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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.

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Presentation on theme: "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."— Presentation transcript:

1 Chapter 8 Heat Transfer & Change of Phase

2 Heat Transfer How does the energy move from a hotter to a colder object? How does the energy move from a hotter to a colder object? Three mechanisms Three mechanisms Conduction Conduction Convection Convection Radiation Radiation

3 Conduction Stir your hot soup with a metal spoon 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 hot Pretty soon you need a pot holder because the end of the spoon you are holding gets hot This is heat transfer by conduction This is heat transfer by conduction Energy travels up the spoon from the end in the hot soup to the end in your hand Energy travels up the spoon from the end in the hot soup to the end in your hand

4 Conduction We sense the movement of energy by the increasing temperature We sense the movement of energy by the increasing temperature This means the atoms and molecules have higher average kinetic energy This means the atoms and molecules have higher average kinetic energy Primarily occurs by the movement of electrons in the material Primarily occurs by the movement of electrons in the material The more easily the electrons can move, the better the conduction The more easily the electrons can move, the better the conduction

5 Conduction Metals have some electrons that are very loosely bound to the atoms in the material Metals have some electrons that are very loosely bound to the atoms in the material These electrons can move easily and can rapidly pick up additional kinetic energy These electrons can move easily and can rapidly pick up additional kinetic energy Metals are good conductors Metals are good conductors Wood and plastic dont have loosely bound electrons, so they are poor conductors Wood and plastic dont have loosely bound electrons, so they are poor conductors

6 Conduction

7 Conduction Air is a poor thermal conductor 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 warm If you stand in the sun on a cold winter day and are shielded from the wind, you stay pretty warm Snow is a poor conductor, while water is better Snow is a poor conductor, while water is better Makes igloos a useful as a house Makes igloos a useful as a house

8 Convection A phenomenon in fluids 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 current Instead of having energy moved by successive collisions of electrons, atoms and molecules, the fluid itself is set into motion called a current These moving fluid currents are convection These moving fluid currents are convection

9 Convection

10 Convection When the radiator heats the air, it becomes less dense and rises When the radiator heats the air, it becomes less dense and rises Cool air moves in to replace the air that rose Cool air moves in to replace the air that rose This generates the air flow This generates the air flow So radiators dont need a fan to stir the air and to distribute heat throughout a room So radiators dont need a fan to stir the air and to distribute heat throughout a room The rising air cools until its density matches that of the surrounding air The rising air cools until its density matches that of the surrounding air

11 Convection We take advantage of the cooling that occurs during an expansion We take advantage of the cooling that occurs during an expansion We make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space We make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space

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

13 Radiation Energy carried by electromagnetic waves Energy carried by electromagnetic waves Light, microwaves, radio waves, x-rays Light, microwaves, radio waves, x-rays Wavelength is related to vibration frequency Wavelength is related to vibration frequency

14 Radiation

15 Radiation Every object is emitting electromagnetic waves regardless of temperature Every object is emitting electromagnetic waves regardless of temperature Things we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrum Things we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrum Most things emit primarily in the infrared Most things emit primarily in the infrared Night vision goggles, etc. Night vision goggles, etc.

16 Radiation Things also absorb radiation Things also absorb radiation If they didnt, they would run out of energy to emit If they didnt, they would run out of energy to emit Good emitters are also good absorbers Good emitters are also good absorbers Equilibrium established between emission and absorption Equilibrium established between emission and absorption When something cant equilibrate, it gets hotter or colder When something cant equilibrate, it gets hotter or colder

17 Radiation Interior of a car on a sunny day Interior of a car on a sunny day Sunlight comes in as visible light Sunlight comes in as visible light Seats and interior are much cooler so they radiate in the infrared instead of visible Seats and interior are much cooler so they radiate in the infrared instead of visible Glass in the windows blocks infrared so energy cant get out Glass in the windows blocks infrared so energy cant get out Car interior heats up! Car interior heats up!

