Chapter 8 Heat Transfer & Change of Phase. Heat Transfer How does the energy move from a hotter to a colder object? How does the energy move from a hotter

Chapter 8Chapter 8

Heat Transfer & Change Heat Transfer & Change of Phaseof Phase

Heat TransferHeat Transfer

How does the energy move from a How does the energy move from a hotter to a colder object?hotter to a colder object?

Three mechanismsThree mechanisms ConductionConduction ConvectionConvection RadiationRadiation

ConductionConduction

Stir your hot soup with a metal spoonStir your hot soup with a metal spoon Pretty soon you need a pot holder Pretty soon you need a pot holder

because the end of the spoon you are because the end of the spoon you are holding gets hotholding gets hot

This is heat transfer by conductionThis is heat transfer by conduction Energy travels up the spoon from the Energy travels up the spoon from the

end in the hot soup to the end in your end in the hot soup to the end in your handhand


We sense the movement of energy We sense the movement of energy by the increasing temperatureby the increasing temperature

This means the atoms and molecules This means the atoms and molecules have higher average kinetic energyhave higher average kinetic energy

Primarily occurs by the movement of Primarily occurs by the movement of electrons in the materialelectrons in the material

The more easily the electrons can The more easily the electrons can move, the better the conductionmove, the better the conduction


Metals have some electrons that are Metals have some electrons that are very loosely bound to the atoms in the very loosely bound to the atoms in the materialmaterial

These electrons can move easily and can These electrons can move easily and can rapidly pick up additional kinetic energyrapidly pick up additional kinetic energy

Metals are good conductorsMetals are good conductors Wood and plastic don’t have loosely Wood and plastic don’t have loosely

bound electrons, so they are poor bound electrons, so they are poor conductorsconductors



Air is a poor thermal conductorAir is a poor thermal conductor If you stand in the sun on a cold If you stand in the sun on a cold

winter day and are shielded from the winter day and are shielded from the wind, you stay pretty warmwind, you stay pretty warm

Snow is a poor conductor, while Snow is a poor conductor, while water is betterwater is better

Makes igloos a useful as a houseMakes igloos a useful as a house

ConvectionConvection

A phenomenon in fluidsA phenomenon in fluids Instead of having energy moved by Instead of having energy moved by

successive collisions of electrons, successive collisions of electrons, atoms and molecules, the fluid itself atoms and molecules, the fluid itself is set into motion called a currentis set into motion called a current

These moving fluid currents are These moving fluid currents are convectionconvection



When the radiator heats the air, it When the radiator heats the air, it becomes less dense and risesbecomes less dense and rises

Cool air moves in to replace the air that Cool air moves in to replace the air that roserose

This generates the air flowThis generates the air flow So radiators don’t need a fan to stir the air So radiators don’t need a fan to stir the air

and to distribute heat throughout a roomand to distribute heat throughout a room The rising air cools until its density The rising air cools until its density

matches that of the surrounding airmatches that of the surrounding air


We take advantage of the cooling We take advantage of the cooling that occurs during an expansionthat occurs during an expansion

We make refrigerators and air We make refrigerators and air conditioners operate by forcing gas conditioners operate by forcing gas under pressure through a small hole under pressure through a small hole and expanding it into an empty and expanding it into an empty spacespace


Explains why breezes come from the Explains why breezes come from the ocean in the day and from the land ocean in the day and from the land at nightat night

RadiationRadiation

Energy carried by electromagnetic Energy carried by electromagnetic waveswaves

Light, microwaves, radio waves, x-Light, microwaves, radio waves, x-raysrays

Wavelength is related to vibration Wavelength is related to vibration frequencyfrequency

RadiationRadiation

average frequency absolute temperature

RadiationRadiation

Every object is emitting electromagnetic Every object is emitting electromagnetic waves regardless of temperaturewaves regardless of temperature

Things we can Things we can seesee from their own from their own radiation are very hot to have energy radiation are very hot to have energy emitted in the visible region of the emitted in the visible region of the spectrumspectrum

Most things emit primarily in the Most things emit primarily in the infraredinfrared

Night vision goggles, etc.Night vision goggles, etc.

RadiationRadiation

Things also absorb radiationThings also absorb radiation If they didn’t, they would run out of If they didn’t, they would run out of

energy to emitenergy to emit Good emitters are also good Good emitters are also good

absorbersabsorbers Equilibrium established between Equilibrium established between

emission and absorptionemission and absorption When something can’t equilibrate, it When something can’t equilibrate, it

gets hotter or coldergets hotter or colder

RadiationRadiation

Interior of a car on a sunny dayInterior of a car on a sunny day Sunlight comes in as visible lightSunlight comes in as visible light Seats and interior are much cooler Seats and interior are much cooler

so they radiate in the infrared so they radiate in the infrared instead of visibleinstead of visible

Glass in the windows blocks infrared Glass in the windows blocks infrared so energy can’t get outso energy can’t get out

Car interior heats up!Car interior heats up!

