7.1 7.1 WoodstovesWoodstoves

A new topic: Thermodynamics!

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?

TemperatureTemperature

Hot gasHot gas Cold gasCold gas

Temperature is a measure of thermal energy

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.

TemperatureTemperature

Tells you which way heatheat will flow.

heat

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).

Rods in liquid NRods in liquid N22

Ball and ringBall and ring

Liquid NLiquid N22 fun fun

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

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+-

-

-

--

-

-

-

-

-

-

-

--

---

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

Hot water bottles – nice and toasty for a long time

hotcool

See you next class!

For next class: Read Section 7.2