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Transcript
```Conceptual
Physical
Science
5th Edition
Chapter 7:
HEAT TRANSFER AND
CHANGE OF PHASE
understand:
•
•
•
•
•
•
•
•
•
Conduction
Convection
Newton’s Law of Cooling
Climate Change and the Greenhouse Effect
Heat Transfer and Change of Phase
Boiling
Melting and Freezing
Energy and Change of Phase
Heat Transfer
Processes of thermal energy transfer:
• Conduction
• Convection
Conduction
Conduction
• Transfer of internal energy by electron and
molecular collisions within a substance
Heat Transfer: Conduction
Conduction occurs
predominately in solids
where the molecules remain
in relatively restricted
locations.
When you stick a nail into
ice, does cold flow from the
ice to your hand, or heat
from your hand to the ice?
Heat Transfer: Conduction
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.
Heat Transfer: Conduction
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.
Conduction
Insulation
• Doesn’t prevent the flow of internal energy
• Slows the rate at which internal energy flows
Example: Rock wool or fiberglass between walls slows
the transfer of internal energy from a warm
house to a cool exterior in winter, and the
reverse in summer
Conduction Application
• Snow patterns on the
roof of a house show
areas of conduction
and insulation.
• Bare parts show
where heat from
inside has conducted
through the roof and
melted the snow.
Energy Transfer
When thermal insulation, such as spun glass or rock wool,
is placed beneath the roof of a house, then in cold weather
the insulation will
A.
B.
C.
D.
create heat to warm the house.
keep the cold from coming through the roof.
slow the flow of heat from inside the house to the outside.
stop the flow of heat from inside the house to the outside.
Energy Transfer
When thermal insulation, such as spun glass or rock wool,
is placed beneath the roof of a house, then in cold weather
the insulation will
A.
B.
C.
D.
create heat to warm the house.
keep the cold from coming through the roof.
slow the flow of heat from inside the house to the outside.
stop the flow of heat from inside the house to the outside.
Explanation:
No insulation can stop heat flow. Insulation only slows it. (A
fortune awaits the inventor who can come up with the “perfect”
insulator!)
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, Styrofoam,
cork, liquid, gases, air, or materials with trapped air
Conduction
Dramatic example:
Author John Suchocki walks
barefoot without burning his feet
on red-hot coals,due to poor
conduction between the coals
and his feet
Convection
Convection
• Transfer of heat involving only
bulk motion of fluids
Examples:
• Visible shimmer of air above a
hot stove or above asphalt on a
hot day
• Visible shimmers in water due
to temperature difference
Convection
Cooling by expansion
• Opposite to the warming that occurs when air is
compressed
Example: The “cloudy” region above
hot steam issuing from the
nozzle of a pressure cooker is
cool to the touch (a
combination of air
expansion and mixing with
cooler surrounding air).
Careful, the part at the nozzle
that you can’t see is
steam—ouch!
Convection Currents
• Convection currents produced by unequal
heating of land and water.
• During the day, warm air above the land rises,
and cooler air over the water moves in to
replace it.
• At night, the direction of air flow is reversed.
Convection
Reason warm air rises
• Warm air expands, becomes less dense, and is
buoyed upward
• Air rises until its density equals that of the
surrounding air
Example: Smoke from a campfire rises and blends with
the surrounding cool air.
Heat Transfer: Convection
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.
Heat Transfer: Convection
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.
• Transfer of energy via electromagnetic waves
that can travel through empty space
Wavelength of radiation is related to the
frequency of vibration.
Low-frequency
vibrations  long waves
High-frequency vibrations  short waves
• Every object above absolute
• From the Sun’s surface comes
• From the Earth’s surface is
terrestrial radiation in the form of
infrared waves below our
threshold of sight
Wave Frequency - Temperature
(a) A low-temperature (cool)
source emits primarily
low-frequency, long
wavelength waves.
(b) A medium-temperature
source emits primarily
medium-frequency.
(c) A high-temperature
source emits primarily
high-frequency, short
wavelength waves.
• Peak frequency of radiation is proportional to the
absolute temperature of the source ( f ~ T )

Emission and Absorption
The surface of any material both absorbs
When a surface absorbs more energy than it
emits, it is a net absorber, and
temperature tends to rise.
When a surface emits more energy than it
absorbs, it is a net emitter, and
temperature tends to fall.
Emission and Absorption
The ability of a material to absorb and radiate
thermal energy is indicated by its color.
