2Heat TransferHow does the energy move from a hotter to a colder object?Three mechanismsConductionConvectionRadiation
3Conduction Stir your hot soup with a metal spoon Pretty soon you need a pot holder because the end of the spoon you are holding gets hotThis is heat transfer by conductionEnergy travels up the spoon from the end in the hot soup to the end in your hand
4ConductionWe sense the movement of energy by the increasing temperatureThis means the atoms and molecules have higher average kinetic energyPrimarily occurs by the movement of electrons in the materialThe more easily the electrons can move, the better the conduction
5ConductionMetals have some electrons that are very loosely bound to the atoms in the materialThese electrons can move easily and can rapidly pick up additional kinetic energyMetals are good conductorsWood and plastic don’t have loosely bound electrons, so they are poor conductors
7Conduction Air is a poor thermal conductor If you stand in the sun on a cold winter day and are shielded from the wind, you stay pretty warmSnow is a poor conductor, while water is betterMakes igloos a useful as a house
8Convection A phenomenon in fluids Instead of having energy moved by successive collisions of electrons, atoms and molecules, the fluid itself is set into motion called a currentThese moving fluid currents are convection
10ConvectionWhen the radiator heats the air, it becomes less dense and risesCool air moves in to replace the air that roseThis generates the air flowSo radiators don’t need a fan to stir the air and to distribute heat throughout a roomThe rising air cools until its density matches that of the surrounding air
11ConvectionWe take advantage of the cooling that occurs during an expansionWe make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space
12ConvectionExplains why breezes come from the ocean in the day and from the land at night
13Radiation Energy carried by electromagnetic waves Light, microwaves, radio waves, x-raysWavelength is related to vibration frequency
15RadiationEvery object is emitting electromagnetic waves regardless of temperatureThings we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrumMost things emit primarily in the infraredNight vision goggles, etc.
16Radiation Things also absorb radiation If they didn’t, they would run out of energy to emitGood emitters are also good absorbersEquilibrium established between emission and absorptionWhen something can’t equilibrate, it gets hotter or colder
17Radiation Interior of a car on a sunny day Sunlight comes in as visible lightSeats and interior are much cooler so they radiate in the infrared instead of visibleGlass in the windows blocks infrared so energy can’t get outCar interior heats up!
18Radiation A good absorber reflects very little energy Think about dark pavementA poor absorber reflects a lot of energyThink about snow that doesn’t melt in sunshine even though 1400 watts/meter2 are hitting it
19Radiation At night, objects receive no input energy from the sun But, they are warmer than outer space, so they continue to radiate energyThus, they cool offCan we make ice in the desert without a refrigerator?
20Newton’s Law of Cooling Rate of cooling of an object is proportional to the temperature difference between an object and its surroundingsWorks both ways, cooling and heatingRate of heating also depends on the temperature difference
22SolidsAtoms or molecules are well ordered into a crystalline lattice for most materialsExceptions are things like glass which is really a very slow flowing liquidMetallic Solids don’t need all their electrons for bondingInsulators need all their electrons for bonding
23Liquids Pretty incompressible Atoms or molecules loosely bonded to neighbors, but in close proximityAllows fluid flow
24GasesCompressibleAtoms or molecules are widely separated and have little interaction with each other aside from elastic collisions
25PlasmasTemperature is so high that outer electrons are separated from their underlying atomsNo molecules existParticles are electrically chargedSun, stars, inside a fluorescent bulb
26Changing States For a given substance, all four states are possible Increase temperature and a PHASE CHANGE occursSolid > Liquid > Gas > Plasma
30EvaporationLiquid cools because it loses higher energy molecules or atomsAir cools because the molecules leaving give up a lot of energy to break free, so they have much lower energy after they gain their freedomSolid > Gas directly is sublimation
32Condensation The opposite of evaporation Evaporation and Condensation are a SURFACE phenomenonInteraction of molecules at the liquid-air interface
33BoilingMolecules break free of the liquid bonds and form little bubblesIf the vapor pressure in the bubble is large enough to resist the pressure of the liquid, and the air pressure above the interface they rise and escape to the outsideRemember, liquid is cooling and air is cooling
36Melting and Freezing Change between liquid and solid Bonds between molecules break when solid meltsTakes energy to break the bondsFreezing is the opposite of meltingMolecules move slowly enough that bonds can form
41Energy and Phase Change For Pure Water334 joules/gram2256 joules/gram
42Boiling PotatoesSo, when you want garlic mashed potatoes and you take potatoes and cut them up and put them in boiling water, do they cook faster when the water is really bubbling compared to when the water is just barely bubbling???