Woodstoves Their surfaces are usually dark in color Theyll burn you - PDF document

Woodstoves 1 Woodstoves 2 Observations about Woodstoves  They burn wood in enclosed fireboxes  They often have long chimney pipes Woodstoves  Their surfaces are usually dark in color  They’ll burn you if you touch them  Heat rises off their surfaces  They warm you when you stand near them Turn off all electronic devices Woodstoves 3 Woodstoves 4 5 Questions about Woodstoves Question 1 1. What are thermal energy and heat? Q: What are thermal energy and heat? 2. How does a woodstove produce thermal energy? A: Disordered energy and its transfer mechanism 3. Why does heat flow from the stove to the room?  Thermal energy is 4. Why is a woodstove better than an open fire?  disordered energy within an object’s particles 5. How does a woodstove heat the room?  the kinetic and potential energies of those particles  responsible for temperature  Heat is energy flowing between objects  due to a difference in their temperatures Woodstoves 5 Woodstoves 6 Question 2 Chemical Forces and Bonds Q: How does a woodstove produce thermal energy?  Atoms interact via electromagnetic forces A: It converts chemical energy into thermal energy  The chemical forces between two atoms are  attractive at long distances  Fire releases chemical potential energy  repulsive at short distances  Wood and air consist of molecules  zero at a specific equilibrium separation  Molecules are bound by chemical bonds  Atoms at their equilibrium separation  When bonds rearrange, they can release energy  are in a stable equilibrium  Burning rearranges bonds and releases energy!  are bound together by an energy deficit  Their energy deficit is a chemical bond •1

Woodstoves 7 Woodstoves 8 A Few Names Chemical Reactions  Molecule: atoms joined by chemical bonds  Breaking old bonds takes work  Chemical bond: a chemical-force linkage  Forming new bonds does work  Bond strength: the work needed to break bond  If new bonds are stronger than the old bonds,  chemical potential energy  thermal energy  Reactants: starting molecules  However, breaking old bonds requires energy  Products: ending molecules  reaction requires activation energy to start Woodstoves 9 Woodstoves 10 When Wood Burns… Question 3  When you ignite wood, Q: Why does heat flow from the stove to the room?  the reactants are carbohydrates and oxygen A: Because the stove is hotter than the room  the products are water and carbon dioxide  Heat naturally flows from hotter to colder  the activation energy comes from a burning match  Microscopically, thermal energy moves both ways  This reaction releases energy as thermal energy  Statistically, the net flow is from hotter to colder  At thermal equilibrium, temperatures are equal  no heat flows between those objects  Temperature measures the average thermal kinetic energy per particle (slightly oversimplified) Woodstoves 11 Woodstoves 12 Question 4 Question 5 Q: Why is a woodstove better than an open fire? Q: How does a woodstove heat the room? A: It releases heat, but not smoke, into the room A: It uses all three heat transfer mechanisms  An open fire is energy efficient, but has problems  Those heat transfer mechanisms are  Smoke is released into the room  conduction: heat flows through materials  Fire uses up the room’s oxygen  convection: heat flows via moving fluids  Can set fire to the room  radiation: heat flows via electromagnetic waves  A fireplace is cleaner, safer, but less efficient  All three transfer heat from hot to cold  A woodstove can be clean, safe, and efficient  A woodstove is a heat exchanger  It separates air used by the fire from room air  It transfers heat without transferring smoke •2

Woodstoves 13 Woodstoves 14 Conduction and Woodstoves Convection and Woodstoves  In conduction, heat flows but atoms stay put  In convection, heat flows with a fluid’s atoms  In an insulator,  Fluid warms up near a hot object  Flowing fluid carries thermal energy with it  adjacent atoms jiggle one another  Fluid cools down near a cold object  atoms do work and exchange energies  Overall, heat flows from hot to cold  on average, heat flows from hot to cold atoms  Buoyancy drives natural convection  In a conductor,  Warmed fluid rises away from hot object  mobile electrons carry heat long distances  Cooled fluid descends away from cold object  heat flows quickly from hot to cold spots  Convection circulates hot air around the room  Conduction moves heat through stove’s walls Woodstoves 15 Woodstoves 16 Radiation and Woodstoves Stefan-Boltzmann Law  In radiation, heat flows via electromagnetic waves (radio waves,  Emissivity is a surface’s emission-absorption efficiency microwaves, light, …)  0  perfect inefficiency: white, shiny, or clear  Range of waves depends on temperature  1  perfect efficiency: black  cold: radio wave, microwaves, infrared light  The amount of heat a surface radiates is  hot: infrared, visible, and ultraviolet light  Higher temperature  more radiated heat  Blacker surface  more radiated heat  Black emits and absorbs radiation perfectly where temperature is measured on an absolute scale Woodstoves 17 Woodstoves 18 What About Campfires? Summary about Wood Stoves  No conduction, unless you touch hot coals  Use all three heat transfer mechanisms  No convection, unless you are above fire  Have tall chimneys for heat exchange  Lots of radiation:  Are dark-coated to encourage radiation  your face feels hot because radiation reaches it  Are sealed to keep smoke out of room air  your back feels cold because no radiation reaches it •3