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Chapter 16Heat Transfer by Radiation
Greenhouse Effect & Global Warming
Lecture 18
Chapter 17Phase Change
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Emission of Radiant Energy
All objects radiate; higher the temperature, the higher the frequency.
At room temperature, the radiated light is at frequencies too low for our eyes to see.
Special cameras are sensitive to this infrared radiation.
Attics in this house were kept warm for growing marijuana.
98º
75º
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Reflection of Radiant EnergyWhite and “silver” objects reflect light, black objects
and “holes” don’t.
Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out.
White tubes look black inside.
Black objects are also the best emitters of radiation. White objects emit less radiation, and perfectly reflective objects don’t emit at all. (Space blanket.)
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Controlling Heat TransferThermos bottle eliminates conduction
and convection by having double-walled sides with vacuum.
Silvered interior walls minimize heat transfer by radiation.
RadiationIf you are in sunlight, Sun’s radiation will warm you. The intensity of solar radiation is 1000 W/m2. In general, you will not be perfectly perpendicular to the Sun’s rays, and will absorb energy at a rate that depends on your angle to the sun’s rays.
Solar Power Example• You receive average solar power of 300 W/m2. If
you convert the solar power to electrical power with 8% efficiency, how much collecting area do you need to produce 2 kW electrical power?
• Electrical power from sun = (0.08)*300W/m2
= 24 W/m2
• Area needed = 2000 W/(24 W/m2) = 83 m2
SeasonsThis angle effect is also responsible for the seasons.
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Greenhouse EffectGlass is transparent to sunlight (short-wavelength).
Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat.
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Earth’s Greenhouse Effect
Earth’s atmosphere acts as a greenhouse, trapping solar energy.
Most of the trapping is due to carbon dioxide and water vapor, which is why they’re called “greenhouse gasses.”
7-Oct-10 Physics 1 (Garcia) SJSU
Global Temperature VariationsTemperatures increased from 1910 to 1940.Temperatures then cooled for 40 years until they
started rising again in the 1980’s.
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Greenhouse Carbon DioxideOver past 1000 years temperatures nearly
constant until CO2 emissions increased starting with the industrial revolution.
Industrial revolution begins
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Cars & Carbon DioxideOne gallon of gasoline has about 5.2 lb of carbon.
At 26 miles per gallon, that’s 0.2 lb of carbon per mile.
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Anthropogenic* Global WarmingRising temperatures are due to human production of greenhouse gases.
*Caused by humans
Tem
pera
ture
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Consequences of Global Warming
Weather modifications
Species extinctions
Melting of Polar Ice Caps
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Solutions to Global Warming
Many simple, small changes together could make a significant difference.
Efficient Appliances
Building Insulation
Efficient Lighting
Phases of MatterFour Phases of Matter:• Solid• Liquid• Gas• Plasma
Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas).
Ice WaterSteam Plasma
EvaporationEvaporation is a change of phase from liquid to
gas that takes place at the surface of a liquid.
GAS
LIQUID
A random molecule at the surface acquires enough energy to escape the attraction force among the molecules (which holds the liquid together).
Evaporative CoolingBecause only the most energetic molecules can
escape the surface, evaporation removes internal energy from the liquid, that is, evaporation cools.
WETCLOTH
Wet towel cools headWET
TONGUE
Wet tongue cools dog
WETBODY &TOWEL
Wetness cools person
Brr
HEAT
HEATHEAT
Condensation
Condensation is the reverse of evaporation, a change of phase from gas to liquid that takes place at the surface of a liquid.
GAS
LIQUID
A random molecule from the gas strikes the surface and sticks instead of bouncing back into the gas.
Condensation heats.
Hot and HumidA 90 degree day in a
dry climate is more comfortable than a 90 degree day in a humid place. In a dry climate you’re cooled by evaporation; in a wet climate, you’re heated by condensation. Heat index is the apparent temperature a person feels for a given humidity.
Fog & CloudsWarm air rises. As it rises, it expands. As it expands, it
cools. As it cools, vapor molecules condense into water droplets. This forms a cloud (or fog if warm, moist air cools near the ground).
Warm
Cool
Warm breath feels cool when it expands
Water vapor(gas) is invisible
As vapor expands, it coolsand tiny, visible, water droplets (liquid) condense.
Tiny bubbles grow due to evaporation at their surface
BoilingWhen the temperature of a liquid is high enough
that evaporation occurs everywhere, not just the surface, then the liquid boils.
The temperature required depends on the pressure; lower the pressure, the lower the boiling temperature (boiling point).
Demo: Low Pressure Boiling
Water boils at room temperature if the pressure is low.
Cooking at high altitudes is difficult due to this effect; coffee brewed in the mountains always tastes lukewarm.
MeltingMelting is the change
of phase from solid to liquid.
Melting is a cooling process; the solid must absorb heat to melt.
Sublimation
Sublimation is change of phase from solid to gas without passing through liquid phase.
Solid carbon dioxide (dry ice) sublimates at a chilly -109 °F.
Put dry ice into warm water to create dense fog of tiny water droplets.
Freezing
Freezing is the opposite of melting, that is, the change of phase from liquid to solid.
Heat must be removed from a liquid in order to freeze it into a solid.
Lava (liquid) freezes into rock (solid), heating the seawater.
Seawater (liquid) boils into vapor (gas), cooling the lava.
Energy & Changes of Phase
Heats of Fusion & Vaporization
Heating a gram of water
80 cal 100 cal 540 cal 720 calHeat of Fusion
Heat Capacity Heat of Vaporization
Total Energy
Check YourselfIs boiling a cooling or a warming process?Boiling is a cooling process.So can you cool your hand by putting it in
boiling water?NO! Ouch!So why is boiling a cooling process?Because when a liquid boils it cools by itself
releasing its most energetic molecules, just as with cooling by evaporation.
Chapter 18The Laws of Thermodynamics
The First Law of Thermodynamics
The heat energy added to a closed system equals the increase in internal energy of the system plus the work done by the system on its surroundings.
This is the law of conservation of energy, written in a form useful to systems involving heat transfer.
Qin = ∆ (int. energy) + Wby
Heat versus Work• Both heat and work represent the flow of energy from
one system to another. However, heat, as “disordered”energy flow, has some special properties:
• Heat always flows spontaneously from a warmer object to a cooler one, although the opposite would not violate conservation of energy.
• It is easy to convert work to thermal energy via friction (rubbing hands together), but conversion of thermal energy to work requires an engine and can’t be done completely.
• Heat carries entropy (disorder) with it, but work does not.
A lead block weighing 1 kg is dropped from a height of 1m. It does not bounce.
What is the change in thermal energy of the block and floor?
a. 0 b. 4.9 Jc. 9.8 J d. cannot be determined
For isolated system (block+Earth), energy is conserved: Won = 0 Etot = constant
As time proceeds, the energy changes form:
h
vv = 0
Start with v = 0PE = mgh
mgh = ½ mv2
v = (2gh)1/2
mgh = ∆Uthermal = 9.8J
kinetic energyconverted tothermal energy inblock and floor
Potential energy Kinetic energy Thermal energy
Irreversible process
Key Points of Lecture 18Key Points of Lecture 18
Before Monday, read Hewitt Chap. 17.
Homework Assignment #13 is due before 11:00 PM on Tuesday, Oct. 12.
• Heat Transfer by Radiation• Greenhouse Effect & Global Warming• Phase Change • Latent Heat• First Law of Thermodynamics
