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
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