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7.1 Woodstoves A new topic: Thermodynamics!
A new topic: Thermodynamics!
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Woodstoves: super efficient fires that you can use inside
Why is it so warm above the stove / fire? Why can you hold a poker / skewer? Why does your back get cold? What is heat and temperature?
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New ideas for today Heat and temperature
Convection, Conduction, Radiation Specific heat
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Outline What is temperature and heat? How is thermal energy transferred? What happens when something is heated?
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Temperature Hot gas Cold gas Temperature is a
Demo: freeze things in LN2 Demo: ball and ring Temperature is a measure of thermal energy
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dilution refrigerator
supernova core 100,000,000,000 K core of sun 15,000,000 K surface of sun 6,000 K lava 1,200 K Room temperature 294 K triple point cell K ice 273 K liquid nitrogen 77 K dry ice 164 K 1848 William Thomson a.k.a. Lord Kelvin liquid He 4 K 0 K dilution refrigerator 0.003 K absolute zero Universe 2.7 K
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Thermal Energy is “disordered” energy gives rise to temperature
is actually the kinetic and potential energies of constituent particles does not include “ordered” energies: The kinetic energy of the bulk object moving or rotating. The potential energy of outside interactions.
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Temperature heat Tells you which way heat will flow.
demo: set up thermal conductivity and rods in LN2 Tells you which way heat will flow.
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Heat Is the energy that flows between objects because of their difference in temperatures Heat is thermal energy on the move Technically: Objects don’t contain heat
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Heat and Temperature Touching objects causes an exchange of thermal energy Microscopically, energy flows both ways On average, energy flows one way Temperature predicts energy flow direction Energy flows from hotter to colder If there is no flow of thermal energy, then objects are in thermal equilibrium (same temperature).
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Rods in liquid N2 Ball and ring Liquid N2 fun
Demo: thermal conductivity rods
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Clicker question What happens when I touch a copper rod stuck in the dewar of liquid Nitrogen? heat cold heat (A) (B) (C)
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Outline What is temperature and heat? How is thermal energy transferred? What happens when something is heated?
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How could you heat your house using fire?
Open fire Fireplace Do I really have to tell you the problem with this? Good: smoke mostly leaves, house not on fire, heat flows to room Bad: house actually gets colder!
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Woodstove Good: Heat flows into room Smoke leaves Airflow controlled
A woodstove is a heat exchanger Transfers heat without transferring smoke Most of the heat removed before smoke leaves
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Heat Transfer Mechanisms
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Heat Transfer Mechanisms
Conduction: heat flow through materials Convection: heat flow through moving fluids Radiation: heat flow through light All three transfer heat from hot to cold
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Conduction Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+ Cu+
- Cu+ Cu+ Conduction - Cu+ - - Cu+ Cu+ - Cu+ - - - Cu+ Cu+ Cu+ - - Cu+ - Cu+ Cu+ - - Cu+ Cu+ Cu+ - - Demo: conduction: wood vs copper Demo: four fingers - Cu+ - Cu+
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Conduction Heat flows through material, atoms don’t! In an insulator:
Rods and marbles Melting ice Heat flows through material, atoms don’t! In an insulator: adjacent atoms jiggle one another atoms do work, yielding microscopic exchanges of energy (typically slow) In a conductor, mobile electrons carry heat heat flows quickly, over long distances via mobile electrons Demo: amazing ice melting blocks
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Convection Hot air balloon Convection screen Convection of water
Convection of air Demo: convection: chimney model Demo: convection tube
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Convection Fluid transports heat (thermal energy stored in atoms)
Fluid warms up near a hot object Fluid cools down near a cold object Natural buoyancy* drives convection Warmed fluid rises away from hot object Cooled fluid descends away from cold object *”Lighter things float”… Demo: hot air balloon Demo: test tube with water held over bunsen burner
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Electric heater Radiation Light the match Heat transferred by electromagnetic waves (radio, infrared, microwaves, light, …) Higher temperature yields more radiated heat Demo: light match Demo: electric heater Infrared radiation
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Woodstoves Conduction moves heat through the stove’s metal walls
Convection circulates hot air around the room Radiation transfers heat directly as light
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Which would make the best thermos?
Clicker question Which would make the best thermos? Black interior, space between flask and case filled with air Black interior, evacuated space Shiny interior, gas-filled space Shiny interior, evacuated space
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Outline What is temperature and heat? How is thermal energy transferred? What happens when something is heated?
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Specific Heat Heat required per kg to raise temperature by 1 C
Specific heat of H2O Specific Heat Heat required per kg to raise temperature by 1 C Material property Try heating an empty vs full pot Water has a very high specific heat! Demo: boil water in paper cup 4 times bigger than aluminum 4190
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cool hot Hot water bottles – nice and toasty for a long time
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For next class: Read Section 7.2
See you next class! Demo: fire-extinguisher rocket
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