Transcript Hewitt/Lyons/Suchocki/Yeh, Conceptual Integrated Science

Hewitt/Lyons/Suchocki/Yeh
Conceptual Integrated
Science
Chapter 6
HEAT
Copyright © 2007 Pearson Education, Inc., publishing as Pearson Addison Wesley
This lecture will help you
understand:
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The Kinetic Theory of Matter
Temperature
Absolute Zero
What Is Heat?
Quantity of Heat
The Laws of Thermodynamics
Entropy
Specific Heat Capacity
Thermal Expansion
Expansion of Water
Heat Transfer: Conduction
Heat Transfer: Convection
Heat Transfer: Radiation
Emission of Radiant Energy
Absorption of Radiant Energy
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6.1 The Kinetic Theory of Matter
Kinetic Theory of Matter:
Matter is made up of
tiny particles (atoms or
molecules) that are
always in motion.
Thermal Energy:
The total energy (kinetic
and potential) of the
submicroscopic particles
that make up a
substance.
Types of motion of particles in matter.
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6.2 Temperature
Temperature
is defined as the measure of
hotness or coldness of an
object (degrees Celsius, or
degrees Fahrenheit, or kelvins).
is related to the average
translational kinetic energy per
Can we trust our sense of hot
molecule in a substance.
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and cold? Will both fingers feel
the same temperature when
they are put in the warm water?
Try this and feel it for yourself!
Temperature
Thermometer
is an instrument that
measures temperature by
comparing the expansion
and contraction of a liquid
as it gains or loses thermal
energy.
Infrared thermometer
measures temperature by
the radiation a substance
emits.
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Temperature
Temperature has no upper limit.
Temperature of a substance is
registered on a liquid-base
thermometer when the substance
has reached thermal equilibrium
with the thermometer.
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Temperature
Three different temperature
scales differ in zero point and
divisions:
• Celsius scale
freezing point of water: 0C
boiling point of water: 100C
division: 100 degree units
• Fahrenheit scale
freezing point of water: 32 F
boiling point of water: 212 F
division: 180 degree units
• Kelvin scale (used in scientific
research)
freezing point of water: 273 K
boiling point of water: 373 K
division: same-size increments
as Celsius scale
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Temperature
CHECK YOUR NEIGHBOR
There is twice as much molecular kinetic energy in 2 liters
of boiling water as in 1 liter of boiling water. Which will be
the same for both?
A.
B.
C.
D.
Temperature.
Thermal energy.
Both of the above.
None of the above.
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Temperature
CHECK YOUR ANSWER
There is twice as much molecular kinetic energy in 2 liters
of boiling water as in 1 liter of boiling water. Which will be
the same for both?
A.
B.
C.
D.
Temperature.
Thermal energy.
Both of the above.
None of the above.
Explanation:
Average kinetic energy of molecules is the same, which means
temperature is the same for both.
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• One Kelvin is the same as one degree
Celsius!
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6.3 Absolute Zero
Absolute zero or zero K
is the lowest limit of
temperature at –273C where
molecules have lost all
available kinetic energy. A
substance cannot get any
colder.
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6.4 What Is Heat?
Heat
is defined as a flow of
thermal energy due to a
temperature difference.
The direction of heat flow is
from a
higher-temperature
substance to a
lower-temperature
substance.
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• The temperature of the sparks
is very high, about 2000°C.
That’s a lot of thermal energy
per molecule of spark.
However, because there are
few molecules per spark,
internal energy is safely small.
Temperature is one thing;
transfer of energy is another.
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What Is Heat?
CHECK YOUR NEIGHBOR
If a red hot thumbtack is immersed in warm water, the
direction of heat flow will be from the
A.
B.
C.
D.
warm water to the red hot thumbtack.
red hot thumbtack to the warm water.
no heat flow.
not enough information.
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What Is Heat?
CHECK YOUR ANSWER
If a red hot thumbtack is immersed in warm water, the
direction of heat flow will be from the
A.
B.
C.
D.
warm water to the red hot thumbtack.
red hot thumbtack to the warm water.
no heat flow.
not enough information.
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Quantity of Heat
Heat is measured in units of energy—
joules or calories.
calorie
is defined as the amount of heat
needed to raise the temperature of 1
gram of water by 1°C.