Good absorbers and good
emitters are dark in color.
Poor absorbers and poor
emitters are reflective or
light in color.
Emission and Absorption
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 tends to cool.
A surface colder than its surroundings will be
a net absorber and tends to warm.
Emission and Absorption
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.
Emission and Absorption
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!
• Darkness is often due to reflection of light back
and forth many times partially absorbing with
each reflection
• Good reflectors are poor absorbers
Which is the better statement?
A.
B.
C.
D.
A black object absorbs energy well.
An object that absorbs energy well is black.
Both say the same thing, so both are equivalent.
Both are untrue.
Which is the better statement?
A.
B.
C.
D.
A black object absorbs energy well.
An object that absorbs energy well is black.
Both say the same thing, so both are equivalent.
Both are untrue.
Explanation:
This is a cause-and-effect question. The color black doesn’t draw
in and absorb energy. It’s the other way around—any object that
does draw in and absorb energy, will, by consequence, be black
in color.
Which of the following does NOT emit radiation?
A.
B.
C.
D.
A lit fluorescent lamp.
A lit incandescent lamp.
A burned out incandescent lamp.
None of the above.
Which of the following does NOT emit radiation?
A.
B.
C.
D.
A lit fluorescent lamp.
A lit incandescent lamp.
A burned out incandescent lamp.
None of the above.
Explanation:
Everything continually emits radiation—and everything continually
absorbs radiation. When emission is greater than absorption,
temperature of the emitter drops. When absorption is greater than
emission, temperature increases. Everything is emitting and
Newton’s Law of Cooling
Newton’s Law of Cooling
• Approximately proportional to the temperature difference
T between the object and its surroundings
• In short: Rate of cooling ~ T
Examples:
• Hot apple pie cools more quickly in a freezer than if
left on the kitchen table
• Warmer house more quickly leaks thermal energy to
the outside than a cooler house
Newton’s Law of Cooling
Newton’s Law of Cooling (continued)
• Applies to rate of warming
– Object cooler than its surroundings warms up at a
rate proportional to T
Example: Frozen food warm quicker in a warm room
than in a cold room
Newton’s Law of Cooling
It is commonly thought that a can of beverage will cool
faster in the coldest part of a refrigerator. Knowledge of
Newton’s law of cooling
A.
B.
C.
D.
supports this knowledge.
shows this knowledge is false.
may or may not support this knowledge.
may or may not contradict this knowledge.
Newton’s Law of Cooling
It is commonly thought that a can of beverage will cool
faster in the coldest part of a refrigerator. Knowledge of
Newton’s law of cooling
A.
B.
C.
D.
supports this knowledge.
shows this knowledge is false.
may or may not support this knowledge.
may or may not contradict this knowledge.
Climate Change and the
Greenhouse Effect
Greenhouse Effect
• Named for a similar temperature-raising effect in
florists’ greenhouses
Climate Change and the
Greenhouse Effect
Understanding the greenhouse effect
requires two concepts:
• All things radiate at a frequency (and
therefore wavelength) that depends on the
temperature of the emitting object
• transparency of things depends on the
Climate Change and the
Greenhouse Effect
Climage Change
• Energy absorbed from the Sun
• Part reradiated by Earth as longer-wavelength
Climate Change and the
Greenhouse Effect
Climate Change (continued)
• Terrestrial radiation absorbed by atmospheric
gases and re-emitted as long-wavelength
• Reradiated energy unable to escape, so
warming of Earth occurs
• Long-term effects on climate are of present
concern
Climate Change and the Greenhouse Effect
If Earth radiated more energy than it absorbs from the Sun,
Earth’s average temperature would
A.
B.
C.
D.
decrease.
increase.
likely not change.
None of these.
Climate Change and the Greenhouse Effect
If Earth radiated more energy than it absorbs from the Sun,
Earth’s average temperature would
A.
B.
C.
D.
decrease.
increase.
likely not change.
None of these.
Explanation:
The Sun emits short-wavelength radiation and the Earth absorbs it. Earth
emits long-wavelength radiation, and we have an energy balance that
determines Earth’s temperature. If Earth radiates more than it absorbs,
then like any system, its temperature would decrease. That’s the physics!
Phases of Matter
• Matter exists in the three common phases:
solid, liquid, and gas (a fourth phase of matter
is plasma).
• When matter changes from one phase to
another, energy is transferred.
