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NATS 101 Lecture 4 Temperature and Heat Transfer

NATS 101 Lecture 4 Temperature and Heat Transfer. 4” by 6” Cards. Today’s assignment Name, SID – legible penmanship What worked What didn’t and needs clarification. What is Temperature?. Microscopic View Energy due to random jiggling of molecules

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NATS 101 Lecture 4 Temperature and Heat Transfer

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  1. NATS 101Lecture 4Temperature and Heat Transfer

  2. 4” by 6” Cards • Today’s assignment Name, SID – legible penmanship What worked What didn’t and needs clarification

  3. 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

  4. Temperature and Density Consider volume of air If 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

  5. Temperature Scales • Fahrenheit (oF) - relative US public standard • Celsius (oC)- relative Freezing point 0oC Boiling point 100oC oC= 5/9 (oF-32) • Kelvin (K)- absolute K= oC+273 Ahrens p27

  6. 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

  7. 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

  8. 0oC 1 cm Heat Transfer Water Air Metal 1oC 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!

  9. Heat Conductivity

  10. Cool Warm Warm gravity gravity Cool Warm Cool heat below - convection heat side - convection heat top - no convection 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!

  11. Convection Movies 2D Convection Tank Animation 2D Convection Model Ra=10**6 2D Convection Model Ra=10**7 IC1 2D Convection Model Ra=10**7 IC2 3D Rayleigh-Benard Convection Model

  12. 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 SLIGHTLYHIGHER ENERGY STATE water molecules move very rapidly and are not bound together EXTEMELYHIGH ENERGY STATE Large Energy Change Small Energy Change

  13. Latent Heat Surroundingaircools Less Ordered Phase More Ordered Phase -620 cal/gm -80 cal/gm -540 cal/gm Strong attraction between molecules Weak attraction between molecules Some attraction +540 cal/gm +80 cal/gm +620 cal/gm Surroundingairwarms Ahrens, p 28

  14. Modes of Heat Transfer Latent Heat Williams, p. 19

  15. Take Home Concepts • Heat-Energy transfer due to temperature differences Three modes of heat transfer Conduction – molecule to molecule Convection – transport of fluid Radiation – electromagnetic waves Next lecture! • Latent Heat – energy of phase changes

  16. Reading Assignment • Ahrens Pages 30-34 Problems 2.7, 2.9

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