4.18 joules = 1 calorie
so 4.18 joules of heat will change
the temperature of
1 gram of water by 1°C
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Although the same
quantity of heat is added
to both containers, the
temperature increases
more in the container
with the smaller amount
of liquid.
If you add 1 calorie of
heat to 1 gram of
water, you’ll raise its
temperature by 1°C.
Quantity of Heat
Energy rating of food and fuel
is measured by energy
released when they are
metabolized.
Kilocalorie:
Heat unit for labeling food
One kilocalorie or Calorie (with
a capital C) is the heat needed to
change the temperature of
1 kilogram of water by 1°C.
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To the weight watcher, the
peanut contains 10
Calories, to the physicist, it
releases 10,000 calories (or
41,840 joules) of energy
when burned or digested.
Quantity of Heat
CHECK YOUR NEIGHBOR
The same quantity of heat is added to different amounts of
water in two equal-size containers. The temperature of the
smaller amount of water
A.
B.
C.
D.
decreases more.
increases more.
does not change.
not enough information.
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Quantity of Heat
CHECK YOUR ANSWER
The same quantity of heat is added to different amounts of
water in two equal-size containers. The temperature of the
smaller amount of water
A.
B.
C.
D.
decreases more.
increases more.
does not change.
not enough information.
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6.5 The Laws of
Thermodynamics
First Law of
Thermodynamics:
Whenever heat flows into
or out of a system, the
gain or loss of thermal
energy equals the amount
of heat transferred.
When thermal energy
transfers as heat, it does
so without net loss or
gain.
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When Pete pushes down on the
piston, he does work on the air
inside. What happens to the
air’s temperature?
The Laws of Thermodynamics
CHECK YOUR NEIGHBOR
When work is done on a system, compressing air in a tire
pump, for example, the temperature of the system
A.
B.
C.
D.
increases.
decreases.
remains unchanged.
is no longer evident.
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The Laws of Thermodynamics
CHECK YOUR ANSWER
When work is done on a system, compressing air in a tire
pump, for example, the temperature of the system
A.
B.
C.
D.
increases.
decreases.
remains unchanged.
is no longer evident.
Explanation:
In accord with the first law of thermodynamics, work input
increases the energy of the system.
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The Laws of Thermodynamics
Second Law of Thermodynamics:
Heat never spontaneously flows from a
lower-temperature substance to a highertemperature substance.
Heat can be made to flow the opposite way
only when work is done on the system or
by adding energy from another source.
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The Laws of Thermodynamics
Third Law of Thermodynamics:
No system can reach absolute zero.
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Entropy
Entropy
is a measure of the disorder
of a system.
Whenever energy freely
transforms from one form to
another, the direction of
transformation is toward a
state of greater disorder
and, therefore, toward one
of greater entropy.
The greater the disorder
 the higher the
entropy.
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Entropy
Second law of
thermodynamics —
restatement:
Natural systems tend to
disperse from
concentrated and
organized-energy states
toward diffuse and
disorganized states.
Energy tends to degrade •
and disperse with time.
The total amount of entropy
in any system tends to
increase with time.
Work is needed to transform a state of
disorderly, diffuse energy to a state of more
concentrated energy. The Sun supplies the
energy to do this work when plants transform
liquids and vapors into sugar molecules– a
plants storehouse of usable, concentrated
energy.
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Entropy
CHECK YOUR NEIGHBOR
Your room gets messier each week. In this case, the
entropy of your room is
A.
B.
C.
D.
increasing.
decreasing.
hanging steady.
nonexistent.
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Entropy
CHECK YOUR ANSWER
Your room gets messier each week. In this case, the
entropy of your room is
A.
B.
C.
D.
increasing.
decreasing.
hanging steady.
nonexistent.
Comment:
If your room became more organized each week, then entropy
would decrease in proportion to the effort expended.
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Specific Heat Capacity
Specific heat capacity
is defined as the quantity
of heat required to change
the temperature of 1 unit
mass of a substance by
1°C.
• thermal inertia that
indicates the resistance of a
substance to a change in
temperature.
• sometimes called specific
heat.
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The filling of a hot apple pie may
be too hot to eat, even though the
crust is not. The crust cools
quickly because it has a lower
heat capacity!
Specific Heat
CHECK YOUR NEIGHBOR
Which has the higher specific heat, water or land?
