PH 104 w/ dr.G
Lec 18
HEAT TRANSFER
Conduction
How does fur keep animals warm?
Convection
How do we get sea breezes and land breezes?
Radiation Thermos: What is the purpose of the mirrormirror-like interior?
Newton’
Newton’s Law of Cooling How do they get time of death?
The Greenhouse Effect What are “greenhouse gasses”
gasses”?
HEAT TRANSFER
Conduction
HEAT TRANSFER
Conduction
y = heat transfer by contact : heat flow
through (static) material
y RATE of heat flow: depends on
quantity and composition:
y High rate: Good conductor
(poor insulator)
y Good conductors of electricity
(example: metals)
HEAT TRANSFER
How does fur keep animals warm?
y = heat transfer by contact : heat flow
through (static) material
y RATE of heat flow: depends on
quantity and composition:
Convection
y Occurs in liquids and gasses
y In atmosphere:
y “Hot air rises”:
y Hot air expands: for constant pressure,
temp ~ vol
y Expansion means
y Poor electrical conductors
(examples: wood, ice, vacuum)
y Air: captured by fur, wool,
feathers, fiberglass insulation
HEAT TRANSFER
How do we get sea breezes and land breezes?
y = heat transfer by transfer of material: in fluids
y Low rate: Good insulator (poor
conductor)
Convection
How does fur keep animals warm?
y Less-dense hot air: feels buoyant force!
y Cooler air replaces rising hot air
y Rising hot air, moving cool air =
HEAT TRANSFER
How do we get sea breezes and land breezes?
y = heat transfer by transfer of material: in fluids
Radiation
Thermos: What is the purpose of the mirror-like interior?
y = heat transfer via radiation (radiant energy) (not
radioactivity)
y = spreadth of
y only way through vacuum of space!
y EMISSION : by anything above absolute zero temperature.
y All frequencies (wavelengths/colors)
y Max frequency ~ temperature
y Hotter objects:
y Beach: daytime: sand gets hot first: hot air over land rises:
y cooler air over water moves inland:
y Beach: nighttime: sand cools first: reverse!
y Cooler objects: more infrared (lower frequency)
y ABSORPTION : efficient emitter =
y REFLECTION : efficient absorber = poor reflector, + vice
versa
y Thermos: Highly reflective surface: traps heat inside!
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HEAT TRANSFER
Newton’s Law of Cooling How do they estimate time of death?
y = Rate of cooling ~ temperature difference with surroundings.
y (Conduction, Convection, Radiation, … and combination)
y Empirical: Proportionality depends on situation
y CSI: estimating time of death
y Proportionality constant:
y Body temperature versus ambient temperature
HEAT TRANSFER
The Greenhouse Effect What are “greenhouse gasses”?
y Greenhouse:
y Greenhouse glass: transparent to high-frequency
y Opaque to reflected low-freq
y Atmosphere: traps warm infrared “inside”
y Greenhouse gasses: transparent to high-frequency em waves
y Opaque to reflected low-freq infrared: trapped!
y Mainly: carbon dioxide from industrial processes
HEAT TRANSFER
The Greenhouse Effect What are “greenhouse gasses”?
y Greenhouse: traps warm air inside, keeps cool air outside
y Greenhouse glass: transparent to high-frequency em waves
y Opaque to reflected low-freq infrared: trapped
y Atmosphere: traps warm infrared “inside”
y Greenhouse gasses: transparent to high-frequency em waves
y Opaque to reflected low-freq infrared: trapped!
y Mainly: carbon dioxide from industrial processes
y Extreme Greenhouse Effect:
y Main component of atmosphere:
y Surface temperature:
y Max atmospheric pressure: 90 times Earth’s !
HEAT TRANSFER
Conduction
Fur holds heat in better because it’
it’s an INSULATOR.
Convection
Sea and land breezes due to hot air rising, cold falling.
Radiation Thermos: mirror interior holds in RADIATED heat
Newton’
Newton’s Law of Cooling Cooling rate ~ body minus ambient
The Greenhouse Effect CO2 opaque to infrared from ground
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