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NATS 101

Lecture 4Temperature 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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Modes of Heat Transfer

Williams, p. 19

Latent Heat

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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

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Reading Assignment

• Ahrens

Pages 30-34

Problems 2.7, 2.9