A.
B.
C.
D.
Water.
Land.
Both of the above are the same.
None of the above.
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Specific Heat
CHECK YOUR ANSWER
Which has the higher specific heat, water or land?
A.
B.
C.
D.
Water.
Land.
Both of the above are the same.
None of the above.
Explanation:
A substance with small temperature changes for large heat
changes has a high specific heat capacity. Water takes much
longer to heat up in the sunshine than does land. This difference
is a major influence on climate.
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6.7 Thermal Expansion
Thermal Expansion
When the temperature
of a substance is
increased, its particles
jiggle faster and move
farther apart.
All forms of matter
generally expand
when heated and
contract when cooled.
The gap in the roadway of a bridge is
called an expansion joint. It allows the
bridge to expand and contract. (Was
this photo taken on a warm day or a
cold day?)
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Thermal expansion accounts for the creaky noises
often heard in the attics of old houses on cold nights.
The construction materials expand during the day and
contract at night, creaking as they grow and shrink.
• ..\..\..\Videos for
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Thermal Expansion
CHECK YOUR NEIGHBOR
When stringing telephone lines between poles in the
summer, it is advisable to allow the lines to
A.
B.
C.
D.
sag.
be taut.
be close to the ground.
allow ample space for birds.
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Thermal Expansion
CHECK YOUR ANSWER
When stringing telephone lines between poles in the
summer, it is advisable to allow the lines to
A.
B.
C.
D.
sag.
be taut.
be close to the ground.
allow ample space for birds.
Explanation:
Telephone lines are longer in a warmer summer and shorter in a
cold winter. Hence, they sag more on hot summer days than in
winter. If the lines are not strung with enough sag in summer, they
might contract too much and snap during the winter—especially
when carrying ice.
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Expansion of Water
Expansion of Water
When water becomes ice, it expands. Ice
has open-structured crystals resulting from
strong bonds at certain angles that
increase its volume. This make ice less
dense than water.
Liquid water is more dense than
ice because water molecules in a
liquid are closer together than
water molecules frozen in ice,
where they have an open
crystalline structure.
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Close to 0°C, liquid water contains
crystals of ice. The open structure of
these crystals increases the volume
of the water slightly.
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The six-sided structure of a
snowflake is a result of the
six-sided ice crystals that
make it up.
Expansion of Water
Water between 0C and 4C does not expand with
temperature. As the temperature of 0 water
rises, it contracts until it reaches 4C. Thereafter,
it expands.
Water is at its smallest volume and greatest
density at 4C. When 0C water freezes to
become ice, however, it has its largest volume
and lowest density.
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Expansion of Water
Volume changes for a 1-gram sample of
water.
Between 0°C and
4°C, the volume of
liquid water
decreases as
temperature
increases. Above
4°C, water behaves
the way other
substances do. Its
volume increases
as its temperature
increases. The
volumes shown
here are for a 1gram sample.
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• As water cools, it sinks until the entire lake is
4°C. Then as water at the surface is cooled
further, it floats on top and can freeze. Once
ice is formed, temperatures lower than 4°C
can extend down into the lake.
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Expansion of Water
CHECK YOUR NEIGHBOR
When a sample of 0C water is heated, it first
A.
B.
C.
D.
expands.
contracts.
remains unchanged.
not enough information.
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Expansion of Water
CHECK YOUR ANSWER
When a sample of 0C water is heated, it first
A.
B.
C.
D.
expands.
contracts.
remains unchanged.
not enough information.
Explanation:
Water continues to contract until it reaches a temperature of 4C.
With further increase in temperature beyond 4C, water then
expands.
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Expansion of Water
CHECK YOUR NEIGHBOR
When a sample of 4C water is cooled, it
A.
B.
C.
D.
expands.
contracts.
remains unchanged.
not enough information.
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Expansion of Water
CHECK YOUR ANSWER
When a sample of 4C water is cooled, it
A.
B.
C.
D.
expands.
contracts.
remains unchanged.
not enough information.
Explanation:
Parts of the water will crystallize and occupy more space.
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6.8 Heat Transfer
Processes of thermal energy transfer:
• conduction
• convection
• radiation
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Heat Transfer: conduction
• Conduction: Heat is
transferred between substances
that are touching
• Heat transfer is always from
warmer to colder substances
• The rate of heat transfer
increases as the temperature
difference increases
• The rate of heat transfer varies
with the type of substance
– Metal?