Heat Transfer and Change of
Phase
Evaporation
• Change of phase from liquid to gas
Heat Transfer and Change of
Phase
Evaporation process
• Molecules in liquid move randomly at various
speeds, continually colliding with one another
• Some molecules gain kinetic energy while
others lose kinetic energy during collision
• Some energetic molecules escape from the
liquid and become gas
• Average kinetic energy of the remaining
molecules in the liquid decreases, resulting in
cooler water
Evaporation Application
• Pigs have no sweat glands and therefore
cannot cool by the evaporation of perspiration.
• Instead, they wallow in mud to cool
themselves.
Heat Transfer and Change of
Phase
Sublimation
• Form of phase change directly from solid to gas
Examples:
• dry ice (solid carbon dioxide molecules)
• mothballs
• frozen water
Heat Transfer and Change of
Phase
Condensation process
• Opposite of evaporation
• Warming process from a gas to a liquid
• Gas molecules near a liquid surface are attracted to the
liquid
• They strike the surface with increased kinetic energy,
becoming part of the liquid
Condensation Application
• If you’re chilly outside
the shower stall, step
back inside and be
warmed by the
condensation of the
excess water vapor in
the shower
• Evaporation cools
you. Condensation
warms you!
Evaporation-Condensation Toy
The toy drinking bird operates by the evaporation of
ether inside its body and by the evaporation of
water from the outer surface of its head. The lower
body contains liquid ether, which evaporates at
room temperature.
(a) vaporizes, (b) creates pressure, ether goes up the tube,
(c) tips over, condensed ether runs back to body.
(d) each pivot wets the head and the cycle repeats.
Energy Transfer
When a liquid changes phase to a gas, it
A.
B.
C.
D.
absorbs energy.
emits energy.
neither absorbs nor emits energy.
becomes more conducting.
Energy Transfer
When a liquid changes phase to a gas, it
A.
B.
C.
D.
absorbs energy.
emits energy.
neither absorbs nor emits energy.
becomes more conducting.
Boiling
Boiling process
• Rapid evaporation occurs beneath the surface of
a liquid
Boiling
Boiling process (continued)
• evaporation beneath the surface forms vapor bubbles
• bubbles rise to the surface
• if vapor pressure in the bubble is less than the
surrounding pressure, then the bubbles collapse
• hence, bubbles don’t form at temperatures below boiling
point when vapor pressure is insufficient
Boiling
• Heating warms the
water from below.
• Boiling cools the
water from above.
Pressure Cooker
• The tight lid of a
pressure cooker holds
pressurized vapor
above the water
surface, which inhibits
boiling.
• In this way, the boiling
point of water is
greater than 100°C.
Boiling
When a liquid is brought to a boil, the boiling process tends
to
A.
B.
C.
D.
resist a further change of phase.
heat the liquid.
cool the liquid.
Boiling
When a liquid is brought to a boil, the boiling process tends
to
A.
B.
C.
D.
resist a further change of phase.
heat the liquid.
cool the liquid.
Equilibrium
(a) In a mixture of ice and water at 0°C, ice crystals gain
and lose water molecules at the same time. The ice and
water are in thermal equilibrium.
(b) This gaining-and-losing process is inhibited when salt is
added to the water. Then with fewer water molecules at
the interface, fewer enter the ice.
Can Crunch!
A spectacular classroom (or home) demo is placing a sodapop can with a bit of water in it on a hot stove. Soon steam
comes from the opening. Invert the can into a bath of water
and atmospheric pressure crushes the can. The key to this
dramatic whopping of the can is mainly
A.
B.
C.
D.
temperature reduction of the steam in the can.
rapid condensation of steam in the can.
reduced pressure due to cooling of steam in the can.
increase in atmospheric pressure on the can.
Can Crunch!
A spectacular classroom (or home) demo is placing a sodapop can with a bit of water in it on a hot stove. Soon steam
comes from the opening. Invert the can into a bath of water
and atmospheric pressure crushes the can. The key to this
dramatic whopping of the can is mainly
A.
B.
C.
D.
temperature reduction of the steam in the can.
rapid condensation of steam in the can.
reduced pressure due to cooling of steam in the can.
increase in atmospheric pressure on the can.
Explanation:
Quick reduction in pressure occurs as steam molecules rapidly condense
upon the water in the bath (they don’t condense on the hot inner metal
walls). With lowered pressure inside, the outside atmospheric pressure
produces the whop! Similarly, the condensation cycle of steam turbines
reduces pressure on the back side of turbine blades. Greater steam
pressure on the front side of the blades turns the turbine.