WoodstovesWoodstoves: super efficient fires that you can use inside
•Why does your back get cold?•What is heat and temperature?
•Why is it so warm above the stove / fire?
•Why can you hold a poker / skewer?
New ideas for todayNew ideas for today•Heat and temperature•Convection, Conduction, Radiation•Specific heat
OutlineOutline• What is temperature and heat?• How is thermal energy transferred?• What happens when something is
heated?
1848William Thomsona.k.a. Lord Kelvin
0 K
supernova core100,000,000,000 K
core of sun15,000,000 Ksurface of sun
6,000 Klava
1,200 K
ice273 K
Room temperature294 K
dry ice164 K
liquid nitrogen77 K
liquid He4 K dilution refrigerator
0.003 K
Universe2.7 K
triple point cell273.16 K
absolute zero
Thermal EnergyThermal 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.
HeatHeat
• 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
Heat and TemperatureHeat 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).
Clicker questionClicker questionWhat happens when I touch a copper rod stuck in the dewar of liquid Nitrogen?
heat cold heat
(A) (B) (C)
OutlineOutline• What is temperature and heat?• How is thermal energy transferred?• What happens when something is
heated?
How could you heat your house using fire?How could you heat your house using fire?
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!
Open fire Fireplace
WoodstoveWoodstove
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
Heat Transfer MechanismsHeat Transfer Mechanisms• Conduction:Conduction:
heat flow through materials• Convection:Convection:
heat flow through moving fluids• Radiation:Radiation:
heat flow through light
All three transfer heat All three transfer heat from hot to coldfrom hot to cold
ConductioConductionn
Cu+
Cu+
Cu+
Cu+
Cu+
Cu+
Cu+
Cu+
Cu+
Cu+Cu+
Cu+
Cu+ Cu+
Cu+
Cu+ Cu+-
-
-
--
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ConductionConduction
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
Rods and marblesRods and marbles
Melting iceMelting ice
ConvectioConvectionn
Hot air balloonHot air balloon
Convection screenConvection screen
Convection of waterConvection of water
Convection of airConvection of air
ConvectionConvection• 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”…
RadiatioRadiationn• Heat transferred by
electromagnetic waves(radio, infrared, microwaves, light, …)
• Higher temperature yields more radiated heat
Infrared radiation
Electric heaterElectric heater
Light the matchLight the match
WoodstovesWoodstoves• ConductionConduction
– moves heat through the stove’s metal walls
• ConvectionConvection– circulates hot air around the room
• RadiationRadiation– transfers heat directly as light
Clicker questionClicker question
Which would make the best thermos?
(A) Black interior, space between flask and case filled with air
(B) Black interior, evacuated space
(C) Shiny interior, gas-filled space(D) Shiny interior, evacuated
space
OutlineOutline• What is temperature and heat?• How is thermal energy transferred?• What happens when something is
heated?
Specific HeatSpecific Heat
•Heat required per kg to raise temperature by 1 C
•Material property
Water has a very high specific heat!
4190J
kg K4 times bigger than aluminum
Try heating an empty vs full pot
Specific heat of HSpecific heat of H22OO
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