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
ConductionConduction
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
ConductionConduction
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
ConductionConduction
ConductionConduction
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
ConvectionConvection
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
ConvectionConvection
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
ConvectionConvection
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
Energy and Phase Energy and Phase ChangeChange
T
Time
Adding heat at a constant rate
Melting
Boiling
Energy and Phase Energy and Phase ChangeChange
Energy and Phase Energy and Phase ChangeChange
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