18 Radiation A good absorber reflects very little energy A good absorber reflects very little energy Think about dark pavement Think about dark pavement A poor absorber reflects a lot of energy A poor absorber reflects a lot of energy Think about snow that doesnt melt in sunshine even though 1400 watts/meter 2 are hitting it Think about snow that doesnt melt in sunshine even though 1400 watts/meter 2 are hitting it

19 Radiation At night, objects receive no input energy from the sun At night, objects receive no input energy from the sun But, they are warmer than outer space, so they continue to radiate energy But, they are warmer than outer space, so they continue to radiate energy Thus, they cool off Thus, they cool off Can we make ice in the desert without a refrigerator? Can we make ice in the desert without a refrigerator?

20 Newtons Law of Cooling Rate of cooling of an object is proportional to the temperature difference between an object and its surroundings Rate of cooling of an object is proportional to the temperature difference between an object and its surroundings Works both ways, cooling and heating Works both ways, cooling and heating Rate of heating also depends on the temperature difference Rate of heating also depends on the temperature difference

21 Phases of Matter Four States Four States Solid Solid Liquid Liquid Gas Gas Plasma Plasma

22 Solids Atoms or molecules are well ordered into a crystalline lattice for most materials Atoms or molecules are well ordered into a crystalline lattice for most materials Exceptions are things like glass which is really a very slow flowing liquid Exceptions are things like glass which is really a very slow flowing liquid Metallic Solids dont need all their electrons for bonding Metallic Solids dont need all their electrons for bonding Insulators need all their electrons for bonding Insulators need all their electrons for bonding

23 Liquids Pretty incompressible Pretty incompressible Atoms or molecules loosely bonded to neighbors, but in close proximity Atoms or molecules loosely bonded to neighbors, but in close proximity Allows fluid flow Allows fluid flow

24 Gases Compressible Compressible Atoms or molecules are widely separated and have little interaction with each other aside from elastic collisions Atoms or molecules are widely separated and have little interaction with each other aside from elastic collisions

25 Plasmas Temperature is so high that outer electrons are separated from their underlying atoms Temperature is so high that outer electrons are separated from their underlying atoms No molecules exist No molecules exist Particles are electrically charged Particles are electrically charged Sun, stars, inside a fluorescent bulb Sun, stars, inside a fluorescent bulb

26 Changing States For a given substance, all four states are possible For a given substance, all four states are possible Increase temperature and a PHASE CHANGE occurs Increase temperature and a PHASE CHANGE occurs Solid > Liquid > Gas > Plasma Solid > Liquid > Gas > Plasma

27 Phase Changes Evaporation Evaporation Condensation Condensation Boiling Boiling Melting Melting Freezing Freezing

28 Water Molecule Two Hydrogen Atoms, One Oxygen Atom Two Hydrogen Atoms, One Oxygen Atom + + - -

29 Evaporation

30 Evaporation Liquid cools because it loses higher energy molecules or atoms Liquid cools because it loses higher energy molecules or atoms Air cools because the molecules leaving give up a lot of energy to break free, so they have much lower energy after they gain their freedom Air cools because the molecules leaving give up a lot of energy to break free, so they have much lower energy after they gain their freedom Solid > Gas directly is sublimation Solid > Gas directly is sublimation

31 Condensation

32 Condensation The opposite of evaporation The opposite of evaporation Evaporation and Condensation are a SURFACE phenomenon Evaporation and Condensation are a SURFACE phenomenon Interaction of molecules at the liquid-air interface Interaction of molecules at the liquid-air interface

33 Boiling Molecules break free of the liquid bonds and form little bubbles Molecules break free of the liquid bonds and form little bubbles If 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 outside If 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 outside Remember, liquid is cooling and air is cooling Remember, liquid is cooling and air is cooling

34 Boiling Boiling

35 Boiling

36 Melting and Freezing Change between liquid and solid Change between liquid and solid Bonds between molecules break when solid melts Bonds between molecules break when solid melts Takes energy to break the bonds Takes energy to break the bonds Freezing is the opposite of melting Freezing is the opposite of melting Molecules move slowly enough that bonds can form Molecules move slowly enough that bonds can form

37 Melting and Freezing Why does salt melt ice? Why does salt melt ice?

38 Energy and Phase Change T Time Adding heat at a constant rate

39 Energy and Phase Change T Time Adding heat at a constant rate Melting Boiling

40 Energy and Phase Change

41 For Pure Water 334 joules/gram2256 joules/gram

42 Boiling Potatoes So, 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??? So, 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???

43 Boiling Potatoes


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