NATS 101

Lecture 4Temperature and Heat Transfer

4 by 6 CardsTodays assignmentName, SID legible penmanship What workedWhat didnt and needs clarification

Surface Station ModelTemperate (oF)Pressure (mb) Last three digitsDew Point (later) MoistureWind Barb Direction and SpeedCloud Cover Tenths total coverage

Upper Air Station ModelTemperate (oC)Height (m or dam) Three digits givenLeading and/or last digit(s) missingDew Point Depression Moisture (later)Wind Barb Direction and Speed-311098Always missing digit(s)

Practice Surface StationTemperate (oF)Pressure (mb) Last three digitsDew Point (later) MoistureWind Barb Direction and SpeedCloud Cover Tenths total coverage

Practice Surface StationSea Level PressureLeading 10 or 9 is not plotted Examples:1013.8 plotted as 138998.7 plotted as 9871036.0 plotted as 360

What is Temperature?Microscopic ViewEnergy due to random jiggling of moleculesRelated to average molecular speed; 500 m/s at room temperature for air

Temperature and DensityConsider volume of airIf air is warmed:The molecules will move faster, have stronger collisions, and tend to become spaced farther apartVolume increases, so density decreasesWarmer less dense

Temperature ScalesFahrenheit (oF) - relativeUS public standard Celsius (oC) - relativeFreezing point 0oCBoiling point 100oCoC= 5/9 (oF-32)Kelvin (K) - absoluteK= oC+273Ahrens p27

What is Heat?Heat-Energy in the process of being transferred from a warmer object to a cooler objectConsider a pot of water on a hot burner. Consider the following questions:Williams, p. 19

Heat Transfer QuestionsWhat 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

ConductionHeat 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 otherCheap Experiment: Touch your chair!

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

Specific Heat Capacity Heat required to raise temperature of 1 gm of substance 1oC.Metal has lower heat capacity than water!

Material

Specific Heat Capacity

(Cal gm-1 oC-1)

Still Water

1.0

Granite (Rock)

0.19

Iron (Metal)

0.11

ConvectionHeat transfer due to vertical exchange of massOccurs in fluids (liquids, gases) because of gravityWarm, buoyant air rises - Cool, dense air sinks Convection warms top of liquid!

Convection Movies2D Convection Tank Animation 2D Convection Model Ra=10**62D Convection Model Ra=10**7 IC12D Convection Model Ra=10**7 IC23D Rayleigh-Benard Convection Model

Energy States and Water Phaseswater molecules are tightly packed in a crystal alignment that prevents them from changing shape LOW ENERGY STATEattractive forces btw molecules weaken and individual molecules can move around each other, but they can not break awaySLIGHTLY HIGHER ENERGY STATEwater molecules move very rapidly and are not bound together EXTEMELY HIGH ENERGY STATESmall Energy ChangeLarge Energy Change

Latent HeatAhrens, p 28Weak attraction between molecules+540 cal/gm+80 cal/gm-540 cal/gm-80 cal/gm-620 cal/gm+620 cal/gmStrong attraction between moleculesSome attractionMore Ordered PhaseLess Ordered PhaseSurrounding air warmsSurrounding air cools

Modes of Heat TransferWilliams, p. 19Latent Heat

Take Home ConceptsHeat-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

Reading AssignmentAhrensPages 30-34Problems 2.7, 2.9