1 NATS 101 Lecture 4 Temperature and Heat Transfer
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4” by 6” Cards
• Today’s assignmentName, SID – legible penmanship
What worked
What didn’t and needs clarification
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What is Temperature?
Microscopic View
Energy due to random jiggling of molecules
Related to average molecular speed; 500 m/s at room temperature for air
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Temperature and DensityConsider volume of airIf air is warmed:The molecules will
move faster, have “stronger” collisions, and tend to become spaced farther apart
Volume increases, so density decreases
Warmer less dense
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Temperature Scales
• Fahrenheit (oF) - relative
US public standard • Celsius (oC) - relative
Freezing point 0oC
Boiling point 100oCoC= 5/9 (oF-32)
• Kelvin (K) - absolute
K= oC+273Ahrens p27
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What is Heat?
Heat-Energy in the process of being transferred from a warmer object to a cooler object
Consider a pot of water on a hot burner.
Consider the following questions:
Williams, p. 19
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Heat Transfer Questions
What causes the…
Pan bottom and handle to get warmer?
Top of the water to become warmer?
Water temperature to not exceed 100oC?
Region away from side of pan to feel warm?
Williams, p. 19
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Conduction
Heat transfer due to collision of molecules.Conduction warms the bottom of the pan!Conductivity - rate of heat transfer across a 1 cm thick
slab of material if one side is kept 1oC warmer than the other
Cheap Experiment: Touch your chair!
1 cmMetal Water AirHeat
Transfer
1oC
0oC
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Heat Conductivity
Material Heat Conductivity (Cal s-1 cm-1 oC-1)
Still Air 6.1 x 10-5
Dry Soil 6.0 x 10-4
Still Water 1.4 x 10-3
Wet Soil 5.0 x 10-2
Granite (Rock) 6.5 x 10-2
Iron (Metal) 0.16
Silver (Metal) 1.01
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Convection
Heat transfer due to vertical exchange of mass
Occurs in fluids (liquids, gases) because of gravity
Warm, buoyant air rises - Cool, dense air sinks
Convection warms top of liquid!
Warm
Cool Warm
Cool
Warm
Cool
heat below - convection heat side - convection heat top - no convection
gravitygravity
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Convection Movies
2D Convection Tank Animation 2D Convection Model Ra=10**6
2D Convection Model Ra=10**7 IC12D Convection Model Ra=10**7 IC2
3D Rayleigh-Benard Convection Model
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Energy States and Water Phases
water molecules are tightly packed in a crystal alignment that prevents them from changing shape
LOW ENERGY STATE
attractive forces btw molecules weaken and individual molecules can move around each other, but they can not break away
SLIGHTLY HIGHER ENERGY STATE
water molecules move very rapidly and are
not bound together
EXTEMELY HIGH ENERGY STATE
Small Energy Change
Large Energy Change
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Latent Heat
Ahrens, p 28
Weak attraction between
molecules+540 cal/gm+80 cal/gm
-540 cal/gm-80 cal/gm
-620 cal/gm
+620 cal/gm
Strong attraction between
molecules
Some attraction
More Ordered
Phase
Less Ordered
Phase
Surrounding air warms
Surrounding air cools
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Take Home Concepts
• Heat-Energy transfer due to temperature differencesThree modes of heat transferConduction – molecule to moleculeConvection – transport of fluidRadiation – electromagnetic waves
Next lecture!• Latent Heat – energy of phase changes