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8/4/2019 PhysicsChpt26

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Unit 8, Chapter 26

CPO Science

Foundations of Physics


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Unit 8: Matter and Energy

26.1 Heat Conduction

26.2 Convection

26.3 Radiation

Chapter 26 Heat Transfer


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Chapter 26 Objectives1. Explain the relationship between temperature and thermal

equilibrium.

2. Explain how heat flows in physical systems in terms ofconduction, convection, and radiation.

3. Apply the concepts of thermal insulators and conductors topractical systems.

4. Describe free and forced convection and recognize theseprocesses in real-life applications.

5. Identify the relationship between wavelength, color, infraredlight, and thermal radiation.

6. Calculate the heat transfer in watts for conduction, convection,

and radiation in simple systems.

7. Explain how the three heat-transfer processes are applied toevaluating the energy efficiency of a house or building.


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Chapter 26 Vocabulary Terms infrared

windchill factor

thermal insulator thermal equilibrium

forced convection

R-value

convection

blackbody spectrum

thermal conductivity

thermal conductor

blackbody heat transfer

thermal radiation

free convection

heat transfer coefficient

conduction


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26.1 Heat Conduction

Key Question:

How does heat passthrough differentmaterials?

*Students read Section 26.1AFTER Investigation 26.1


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26.1 Heat Transfer The science of how heat flows is called heat

transfer.

There are three ways heat transfer works:conduction, convection, and radiation.

Heat flow depends on the temperature difference.


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26.1 Thermal Equilibrium Two bodies are in thermal

equilibrium with each

other when they have thesame temperature.

In nature, heat always

flows from hot to cold untilthermal equilibrium isreached.


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26.1 Heat Conduction Conduction is the transfer of heat through

materials by the direct contact of matter.

Dense metals like copper and aluminum are verygood thermal conductors.


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26.1 Heat Conduction A thermal insulator is a material that conducts

heat poorly.

Heat flows very slowly through the plastic so thatthe temperature of your hand does not rise verymuch.


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26.1 Heat Conduction Styrofoam gets its

insulating ability by

trapping spaces of airin bubbles.

Solids usually arebetter heat conductors

than liquids, andliquids are betterconductors thangases.


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26.1 Heat Conduction The ability to conduct

heat often depends moreon the structure of amaterial than on thematerial itself.

Solid glass is a thermal

conductor when it isformed into a beaker orcup.

When glass is spun intofine fibers, the trapped airmakes a thermal insulator.


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26.1 Thermal Conductivity

The thermal conductivity of a material describeshow well the material conducts heat.


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26.1 ThermalConductivity

Heat conduction insolids and liquidsworks by transferringenergy through

bonds betweenatoms or molecules.


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26.1 Heat Conduction Equation

PH = k A (T2 -T1)L

Area of cross section (m2)

Length (m)

Thermal conductivity

(watts/m

o

C)

Heat flow(watts)

Temperaturedifference (oC)


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26.1 Variables for conduction


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26.1 Calculate Heat Transfer

A copper bar connects two beakers of water at differenttemperatures.

One beaker is at 100C and the other is at 0C.

The bar has a cross section area of 0.0004 m2 and is one-halfmeter (0.5 m) long.

How many watts of heat are conducted through the bar fromthe hot beaker to the cold beaker?

The thermal conductivity of copper is 401 W/mC.


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26.2 Convection

Key Question:

Can moving matter carrythermal energy?



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26.2 Convection Convection is the transfer of

heat by the motion of liquids

and gases. Convection in a gas occursbecause gas expands whenheated.

Convection occurs becausecurrents flow when hot gasrises and cool gas sink.

Convection in liquids alsooccurs because of differences

in density.


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26.2 Convection When the flow of gas or

liquid comes from

differences in density andtemperature, it is calledfree convection.

When the flow of gas orliquid is circulated bypumps or fans it is calledforced convection.


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26.2 Convection

Convection depends onspeed.

Motion increases heattransfer by convection inall fluids.


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26.2 Convection Convection depends on

surface area.

