Heat TransferHeat Transfer

Chapter 22Chapter 22

ConductionConduction Conduction –Conduction – energy transfer from particle to energy transfer from particle to

particle within certain materials, or from one particle within certain materials, or from one material to another when the two are in direct material to another when the two are in direct contactcontact

Conductors –Conductors – materials that conduct heat well materials that conduct heat well Metals are the best conductors (silver, copper, Metals are the best conductors (silver, copper,

aluminum, and iron)aluminum, and iron) Materials composed if atoms with “loose” outer Materials composed if atoms with “loose” outer

electrons are good conductors of heatelectrons are good conductors of heat Insulators –Insulators – materials which delay the transfer materials which delay the transfer

of heat (wood, wool, straw, paper, cork, and of heat (wood, wool, straw, paper, cork, and Styrofoam)Styrofoam)

Cold is simply the absence of heat, only heat is Cold is simply the absence of heat, only heat is transferred through a conductor or insulatortransferred through a conductor or insulator

ConvectionConvection

Convection –Convection – a means of heat transfer a means of heat transfer by movement of the heated substance by movement of the heated substance itself, such as by currents in a fluiditself, such as by currents in a fluid

Convection occurs in all fluids, whether Convection occurs in all fluids, whether liquid or gasliquid or gas

Convection is occurring all around you, Convection is occurring all around you, the atmosphere, the ocean, Earth, the the atmosphere, the ocean, Earth, the sun!sun!

As warm air rises, it expands and coolsAs warm air rises, it expands and cools When the air has been cooled, it will When the air has been cooled, it will

sink back down again and warm up …sink back down again and warm up …

ConvectionConvection

RadiationRadiation

Radiation –Radiation – Energy transmitted by Energy transmitted by electromagnetic waves (i.e. the sun)electromagnetic waves (i.e. the sun)

Radiant Energy –Radiant Energy – any energy, any energy, including heat, that is transmitted by including heat, that is transmitted by radiationradiation

All objects continually emit radiant All objects continually emit radiant energy in a mixture of wavelengthsenergy in a mixture of wavelengths

The lower the temperature, the longer The lower the temperature, the longer the wavelengththe wavelength

Three Mechanisms of Heat Three Mechanisms of Heat TransferTransfer

Absorption of Radiant Absorption of Radiant EnergyEnergy

Absorption and reflection are Absorption and reflection are opposite processes, a good absorber opposite processes, a good absorber of radiant energy will reflect only a of radiant energy will reflect only a little amount of radiant energylittle amount of radiant energy

A perfect absorber will reflect no A perfect absorber will reflect no radiant energy and appear blackradiant energy and appear black

Good reflectors are poor absorbers of Good reflectors are poor absorbers of radiant energyradiant energy

Emission of Radiant EnergyEmission of Radiant Energy

Good absorbers are also good Good absorbers are also good emitters; poor absorbers are poor emitters; poor absorbers are poor emittersemitters

If a good absorber was not also a If a good absorber was not also a good emitter, then black objects good emitter, then black objects would remain warmer than lighter would remain warmer than lighter colored objects and never come to colored objects and never come to thermal equilibrium with themthermal equilibrium with them

Each object is emitting as much Each object is emitting as much energy as it is absorbingenergy as it is absorbing

Absorption and Emission of Absorption and Emission of Radiant EnergyRadiant Energy

Newton’s Law of CoolingNewton’s Law of Cooling The rate of cooling of an object depends The rate of cooling of an object depends

on how much hotter the object is than the on how much hotter the object is than the surroundingssurroundings

The rate of cooling of an object – The rate of cooling of an object – whether by conduction, convection, whether by conduction, convection,

or radiation – is approximately or radiation – is approximately proportional to the temperature proportional to the temperature

difference difference ΔΔT T between the object and between the object and its surroundingsits surroundings

Rate of cooling ~ Rate of cooling ~ ΔΔTT Newton’s law of cooling also holds for Newton’s law of cooling also holds for

heatingheating

Global WarmingGlobal Warming Greenhouse Effect – Greenhouse Effect – The warming effect The warming effect

whose cause is that short-wavelength whose cause is that short-wavelength radiant energy from the sun can enter the radiant energy from the sun can enter the atmosphere and be absorbed by Earth atmosphere and be absorbed by Earth more easily than long-wavelength energy more easily than long-wavelength energy from Earth can leavefrom Earth can leave

Earth absorbs the energy from the sun Earth absorbs the energy from the sun through the atmospherethrough the atmosphere

As the atmosphere gets thicker from carbon As the atmosphere gets thicker from carbon dioxide, it will not allow as much energy to dioxide, it will not allow as much energy to escape into spaceescape into space

Terrestrial Radiation –Terrestrial Radiation – Energy that Earth Energy that Earth radiatesradiates

The Greenhouse EffectThe Greenhouse Effect

AssignmentAssignment

Read Chapter 22 (pg. 325-336)Read Chapter 22 (pg. 325-336) Do Chapter 22 #21-30 (pg. 338)Do Chapter 22 #21-30 (pg. 338) Do Appendix F #1-2 (pg. 681)Do Appendix F #1-2 (pg. 681)