the
temperature of 1 kg of water by 2 K. The equation below
shows how to use specific heat to relate mass, temperature change, and energy:
Math Skills
14. Describe Relationships
How is mass related to the
amount of energy needed
to raise a substance’s
temperature?
energy = specific heat mass temperature change
energy = cm$T
In this equation, the “delta” symbol ($) represents
“change in.” To calculate the change in temperature of a
substance, use the following equation:
$T = final temperature initial temperature
$T = Tf Ti
Now, let’s look at an example of how to use specific
heat to solve problems involving heat and temperature.
How much energy is required to increase the temperature
of 200 kg of water from 25 ºC to 37 ºC?
Unknown:
Known:
mass of water, m = 200 kg energy
Step 1: List
the known and
unknown values.
initial temperature of
water, Ti = 25 ºC
final temperature of water,
Tf = 37 ºC
specific heat of water,
c = 4,186 J/kg•K
Math Skills
15. Calculate How much
energy is required to raise
the temperature of 755 g of
solid iron from 283 K to
403 K? Show your work.
(Hint: Use the information
in the table on the previous
page.)
Step 2: Write the
equations.
TK = TC + 273.16
energy = cm$T
$T = Tf Ti
Step 3: Insert
the known values
and solve for
the unknown
values. Remember
to convert all
measurements to
proper units.
Ti = 25 ºC + 273.16
Ti = 298.16 K
Tf = 37 ºC + 273.16
Tf = 310.16 K
$T = 12 K
energy = (4,186 J/kg•K) × (200 kg) × (12 K)
energy = 10,000,000 J = 1.0 × 104 kJ
So, it takes about 1.0 × 104 kJ of energy to increase the
water’s temperature.
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Energy Transfer continued
What Can Happen When a Substance
Absorbs Heat?
Imagine placing a pot of cold water on a hot stove
burner. The water absorbs heat from the burner. If you
put a thermometer in the water, you would see the
temperature of the water increase. When the water’s temperature reached 100 ºC, the water would start to boil.
Many people think that the temperature of the water continues to rise as it boils. However, this is not the case.
If you watched the thermometer in the boiling water,
you would see that the water’s temperature would not
rise. To understand why this is, you need to know what
can happen to a substance when it absorbs heat.
Heat moving into a substance can cause the particles
in the substance to move faster. If the particles move
faster, the substance’s temperature increases. This happens if the substance is far from its melting or boiling
point. If the substance is at its melting or boiling point,
however, the absorbed energy causes the substance to
change in state.
When a substance is near its boiling point, it still
absorbs energy. However, none of the energy is used to
increase the kinetic energy of its particles. The energy is
used to change the substance from a liquid to a gas. The
same is true for a substance near its melting point. The
energy it absorbs makes it change from a solid to a liquid.
The graph below shows an example of how temperature changes when energy is added to a substance. Notice
that absorbing energy can cause the substance’s temperature to increase, or it can cause the substance to change
state. However, absorbing heat cannot cause both a temperature change and a change in state.
16. Apply Concepts What
type of energy transfer
causes heat to move from
the side of a metal pan into
the water inside the pan?
What kind of energy
transfer occurs when the
heated water rises toward
the top of the pan?
GA
S
LI
Q
UI
D
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17. Interpret What is
happening during the times
that the temperature of the
substance doesn’t change?
Melting point
SO
LI D
Temperature
Boiling point
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Added Energy
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Section 2 Review
Section Vocabulary
convection the movement of matter due to differences in density that are caused by temperature variations; can result in the transfer of
energy as heat
convection current any movement of matter
that results from differences in density; may be
vertical, circular, or cyclical
radiation the energy that is transferred as
electromagnetic waves, such as visible light
and infrared waves
specific heat the quantity of heat required to
raise a unit mass of homogeneous material
1 K or 1 ºC in a specified way given constant
pressure and volume
thermal conduction the transfer of energy as
heat through a material
1.Explain Why are most cooking pots and pans made from metal?
2.Identify Fill in the blanks in the table below.
Example
Type of energy transfer
The moon’s surface has a higher temperature on
the side facing the sun.
radiation
When cold water is poured into a glass, the glass
becomes colder.
Warm ocean water carries heat from the equator
toward the poles.
The pavement in a parking lot becomes hot on a
sunny day.
3.Draw Conclusions Convection occurs within the rock in Earth’s mantle. What can
you conclude about the rock in the mantle based on this information? (Hint: In
what kind of matter can convection occur?)
4.Infer Why can energy move through outer space by radiation, but not by convec-
tion or conduction?
5.Calculate A container holds 2.0 kg of liquid water. The water absorbs 477 kJ of
energy. If the water’s initial temperature was 298 K, what is its final temperature?
Show your work. (Hint: Rearrange the energy equation to solve for DT.)
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