If the surface contactingthe fluid is increased, therate of heat transfer alsoincreases.

Almost all devices madefor convection have finsfor this purpose.


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26.2 Forced Convection

Both free and forced convection help toheat houses and cool car engines.


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26.2 Convection and Sea Breezes

On a smaller scale nearcoastlines, convection is

responsible for sea breezes. During the daytime, land is much

hotter than the ocean.

A sea breeze is created when hot

air over the land rises due toconvection and is replaced bycooler air from the ocean.

At night the temperature reverses

so a land breeze occurs.


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26.2 Convection Currents Much of the Earths climate is regulated by giant

convection currents in the ocean.


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26.2 Heat Convection Equation

PH = h A (T2 -T1)

Area contacting fluids (m2)Heat transfer coefficient(watts/m2oC)

Heat flow(watts)

Temperaturedifference (oC)


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26.2 Calculating convection The surface of a window is a

temperature of 18C (64oF).

A wind at 5C (41o

F) is blowingon the window fast enough tomake the heat transfercoefficient 100 W/m2C.

How much heat is transferredbetween the window and the airif the area of the window is 0.5square meters?


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26.3 Radiation

Key Question:

How does heat from thesun get to Earth?



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26.3 Radiation Radiation is heat transfer by

electromagnetic waves.

Thermal radiation iselectromagnetic waves(including light) produced byobjects because of their

temperature. The higher the temperature

of an object, the morethermal radiation it gives off.


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26.3 Radiant Heat

We do not see thethermal radiation

because it occurs atinfrared wavelengthsinvisible to the humaneye.

Objects glow differentcolors at differenttemperatures.


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26.3 Radiant Heat A rock at room

temperature does notglow.

The curve for 20Cdoes not extend intovisible wavelengths.

As objects heat up they

start to give off visiblelight, or glow.

At 600C objects glowdull red, like the burner

on an electric stove.


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26.3 Radiant Heat As the temperature rises, thermal

radiation produces shorter-wavelength, higher energy light.

At 1,000C the color is yellow-orange, turning to white at 1,500C.

If you carefully watch a bulb on adimmer switch, you see its color

change as the filament gets hotter.

The bright white light from a bulb isthermal radiation from an extremelyhot filament, near 2,600C.


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26.3 Radiant Heat

The graph of powerversus wavelengthfor a perfectblackbody is calledthe blackbody

spectrum.


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26.3 Radiant Heat A perfect blackbody is a surface that reflects

nothing and emits pure thermal radiation.

The white-hot filament of a bulb is agood blackbody because all light fromthe filament is thermal radiation andalmost none of it is reflected from other

sources.

The curve for 2,600C shows thatradiation is emitted over the whole

range of visible light.


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26.3 Radiant Heat A star is a near-perfect

blackbody.

The distribution of energybetween different wavelengths(colors) depends strongly on thetemperature.

Sirius is a hot, young star about

twice as big as the sun and 22times as bright.

Because its temperature ishotter, Sirius appears bluer than

the sun.


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26.3 Radiant Heat The total power emitted as thermal radiation by a

blackbody depends on temperature (T) and

surface area (A

).

Real surfaces usually emit less than the blackbodypower, typically between 10 and 90 percent.

The Kelvin temperature scale is used in theStefan-Boltzmann formula because thermalradiation depends on the temperature above

absolute zero.


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26.3 Stefan-Boltzmann formula

P = s AT4

Surface area (m2)

Stefan-Boltzmannconstant5.67 x 10-8 watts/m2K4)

Power(watts)

Absolute temperature(K)


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26.3 Calculate Radiant Power The filament in a light bulb has a

diameter of 0.5 millimeters anda length of 50 millimeters.

The surface area of the filamentis 4 10-8 m2.

If the temperature is 3,000 K,how much power does thefilament radiate?


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26.3 Radiant Heat When comparing heat

transfer for a pot 10 cm

above a heating elementon a stove, radiant heataccounts for 74%

How is heat transferredwhen the pot sits on theelement?


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Application: Energy-efficient Buildings

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