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Woodstoves Woodstoves and and

lightbulbslightbulbs

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Introductory QuestionIntroductory Question

Which is more effective at heating Which is more effective at heating a room:a room:

A.A. a black woodstovea black woodstove

B.B. a shiny chrome-plated woodstovea shiny chrome-plated woodstove

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Question 1Question 1

What are thermal energy and heat?What are thermal energy and heat?

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Having Thermal EnergyHaving Thermal Energy

Thermal energy isThermal energy is disordered energy within an objectdisordered energy within an object kinetic and potential energies of atomskinetic and potential energies of atoms is responsible for temperatureis responsible for temperature

Thermal energy doesn’t include Thermal energy doesn’t include order energiesorder energies kinetic energy of an object moving or kinetic energy of an object moving or

rotatingrotating potential energy of outside interactionspotential energy of outside interactions

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Transferring HeatTransferring Heat

Heat isHeat is energy that flows between objects energy that flows between objects

because of their difference in because of their difference in temperaturetemperature

thermal energy on the movethermal energy on the move

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Question 2Question 2

How does a woodstove produce How does a woodstove produce thermal energy?thermal energy?

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Burning WoodBurning Wood

Fire releases chemical potential Fire releases chemical potential energyenergy Wood and air consist of moleculesWood and air consist of molecules Molecules are bound by chemical bondsMolecules are bound by chemical bonds When bonds rearrange, they can When bonds rearrange, they can

release energyrelease energy Burning rearranges bonds and releases Burning rearranges bonds and releases

energy!energy!

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Chemical Forces and Chemical Forces and BondsBonds

Atoms interact via electromagnetic forcesAtoms interact via electromagnetic forces The chemical forces between two atoms The chemical forces between two atoms

areare attractive at long distancesattractive at long distances repulsive at short distancesrepulsive at short distances zero at a specific equilibrium separationzero at a specific equilibrium separation

Atoms at the equilibrium separationAtoms at the equilibrium separation are in a stable equilibriumare in a stable equilibrium and are bound together by an energy deficitand are bound together by an energy deficit

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Chemical ReactionsChemical Reactions

Breaking old bonds takes workBreaking old bonds takes work Forming new bonds does workForming new bonds does work If new bonds are stronger than old,If new bonds are stronger than old,

chemical potential energy chemical potential energy thermal thermal energyenergy

Breaking old bonds requires energyBreaking old bonds requires energy reaction requires activation energy to reaction requires activation energy to

startstart

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Question 3Question 3

Why does heat flow from the stove to Why does heat flow from the stove to the room?the room?

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Heat and TemperatureHeat and Temperature

Heat always flows from hotter to colderHeat always flows from hotter to colder This flow direction is an overall statistical This flow direction is an overall statistical

resultresult Microscopically, thermal energy moves both Microscopically, thermal energy moves both

waysways At thermal equilibriumAt thermal equilibrium

the temperatures of the objects are equalthe temperatures of the objects are equal and no heat flows between those objectsand no heat flows between those objects

Temperature is approximately the average Temperature is approximately the average thermal kinetic energy per particlethermal kinetic energy per particle

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Question 4Question 4

Why is a woodstove better than an Why is a woodstove better than an open fire?open fire?

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An Open FireAn Open Fire

Burns wood to release thermal Burns wood to release thermal energyenergy

It has good features:It has good features: Heat flows from hot fire to cold roomHeat flows from hot fire to cold room

But it also has bad features:But it also has bad features: Smoke enters roomSmoke enters room Fire uses up room’s oxygenFire uses up room’s oxygen Can set fire to roomCan set fire to room

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A WoodstoveA Woodstove

Burns wood to release thermal Burns wood to release thermal energyenergy

It has good features:It has good features: Heat flows from hot fire to cold roomHeat flows from hot fire to cold room All the smoke goes up chimney pipeAll the smoke goes up chimney pipe New oxygen enters room through New oxygen enters room through

crackscracks Relatively little fire hazardRelatively little fire hazard Transfers heat efficiently to roomTransfers heat efficiently to room

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Question 5Question 5

How does a woodstove heat the How does a woodstove heat the room?room?

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Heat Transfer Heat Transfer MechanismsMechanisms

ConductionConduction: heat flow through : heat flow through materialsmaterials

ConvectionConvection: heat flow via moving : heat flow via moving fluidsfluids

RadiationRadiation: heat flow via light waves: heat flow via light waves All three transfer heat from hot to All three transfer heat from hot to

coldcold

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Conduction and Conduction and WoodstovesWoodstoves

Heat flows but atoms don’tHeat flows but atoms don’t In an insulator,In an insulator,

adjacent atoms jiggle one anotheradjacent atoms jiggle one another atoms do work and exchange energiesatoms do work and exchange energies on average, heat flows from hot to cold atomson average, heat flows from hot to cold atoms

In a conductor,In a conductor, mobile electrons carry heat long distancesmobile electrons carry heat long distances heat flows quickly from hot to cold spotsheat flows quickly from hot to cold spots

