Transmission of HeatTransmission of Heat

ConductionConduction

Heat transfer due to direct contactHeat transfer due to direct contact Either between different materials in thermal Either between different materials in thermal

contact or different parts of the same contact or different parts of the same materialmaterial

Materials that conduct heat well are good Materials that conduct heat well are good conductorsconductors, usually metals, usually metals

Collisions between atoms, molecules, and Collisions between atoms, molecules, and electrons transfer energyelectrons transfer energy

ConductionConduction

Metals make good conductors because of Metals make good conductors because of many “loose” outer electronsmany “loose” outer electrons

Materials that don’t conduct well are Materials that don’t conduct well are insulatorsinsulators

Liquids & gases usually good insulatorsLiquids & gases usually good insulators Insulating materials often have enclosed air Insulating materials often have enclosed air

spaces to delay heat transferspaces to delay heat transfer

ConvectionConvection

Heat transfer due to current in a fluidHeat transfer due to current in a fluid Warm fluid actually moves, changes Warm fluid actually moves, changes

positionposition Warmer areas expand, are less dense and Warmer areas expand, are less dense and

riserise Convection currents create wind, weather Convection currents create wind, weather

patterns, sea breezespatterns, sea breezes

RadiationRadiation

Transfer of energy by electromagnetic Transfer of energy by electromagnetic waveswaves

Can transfer heat through vacuum of spaceCan transfer heat through vacuum of space Objects emit energy depending on temp -- Objects emit energy depending on temp --

low temp means long waves, low energylow temp means long waves, low energy High temp objects emit high energy short High temp objects emit high energy short

waveswaves

RadiationRadiation

Infrared waves are absorbed by our skin and Infrared waves are absorbed by our skin and produce sensation of heatproduce sensation of heat

Heat lamps emit infrared radiationHeat lamps emit infrared radiation Hotter objects emit visible light as well, Hotter objects emit visible light as well,

starting at red (500 starting at red (500 ooC)C) moving to white moving to white hot (1200 hot (1200 ooC)C) as more colors are added to as more colors are added to radiationradiation

Absorption of Radiant EnergyAbsorption of Radiant Energy

Radiation either absorbed or reflected Radiation either absorbed or reflected Good absorbers are poor reflectors and Good absorbers are poor reflectors and

appear darkappear dark Good reflectors appear light, absorb little Good reflectors appear light, absorb little

energy and remain coolenergy and remain cool

Emission of Radiant EnergyEmission of Radiant Energy

Good absorbers are good emitters of energy Good absorbers are good emitters of energy and vice versaand vice versa

Dark objects absorb and emit energy wellDark objects absorb and emit energy well Rate of cooling is proportional to temp Rate of cooling is proportional to temp

difference with surroundings: difference with surroundings: Newton’s Newton’s Law of CoolingLaw of Cooling

Same law applies to heatingSame law applies to heating

Greenhouse EffectGreenhouse Effect

Earth absorbs radiant energy from sunEarth absorbs radiant energy from sun Earth emits terrestrial radiation into spaceEarth emits terrestrial radiation into space If rates of absorption and radiation are If rates of absorption and radiation are

equal, temp remains constantequal, temp remains constant Greenhouse effectGreenhouse effect is effect of atmospheric is effect of atmospheric

gases on balance of absorption-emissiongases on balance of absorption-emission

Greenhouse EffectGreenhouse Effect

High energy short waves from sun reach High energy short waves from sun reach earth and are absorbedearth and are absorbed

Earth emits longer waves (because it’s Earth emits longer waves (because it’s cooler)cooler)

Longer waves are absorbed by carbon Longer waves are absorbed by carbon dioxide and water vapor in atmospheredioxide and water vapor in atmosphere

Energy is trapped by atmosphere and earth Energy is trapped by atmosphere and earth warmswarms

Greenhouse EffectGreenhouse Effect

If carbon dioxide levels increase, global If carbon dioxide levels increase, global warming can occurwarming can occur

Effect is similar in greenhouse -- glass is Effect is similar in greenhouse -- glass is transparent to short waves from sun but transparent to short waves from sun but reflect long waves back into greenhouse reflect long waves back into greenhouse