– Air?
• Winter coats
• insulation
The transfer of heat from the hot end of the
metal pin to the cool end by molecular contact is
called conduction
Heat Transfer: Conduction
Conduction
occurs predominately in solids where the
molecules remain in relatively restricted
locations.
The tile floor feels colder than the wooden floor,
even though both floor materials are at the same
temperature. This is because tile is a better
conductro of heat than wood, and so heat is more
readily conducted out of the foot touching the tile.
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When you touch a nail stuck in
ice, does cold flow from the ice
to your hand, or does energy
flow from your hand to the ice?
Heat Transfer: Conduction
Example:
When one end of a solid is placed near a heat
source, electrons and adjacent molecules gain
kinetic energy and start to move faster and farther.
They collide with neighboring molecules and
transfer some of their kinetic energy to them.
These molecules then interact with other
neighboring molecules, and thermal energy is
gradually transferred along the solid.
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Heat Transfer: Conduction
Good conductors:
• composed of atoms with “loose” outer electrons
• known as poor insulators
• examples—all metals to varying degrees
Poor conductors:
• delay the transfer of heat
• known as good insulators
• examples—wood, wool, straw, paper, cork,
Styrofoam, liquid, gases, air, or materials with trapped
air
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Heat Transfer: Conduction
No insulator can totally prevent heat from
getting through it.
An insulator reduces the rate at which heat
penetrates.
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• Firewalking author John Suchocki isn’t
burned by the red-hot wooden coals due
to the low heat conductivity of wood
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Heat Transfer: Conduction
CHECK YOUR NEIGHBOR
If you hold one end of a metal bar against a piece of ice,
the end in your hand will soon become cold. Does cold flow
from the ice to your hand?
A.
B.
C.
D.
Yes.
In some cases, yes.
No.
In some cases, no.
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Heat Transfer: Conduction
CHECK YOUR ANSWER
If you hold one end of a metal bar against a piece of ice,
the end in your hand will soon become cold. Does cold flow
from the ice to your hand?
A.
B.
C.
D.
Yes.
In some cases, yes.
No.
In some cases, no.
Explanation:
Cold does not flow from the ice to your hand. Heat flows from your
hand to the ice. The metal is cold to your touch, because you are
transferring heat to the metal.
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Heat Transfer: Convection
Convection:
• occurs in liquids and gases
• involves the movement of
warmer gases or liquids to
cooler surroundings
Two characteristics of
convection:
• the ability of flow—
carrying thermal energy
with the fluid
• the ability of warm fluid
to rise in cooler
surroundings
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Convection currents in air and in
fluid.
• Convection: transfer of heat by movement
of a fluid
• Remember air is a fluid
– Convection – transfer of heat by vertical fluid
motion (see below)
• Although we can’t see air, there are signs that tell us
where the air is rising. On a calm day, you can watch a
hawk circle and climb high above level ground while its
wings remain motionless. A rising thermal carries the
hawk upward as it scans the terrain for prey. If the water
vapor inside the rising thermal condenses into liquid
cloud droplets, the thermal becomes visible to us as a
puffy cumulus cloud. Flying in a light aircraft beneath
these clouds usually produces a bumpy ride, as
passengers are jostled around by the rising and sinking
air associated with convection.
Sea Breeze
Convection currents produced by unequal heating of land and water (a)
during the day, warm air above the land rises, and cooler air over
the water moves into replace it. (b) at night, the direction of the air
flow is reversed, because now the water is warmer than the land.
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Surface heating and lifting of air along a sea breeze combine to form thunderstorms
almost daily during the summer in southern Florida.
Figure 7.6: Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that
enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of
high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail.
Land Sea Breeze animations
• http://www.classzone.com/books/earth_science/t
erc/content/visualizations/es1903/es1903page0
1.cfm?chapter_no=visualization
• Satellite of sea breeze:
http://www.met.tamu.edu/class/atmo151/tut/seab
r/sea18.html
• Lake breeze
http://itg1.meteor.wisc.edu/wxwise/AckermanKn
ox/chap12/great_lake_breeze.html
Heat Transfer: Convection
CHECK YOUR NEIGHBOR
Although warm air rises, why are mountaintops cold and
snow covered, while the valleys below are relatively warm
and green?