Conduction moves heat through stove’s Conduction moves heat through stove’s wallswalls

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Convection and Convection and WoodstovesWoodstoves

Fluid transports heat stored in its Fluid transports heat stored in its atomsatoms Fluid warms up near a hot objectFluid warms up near a hot object Flowing fluid carries thermal energy Flowing fluid carries thermal energy

with itwith it Fluid cools down near a cold objectFluid cools down near a cold object Overall, heat flows from hot to coldOverall, heat flows from hot to cold

Convection circulates hot air around Convection circulates hot air around the roomthe room

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Radiation and Radiation and WoodstovesWoodstoves

Heat flows by electromagnetic wavesHeat flows by electromagnetic waves(radio waves, microwaves, light, …)(radio waves, microwaves, light, …)

Wave types depend on temperatureWave types depend on temperature cold: radio wave, microwaves, infrared lightcold: radio wave, microwaves, infrared light hot: infrared, visible, and ultraviolet light hot: infrared, visible, and ultraviolet light

Higher temperature Higher temperature more radiated heat more radiated heat Black emits and absorbs light bestBlack emits and absorbs light best

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Stefan-Boltzmann LawStefan-Boltzmann Law

The amount of heat a surface The amount of heat a surface radiatesradiates is is

where emissivity is emission efficiencywhere emissivity is emission efficiency EmissivityEmissivity

0 is worst efficiency: white, shiny, or clear0 is worst efficiency: white, shiny, or clear 1 is best efficiency: black1 is best efficiency: black

Radiation transfers heat to your skin as lightRadiation transfers heat to your skin as light

4

power emissivity Stefan-Boltzmann constant

temperature surface area

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Introductory Question Introductory Question (Revisited)(Revisited)

Which is more effective at heating Which is more effective at heating a room:a room:

A.A. a black woodstovea black woodstove

B.B. a shiny chrome-plated woodstovea shiny chrome-plated woodstove

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Thermal RadiationThermal Radiation

All materials emit thermal radiationAll materials emit thermal radiation All materials contain electric chargesAll materials contain electric charges Thermal energy causes those charges Thermal energy causes those charges

accelerateaccelerate Accelerating charges emit Accelerating charges emit

electromagnetic waveselectromagnetic waves Hotter temperatures yield shorter Hotter temperatures yield shorter

wavelengthswavelengths

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An Incandescent An Incandescent LightbulbLightbulb

Light is emitted by a hot tungsten Light is emitted by a hot tungsten filamentfilament Electric wires deliver power to the Electric wires deliver power to the

filamentfilament Glass bulb protects the filamentGlass bulb protects the filament Inert gas fill prolongs filament’s lifeInert gas fill prolongs filament’s life

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Black Body SpectrumBlack Body Spectrum

The spectrum and intensity of The spectrum and intensity of electromagnetic waves from a black body electromagnetic waves from a black body depend only on its temperaturedepend only on its temperature

TemperatureTemperature

determines the color.determines the color. Visible efficiencyVisible efficiency

increses with temp.increses with temp. Bulb’s lifetimeBulb’s lifetime

decreses with temp.decreses with temp.

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What determines a lightbulb’s brightness?What determines a lightbulb’s brightness?

• Lightbulb filament maintains zero net powerLightbulb filament maintains zero net power•Its thermal power out must equal Its thermal power out must equal electrical power inelectrical power in•Its radiated power increases with its Its radiated power increases with its electrical powerelectrical power•Its radiated power increases with its Its radiated power increases with its surface areasurface area

• Higher wattage bulbsHigher wattage bulbs• use larger filaments with more surface use larger filaments with more surface areaarea• maintain the usual 2500 maintain the usual 2500 C filament C filament temperaturetemperature• and radiate more visible light as a resultand radiate more visible light as a result

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Why Why do lightbulbs eventually do lightbulbs eventually burn-out?burn-out?

Filament requirements are challengingFilament requirements are challenging Filament must remain solid to high Filament must remain solid to high

temperaturestemperatures Filament must experience minimal sublimationFilament must experience minimal sublimation Filament must be electrically conductingFilament must be electrically conducting

Tungsten metal is the best filament materialTungsten metal is the best filament material Tungsten remains solid to 3422 °CTungsten remains solid to 3422 °C Tungsten sublimes relatively slowly at 2500 °CTungsten sublimes relatively slowly at 2500 °C Tungsten conducts electricityTungsten conducts electricity

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SummarySummary

Woodstoves use all three heat transfer Woodstoves use all three heat transfer mechanismsmechanisms

Have tall chimneys for heat exchangeHave tall chimneys for heat exchange Are black to encourage radiationAre black to encourage radiation Are sealed to keep smoke out of room airAre sealed to keep smoke out of room air

Lightbulbs emit visible thermal radiationLightbulbs emit visible thermal radiation Most of their thermal radiation is invisibleMost of their thermal radiation is invisible They fail when the filament sublimes awayThey fail when the filament sublimes away