L 18 Thermodynamics [3]L 18 Thermodynamics [3]

Heat transferHeat transfer• convection convection • conductionconduction• radiationradiation

• emitters of radiationemitters of radiation• seeing behind closed doorsseeing behind closed doors• Greenhouse effectGreenhouse effect

Heat CapacityHeat CapacityHow to boil waterHow to boil water

Heat flow

• HEAT the energy that flows from one system to another because of temperature differences.

• But how does it flow? Three ways:• convection• conduction• radiation

Convection• heat is carried from place to place by the

bulk movement of either liquids or gases• does not apply to solids• when water is boiled, hot liquid rises and

mixes with cooler liquid, thus the heat is transferred

• Hot air rises:• want heat into lower level of house (winter) • cooled air into upper levels (summer)

Conduction• heat is transferred directly through a material,

with no bulk movement of stuff• only energy moves

iron is a particularlypoor conductor of heat

heat conduction

HOT COLD

Heat Flow

Cross sectional

area A

L

Heat Flow rate depends on A / L

Thermal Conductivity

• The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivity

• there are good thermal conductors(metals) and poor ones (insulators)

Material Thermalconductivity

Copper 400

Silver 420Stainless steel 14

wood 0.15

glass 0.8

wool 0.04

Goose down 0.025

styrofoam 0.01

Thermal Conductivities of Metals

Metal Thermal Conductivity

(W/mK)

Silver 406

Copper 385

Aluminum 205

Brass 109

Iron 80

Steel 50

Grandma’s silver spoons

radiation

• Radiation is the heat transfer by electromagnetic waves – thermal light waves - invisible to the eyes

• thermal radiation is a small part of the electromagnetic spectrum – waves are characterized by their frequency or wavelength

• different colors in the visible correspond to different wavelengths from red to blue

electromagnetic spectrum

radio waves

microwaves,cell phones

visible x-rays

TV thermalradiation

visible electromagnetic waves: LIGHT

visible lightthermal radiation UV radiation

produces sunburn

shorter wavelength more energy

Thermal Radiation• The warmth you feel from

the sun is the sun’s thermal radiation

• It travels through the vacuum of space to reach earth, no material is necessary (takes 8 minutes)

• you can feel its effects even though you cannot see the radiation.

• you can feel the thermal radiation from a fireplace

What produces thermal radiation?

• all objects whose temperature is above absolute zero emit thermal radiation

• The hotter the object, the more radiation it emits, the amount of radiation is ~ T4

• We all continuously emit thermal radiation• We also absorb it from objects and people

around us• If we just emitted radiation we would

eventually cool to absolute zero!

Emission and Absorption are balanced

Thermal radiation spectrum

• The intensity of radiation increases with temperature

• the color shifts toward the blue at higher temperatures

• The UV radiation from the sun is just beyond the violet (11,000 F)

sources of thermal radiation• the incandescent light bulb

( the ones that have a filament) are sources of both visible light and heat.

• when electricity flows through a wire it gets hot.

• it emits radiation even though you can’t see it

• as it gets hotter it glows red then orange then white

tungsten filament,can get very hot and not melt

evacuated glass bulb

Radiation emitted by hot objects

• The hotter they are, the more they emit

• the efficiency with which an object emits thermal radiation is characterized be a parameter called its emissive e

• e is a number between 0 and 1

• a good emitter has an e close to 1

• a poor emitter has an e close to 0

good emitters are good absorbers

• an object that is a good emitter is also a good absorber of thermal radiation

• a poor emitter is also a poor absorber

• generally dark, dull objects are the best emitters/absorbers

• shinny objects are poor emitters/absorbers

• If you do not want the edges of your pie to burn, you wrap it in aluminum foil

good/bad emitters-Leslie’s cube

copper cubefilled with hot

water

this side ispainted black

infraredradiation sensor

Practical considerations• wear light clothing

in summer light clothing absorbs less sunlight

• cover all body parts in winter warm body parts (like your head) emit radiation

thermal radiation• all objects that are at a temperature above

absolute zero emit thermal radiation (waves)• the higher the temp, the more they emit• the color (wavelength) of the emitted waves

goes from redorangeyellow blue as the temperature increases

seeing behind closed doors

Infrared sensorscan pick up temp-erature differencesof 0.05 degrees C.

we can “see” behind closed doors because of the heat signature left by warm objects on walls

Which one is best?

A.silveredB. silvered and

un-evacuated

C. evacuatedD.un-silvered

andun-evacuated

The Greenhouse effect

• http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/

C O 2

Sun’s visible light

infrared radiation is

trapped

Greenhouse effect• the sun’s visible light can penetrate

through the atmosphere to the earth’s surface where it heats it

• the visible light energy is converted to thermal light energy

• the thermal radiation is reflected from CO2 in the atmosphere

Greenhouse effect

• concentrations of CO2 have been increasing

rise in earth’s temperature

• same effect occurs in your car during the day.

The ozone layer• ozone, O3 is a naturally

occurring trace element in the atmosphere

• It absorbs solar ultraviolet radiation, especially the harmful UV-B rays

• it is destroyed by Cfc’s (chlorofluorocarbons)

• loss affects us and environment

How do I boil water?

• How much heat does it take to boil water?

• Simpler question how much heat is required to raise the temperature of water by so many degrees?

• The answer depends on how much water you have and how hot you want to get it

• The answer would be different for a different material, say aluminum.

Heat Capacity or specific heat• The heat capacity is the amount

of heat that is required to raise the temperature of 1 g of a substance by 1 degree C.

• it is measured in Calories• for water it is 1 cal/g °C• heat Q = m • c • temp changehot plate

specific heatmass of sample

Some heat capacities

Substance Specific heat in cal/g °C

water 1

Ethyl alcohol 0.58

Steel 0.11

Aluminum 0.215

lead 0.03