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4. Lecture 3 Temperature, Heat Transfer
NATS 101
Lecture 3 Temperature, Heat Transfer

5. What is Temperature? Microscopic View
Energy due to random jiggling of molecules
Related to average molecular speed; m/s (=1100 mph) at room temperature for air

6. 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

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

8. 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

9. 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

10. 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 cm
Metal
Water
Air
Heat Transfer
1oC
0oC

11. Heat Conductivity

12. heat below - convection heat top - no 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
heat below - convection
heat side - convection
heat top - no convection
gravity

13. Convection Movies 2D Convection Tank Animation 
3D Rayleigh-Benard Convection Model 
Last hour movie of clouds and moist convection
over Catalina mountains

14. 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
Large Energy Change
Small Energy Change

15. Strong attraction between molecules Weak attraction between molecules
Latent Heat
Surrounding air cools
Less Ordered Phase
More Ordered Phase
-620 cal/gm
Strong attraction between molecules
-80 cal/gm
-540 cal/gm
Weak attraction between molecules
Some attraction
+80 cal/gm
+540 cal/gm
+620 cal/gm
Surrounding air warms
Ahrens, p 28

16. Energy Transfer
The explosive TNT--trinitrotoluene-- releases 3,800 calories per gram, roughly 6 times that per gram of water latent heat.
Its energy release may be extremely sudden, but per unit weight, TNT contains less energy than sugar.
Latent Heat
Williams, p. 19

17. 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

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

19. 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
Latent Heat – energy of phase changes