Woodstoves 1 Woodstoves

Woodstoves 2 Introductory Question Which is more effective at heating a room: Which is more effective at heating a room: A. a black woodstove B. a shiny chrome-plated woodstove

Woodstoves 3 Observations about Woodstoves They burn wood inside closed fireboxes They burn wood inside closed fireboxes They often have long chimney pipes They often have long chimney pipes They are usually black They are usually black You get burned if you touch them You get burned if you touch them Heat rises off their surfaces Heat rises off their surfaces It feels hot to stand near them It feels hot to stand near them

Woodstoves 4 5 Questions about Wood Stoves What are thermal energy and heat? What are thermal energy and heat? How does a woodstove produce thermal energy? How does a woodstove produce thermal energy? Why does heat flow from the stove to the room? Why does heat flow from the stove to the room? Why is a woodstove better than an open fire? Why is a woodstove better than an open fire? How does a woodstove heat the room? How does a woodstove heat the room?

Woodstoves 5 Question 1 What are thermal energy and heat? What are thermal energy and heat?

Woodstoves 6 Having Thermal Energy Thermal energy is Thermal energy is disordered energy within an object disordered energy within an object kinetic and potential energies of atoms kinetic and potential energies of atoms is responsible for temperature is responsible for temperature Thermal energy doesn’t include order energies Thermal energy doesn’t include order energies kinetic energy of an object moving or rotating kinetic energy of an object moving or rotating potential energy of outside interactions potential energy of outside interactions

Woodstoves 7 Transferring Heat Heat is Heat is energy that flows between objects because of their difference in temperature energy that flows between objects because of their difference in temperature thermal energy on the move thermal energy on the move Technically, objects don’t contain “heat” Technically, objects don’t contain “heat”

Woodstoves 8 Question 2 How does a woodstove produce thermal energy? How does a woodstove produce thermal energy?

Woodstoves 9 Burning Wood Fire releases chemical potential energy Fire releases chemical potential energy Wood and air consist of molecules Wood and air consist of molecules Molecules are bound by chemical bonds Molecules are bound by chemical bonds When bonds rearrange, they can release energy When bonds rearrange, they can release energy Burning rearranges bonds and releases energy! Burning rearranges bonds and releases energy!

Woodstoves 10 Chemical Forces and Bonds Atoms interact via electromagnetic forces Atoms interact via electromagnetic forces The chemical forces between two atoms are The chemical forces between two atoms are attractive at long distances attractive at long distances repulsive at short distances repulsive at short distances zero at a specific equilibrium separation zero at a specific equilibrium separation Atoms at the equilibrium separation Atoms at the equilibrium separation are in a stable equilibrium are in a stable equilibrium and are bound together by an energy deficit and are bound together by an energy deficit

Woodstoves 11 A Few Names Molecule: atoms joined by chemical bonds Molecule: atoms joined by chemical bonds Chemical bond: a chemical-force linkage Chemical bond: a chemical-force linkage Bond strength: the work needed to break bond Bond strength: the work needed to break bond Reactants: starting molecules Reactants: starting molecules Reaction products: ending molecules Reaction products: ending molecules

Woodstoves 12 Chemical Reactions Breaking old bonds takes work Breaking old bonds takes work Forming new bonds does work Forming new bonds does work If new bonds are stronger than old, If new bonds are stronger than old, chemical potential energy  thermal energy chemical potential energy  thermal energy Breaking old bonds requires energy Breaking old bonds requires energy reaction requires activation energy to start reaction requires activation energy to start

Woodstoves 13 When Wood Burns… When you ignite wood, When you ignite wood, the reactants are carbohydrates and oxygen the reactants are carbohydrates and oxygen the reaction products are water and carbon dioxide the reaction products are water and carbon dioxide the activation energy comes from a burning match the activation energy comes from a burning match This reaction releases energy as thermal energy This reaction releases energy as thermal energy

Woodstoves 14 Question 3 Why does heat flow from the stove to the room? Why does heat flow from the stove to the room?

Woodstoves 15 Heat and Temperature Heat always flows from hotter to colder Heat always flows from hotter to colder This flow direction is an overall statistical result This flow direction is an overall statistical result Microscopically, thermal energy moves both ways Microscopically, thermal energy moves both ways At thermal equilibrium At thermal equilibrium the temperatures of the objects are equal the temperatures of the objects are equal and no heat flows between those objects and no heat flows between those objects Temperature is approximately the average thermal kinetic energy per particle Temperature is approximately the average thermal kinetic energy per particle

Woodstoves 16 Question 4 Why is a woodstove better than an open fire? Why is a woodstove better than an open fire?