A.
B.
C.
D.
Warm air cools when rising.
There is a thick insulating blanket of air above valleys.
Both of the above.
None of the above.
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Heat Transfer: Convection
CHECK YOUR ANSWER
Although warm air rises, why are mountaintops cold and
snow covered, while the valleys below are relatively warm
and green?
A.
B.
C.
D.
Warm air cools when rising.
There is a thick insulating blanket of air above valleys.
Both of the above.
None of the above.
Explanation:
Earth’s atmosphere acts as a blanket, which for one important
thing, keeps Earth from freezing at nighttime.
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Heat Transfer: Radiation
Radiation
is the process by which thermal energy is
transferred by electromagnetic waves.
A thermal energy source such as the Sun converts
some of its energy into electromagnetic waves.
These waves carry energy, which converts back
into thermal energy when absorbed by a
receiver. The energy source radiates energy,
and a receiver absorbs it.
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Heat Transfer: Radiation
Wavelength of radiation is related to the
frequency of vibration.
Low-frequency
vibrations  long waves
High-frequency vibrations  short waves
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Emission of Radiant Energy
Emission of Radiant Energy
All substances at any temperature above absolute zero
emit radiant energy.
Average frequency (f ) of radiant energy is directly
proportional to the absolute temperature T of the
emitter:

(a) A low temperature (cool)
f T

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source emits primarily lowfrequency, long-wavelength
waves. (a) A mediumtemperature source emits
primarily medium-frequency,
medium-wavelength waves.
(c) A high-temperature (hot)
source emits primarily highfrequency, short-wavelength
waves.
• Radiation curves for different temperatures. The peak
frequency of radiant energy is directly proportional to the
absolute temperature of the emitter
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• Both the Sun and the Earth emit radiant
energy.
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• Key points to remember about radiation
– All objects emit radiation (if temperature is greater than 0 K)
– Objects emit radiation over a range of wavelengths
– As an objects temperature increases it emits radiation at shorter
wavelengths
– As an objects temperature increases it emits more radiation
– The sun emits most of its radiation as shortwave radiation
– The earth emits most of its radiation as longwave radiation
• A closer look at the radiation emitted by the sun:
Emission of Radiant Energy
CHECK YOUR NEIGHBOR
If a good absorber of radiant energy were a poor emitter, its
temperature compared with its surroundings would be
A.
B.
C.
D.
lower.
higher.
unaffected.
none of the above.
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Emission of Radiant Energy
CHECK YOUR ANSWER
If a good absorber of radiant energy were a poor emitter, its
temperature compared with its surroundings would be
A.
B.
C.
D.
lower.
higher.
unaffected.
none of the above.
Explanation:
If a good absorber were not also a good emitter, there would be a
net absorption of radiant energy, and the temperature of a good
absorber would remain higher than the temperature of the
surroundings. Nature is not so!
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Absorption of Radiant Energy
Absorption of Radiant Energy:
The ability to absorb and radiate thermal energy
is indicated by the color of the material.
Good absorbers and good
emitters are dark in color.
Poor absorbers and poor
emitters are reflective or
light in color.
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Absorption of Radiant Energy
The surface of any material both absorbs
and emits radiant energy.
When a surface absorbs more energy than it
emits, it is a net absorber, and
temperature rises.
When a surface emits more energy than it
absorbs, it is a net emitter, and
temperature falls.
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Absorption of Radiant Energy
Whether a surface is a net absorber or net
emitter depends on whether its
temperature is above or below that of its
surroundings.
A surface hotter than its surroundings will be
a net emitter and will cool.
A surface colder than its surroundings will
be a net absorber and will warm.
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Absorption of Radiant Energy
CHECK YOUR NEIGHBOR
Which melts faster in sunshine—dirty snow or clean snow?
A.
B.
C.
D.
Dirty snow.
Clean snow.
Both of the above.
None of the above.
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Absorption of Radiant Energy
CHECK YOUR ANSWER
Which melts faster in sunshine—dirty snow or clean snow?
A.
B.
C.
D.
Dirty snow.
Clean snow.
Both of the above.
None of the above.
Explanation:
Dirty snow absorbs more sunlight, whereas clean snow reflects
more.
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