RadiationRadiation

A good absorber reflects very little A good absorber reflects very little energyenergy

Think about dark pavementThink about dark pavement A poor absorber reflects a lot of A poor absorber reflects a lot of

energyenergy Think about snow that doesn’t melt Think about snow that doesn’t melt

in sunshine even though 1400 in sunshine even though 1400 watts/meterwatts/meter22 are hitting it are hitting it

RadiationRadiation

At night, objects receive no input At night, objects receive no input energy from the sunenergy from the sun

But, they are warmer than outer But, they are warmer than outer space, so they continue to radiate space, so they continue to radiate energyenergy

Thus, they cool offThus, they cool off Can we make ice in the desert Can we make ice in the desert

without a refrigerator?without a refrigerator?

Newton’s Law of CoolingNewton’s Law of Cooling

Rate of cooling of an object is Rate of cooling of an object is proportional to the proportional to the temperature difference temperature difference between an object and its between an object and its surroundingssurroundings

Works both ways, cooling and heatingWorks both ways, cooling and heating Rate of heating also depends on the Rate of heating also depends on the

temperature differencetemperature difference

Phases of MatterPhases of Matter

Four StatesFour States SolidSolid LiquidLiquid GasGas PlasmaPlasma

SolidsSolids

Atoms or molecules are well ordered Atoms or molecules are well ordered into a crystalline lattice for most into a crystalline lattice for most materialsmaterials

Exceptions are things like glass which Exceptions are things like glass which is really a very slow flowing liquidis really a very slow flowing liquid

Metallic Solids don’t need all their Metallic Solids don’t need all their electrons for bondingelectrons for bonding

Insulators need all their electrons for Insulators need all their electrons for bondingbonding

LiquidsLiquids

Pretty incompressiblePretty incompressible Atoms or molecules loosely bonded Atoms or molecules loosely bonded

to neighbors, but in close proximityto neighbors, but in close proximity Allows fluid flowAllows fluid flow

GasesGases

CompressibleCompressible Atoms or molecules are widely Atoms or molecules are widely

separated and have little interaction separated and have little interaction with each other aside from elastic with each other aside from elastic collisionscollisions

PlasmasPlasmas

Temperature is so high that outer Temperature is so high that outer electrons are separated from their electrons are separated from their underlying atomsunderlying atoms

No molecules existNo molecules exist Particles are electrically chargedParticles are electrically charged Sun, stars, inside a fluorescent bulbSun, stars, inside a fluorescent bulb

Changing StatesChanging States

For a given substance, all four states For a given substance, all four states are possibleare possible

Increase temperature and a PHASE Increase temperature and a PHASE CHANGE occursCHANGE occurs

Solid > Liquid > Gas > PlasmaSolid > Liquid > Gas > Plasma

Phase ChangesPhase Changes

EvaporationEvaporation CondensationCondensation BoilingBoiling MeltingMelting FreezingFreezing

Water MoleculeWater Molecule

Two Hydrogen Atoms, One Oxygen Two Hydrogen Atoms, One Oxygen AtomAtom

++

- -

EvaporationEvaporation

EvaporationEvaporation

Liquid cools because it loses higher Liquid cools because it loses higher energy molecules or atomsenergy molecules or atoms

Air cools because the molecules Air cools because the molecules leaving give up a lot of energy to leaving give up a lot of energy to break free, so they have much lower break free, so they have much lower energy after they gain their freedomenergy after they gain their freedom

Solid > Gas directly is sublimationSolid > Gas directly is sublimation

CondensationCondensation

CondensationCondensation

The opposite of evaporationThe opposite of evaporation Evaporation and Condensation are a Evaporation and Condensation are a

SURFACE phenomenonSURFACE phenomenon Interaction of molecules at the Interaction of molecules at the

liquid-air interfaceliquid-air interface

BoilingBoiling

Molecules break free of the liquid Molecules break free of the liquid bonds and form little bubblesbonds and form little bubbles

If the vapor pressure in the bubble is If the vapor pressure in the bubble is large enough to resist the pressure large enough to resist the pressure of the liquid, and the air pressure of the liquid, and the air pressure above the interface they rise and above the interface they rise and escape to the outsideescape to the outside

Remember, liquid is cooling and air Remember, liquid is cooling and air is coolingis cooling

BoilingBoiling

BoilingBoiling

Melting and FreezingMelting and Freezing

Change between liquid and solidChange between liquid and solid Bonds between molecules break Bonds between molecules break

when solid meltswhen solid melts Takes energy to break the bondsTakes energy to break the bonds Freezing is the opposite of meltingFreezing is the opposite of melting Molecules move slowly enough that Molecules move slowly enough that

bonds can formbonds can form

Melting and FreezingMelting and Freezing

Why does salt melt ice?Why does salt melt ice?

Energy and Phase Energy and Phase ChangeChange

T

Time

Adding heat at a constant rate


T

Time

Adding heat at a constant rate

Melting

Boiling



For Pure Water

334 joules/gram 2256 joules/gram

Boiling PotatoesBoiling Potatoes

So, when you want garlic mashed So, when you want garlic mashed potatoes and you take potatoes and potatoes and you take potatoes and cut them up and put them in boiling cut them up and put them in boiling water, do they cook faster when the water, do they cook faster when the water is really bubbling compared to water is really bubbling compared to when the water is just barely when the water is just barely bubbling???bubbling???

Boiling PotatoesBoiling Potatoes

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Chapter 8 Heat Transfer & Change of Phase. Heat Transfer How does the energy move from a hotter to a colder object? How does the energy move from a hotter