Woodstoves 17 An Open Fire Burns wood to release thermal energy Burns wood to release thermal energy It has good features: It has good features: Heat flows from hot fire to cold room Heat flows from hot fire to cold room But it also has bad features: But it also has bad features: Smoke enters room Smoke enters room Fire uses up room’s oxygen Fire uses up room’s oxygen Can set fire to room Can set fire to room

Woodstoves 18 A Fireplace Burns wood to release thermal energy Burns wood to release thermal energy It has good features: It has good features: Heat flows from hot fire to cold room Heat flows from hot fire to cold room Smoke goes mostly up chimney Smoke goes mostly up chimney New oxygen enters room through cracks New oxygen enters room through cracks Less likely to set fire on room Less likely to set fire on room And it has bad features: And it has bad features: Inefficient at transferring heat to room Inefficient at transferring heat to room

Woodstoves 19 A Woodstove Burns wood to release thermal energy Burns wood to release thermal energy It has good features: It has good features: Heat flows from hot fire to cold room Heat flows from hot fire to cold room All the smoke goes up chimney pipe All the smoke goes up chimney pipe New oxygen enters room through cracks New oxygen enters room through cracks Relatively little fire hazard Relatively little fire hazard Transfers heat efficiently to room Transfers heat efficiently to room

Woodstoves 20 Heat Exchangers Woodstove is a heat exchanger Woodstove is a heat exchanger Separates air used by the fire from room air Separates air used by the fire from room air Transfers heat without transferring smoke Transfers heat without transferring smoke

Woodstoves 21 Question 5 How does a woodstove heat the room? How does a woodstove heat the room?

Woodstoves 22 Heat Transfer Mechanisms Conduction: heat flow through materials Conduction: heat flow through materials Convection: heat flow via moving fluids Convection: heat flow via moving fluids Radiation: heat flow via light waves Radiation: heat flow via light waves All three transfer heat from hot to cold All three transfer heat from hot to cold

Woodstoves 23 Conduction and Woodstoves Heat flows but atoms don’t Heat flows but atoms don’t In an insulator, In an insulator, adjacent atoms jiggle one another adjacent atoms jiggle one another atoms do work and exchange energies atoms do work and exchange energies on average, heat flows from hot to cold atoms on average, heat flows from hot to cold atoms In a conductor, In a conductor, mobile electrons carry heat long distances mobile electrons carry heat long distances heat flows quickly from hot to cold spots heat flows quickly from hot to cold spots Conduction moves heat through stove’s walls Conduction moves heat through stove’s walls

Woodstoves 24 Convection and Woodstoves Fluid transports heat stored in its atoms Fluid transports heat stored in its atoms Fluid warms up near a hot object Fluid warms up near a hot object Flowing fluid carries thermal energy with it Flowing fluid carries thermal energy with it Fluid cools down near a cold object Fluid cools down near a cold object Overall, heat flows from hot to cold Overall, heat flows from hot to cold Natural buoyancy drives convection Natural buoyancy drives convection Warmed fluid rises away from hot object Warmed fluid rises away from hot object Cooled fluid descends away from cold object Cooled fluid descends away from cold object Convection circulates hot air around the room Convection circulates hot air around the room

Woodstoves 25 Radiation and Woodstoves Heat flows by electromagnetic waves (radio waves, microwaves, light, …) Heat flows by electromagnetic waves (radio waves, microwaves, light, …) Wave types depend on temperature Wave types depend on temperature cold: radio wave, microwaves, infrared light cold: radio wave, microwaves, infrared light hot: infrared, visible, and ultraviolet light hot: infrared, visible, and ultraviolet light Higher temperature  more radiated heat Higher temperature  more radiated heat Black emits and absorbs light best Black emits and absorbs light best

Woodstoves 26 Stefan-Boltzmann Law The amount of heat a surface radiates is The amount of heat a surface radiates is where emissivity is emission efficiency where emissivity is emission efficiency Emissivity Emissivity 0 is worst efficiency: white, shiny, or clear 0 is worst efficiency: white, shiny, or clear 1 is best efficiency: black 1 is best efficiency: black Radiation transfers heat to your skin as light Radiation transfers heat to your skin as light

Woodstoves 27 What About Campfires? No conduction, unless you touch hot coals No conduction, unless you touch hot coals No convection, unless you are above fire No convection, unless you are above fire Lots of radiation: Lots of radiation: your face feels hot your face feels hot your back feels cold your back feels cold

Woodstoves 28 Introductory Question (Revisited) Which is more effective at heating a room: Which is more effective at heating a room: A. a black woodstove B. a shiny chrome-plated woodstove

Woodstoves 29 Summary about Wood Stoves Use all three heat transfer mechanisms Use all three heat transfer mechanisms Have tall chimneys for heat exchange Have tall chimneys for heat exchange Are black to encourage radiation Are black to encourage radiation Are sealed to keep smoke out of room air Are sealed to keep smoke out of room air