Woodstoves 1

WoodstovesWoodstoves

Woodstoves 2

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

Woodstoves 3

Observations about Observations about WoodstovesWoodstoves

They burn wood inside closed They burn wood inside closed fireboxesfireboxes

They often have long chimney pipesThey often have long chimney pipes They are usually blackThey are usually black You get burned if you touch themYou get burned if you touch them Heat rises off their surfacesHeat rises off their surfaces It feels hot to stand near themIt feels hot to stand near them

Woodstoves 4

5 Questions about Wood 5 Questions about Wood StovesStoves

What are thermal energy and heat?What are thermal energy and heat? How does a woodstove produce How does a woodstove produce

thermal energy?thermal energy? Why does heat flow from the stove to Why does heat flow from the stove to

the room?the room? Why is a woodstove better than an Why is a woodstove better than an

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

Woodstoves 5

Question 1Question 1

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

Woodstoves 6

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

Woodstoves 7

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 Technically, objects don’t contain Technically, objects don’t contain

“heat”“heat”

Woodstoves 8

Question 2Question 2

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

Woodstoves 9

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!

Woodstoves 10

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

Woodstoves 11

A Few NamesA Few Names

Molecule: atoms joined by chemical Molecule: atoms joined by chemical bondsbonds

Chemical bond: a chemical-force Chemical bond: a chemical-force linkagelinkage

Bond strength: the work needed to Bond strength: the work needed to break bondbreak bond

Reactants: starting moleculesReactants: starting molecules Reaction products: ending moleculesReaction products: ending molecules

Woodstoves 12

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

Woodstoves 13

When Wood Burns…When Wood Burns…

When you ignite wood,When you ignite wood, the reactants are carbohydrates and the reactants are carbohydrates and

oxygenoxygen the reaction products are water and the reaction products are water and

carbon dioxidecarbon dioxide the activation energy comes from a the activation energy comes from a

burning matchburning match This reaction releases energy as This reaction releases energy as

thermal energy thermal energy

Woodstoves 14

Question 3Question 3

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

Woodstoves 15

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

Woodstoves 16

Question 4Question 4

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

Woodstoves 17

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

Woodstoves 18

A FireplaceA Fireplace

Burns wood to release thermal energyBurns wood to release thermal energy It has good features:It has good features:

Heat flows from hot fire to cold roomHeat flows from hot fire to cold room Smoke goes mostly up chimneySmoke goes mostly up chimney New oxygen enters room through cracksNew oxygen enters room through cracks Less likely to set fire on roomLess likely to set fire on room

And it has bad features:And it has bad features: Inefficient at transferring heat to roomInefficient at transferring heat to room

Woodstoves 19

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

Woodstoves 20

Heat ExchangersHeat Exchangers

Woodstove is a heat exchangerWoodstove is a heat exchanger Separates air used by the fire from Separates air used by the fire from

room airroom air Transfers heat without transferring Transfers heat without transferring

smokesmoke

Woodstoves 21

Question 5Question 5

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

Woodstoves 22

Heat Transfer Heat Transfer MechanismsMechanisms

Conduction: heat flow through Conduction: heat flow through materialsmaterials

Convection: heat flow via moving Convection: heat flow via moving fluidsfluids

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

coldcold

Woodstoves 23

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

Woodstoves 24

Convection and Convection and WoodstovesWoodstoves

Fluid transports heat stored in its atomsFluid transports heat stored in its atoms Fluid warms up near a hot objectFluid warms up near a hot object Flowing fluid carries thermal energy with itFlowing fluid carries thermal energy with 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

Natural buoyancy drives convectionNatural buoyancy drives convection Warmed fluid rises away from hot objectWarmed fluid rises away from hot object Cooled fluid descends away from cold objectCooled fluid descends away from cold object

Convection circulates hot air around the Convection circulates hot air around the roomroom

Woodstoves 25

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

Woodstoves 26

Stefan-Boltzmann LawStefan-Boltzmann Law

The amount of heat a surface radiates isThe amount of heat a surface radiates 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

Woodstoves 27

What About Campfires?What About Campfires?

No conduction, unless you touch hot No conduction, unless you touch hot coalscoals

No convection, unless you are above No convection, unless you are above firefire

Lots of radiation:Lots of radiation: your face feels hotyour face feels hot your back feels coldyour back feels cold

Woodstoves 28

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

Woodstoves 29

Summary about Wood Summary about Wood StovesStoves

Use all three heat transfer Use all three heat transfer mechanismsmechanisms

Have tall chimneys for heat Have tall chimneys for heat exchangeexchange

Are black to encourage radiationAre black to encourage radiation Are sealed to keep smoke out of Are sealed to keep smoke out of

room airroom air