1. Physics 101: Lecture 24 Conduction, Convection+Radiation
Final
Physics 101: Lecture 24 Conduction, Convection+Radiation 1

2. Review Heat is FLOW of energy Specific Heat Latent Heat Today: Heat
Flow of energy may increase temperature
Specific Heat
DT = Q / (c m)
Latent Heat
heat associated with change in phase
Today: Heat
Conduction
Convection
Radiation 07

3. Question Touch base of chair, and desk top, which feels colder?
A) Base B) Same C) Desk
Both must be the same temperature (room temperature), but metal feels colder because it conducts heat better/faster. 10

4. Heat Transfer: Conduction
Hot molecules have more KE than cold molecules
High-speed molecules on left collide with low-speed molecules on right
energy transferred to lower-speed molecules
heat transfers from hot to cold
I = rate of heat transfer = Q/t [J/s]
I = k A (TH-TC)/L
Q/t = k A T/x
k = “thermal conductivity”
Units: J/s-m-C
good thermal conductors…high k
good thermal insulators … low k
R = L/(Ak) = thermal resistance
L = Dx TH
Hot TC
Cold
Area A
demos 13

5. Question
If you want to maximize the amount of Heat flow which substance should you choose?
1. Aluminum
2. Copper
3. Steel 15

6. Example: Boiling water
A steel pot with an area of 0.05m2 and thickness of 0.006m is placed on top of a burner so that its bottom surface remains at 1500C. How much water boils away each minute?

7. Question
If you want to limit the amount of Heat flow (Insulate) which substance should you choose?
1. Air
2. Wood
3. Glass 15

8. How does an oven work?

9. Heat Transfer Convection
Air heats at bottom
Thermal expansion…density gets smaller
Lower density air rises
Archimedes: low density floats on high density
Cooler air pushed down
Cycle continues with net result of circulation of air
Practical aspects
heater ducts on floor
A/C ducts on ceiling
stove heats water from bottom
“riding the thermals”
heater
demos 27

10. Heat Transfer: Radiation
All things radiate and absorb electromagnetic energy
Iemit = eAT4
Iabsorb = eAT04
Inet = Iemit - Iabsorb = eA(T4 - T04)
if T > T0, object cools down
if T < T0, object heats up T
Surroundings at T0
Hot stove HW 38

11. Earth Homework
The Earth has a surface temperature around 270 K and an emissivity of 0.8, while space has a temperature of around 2 K. What is the net power radiated by the earth into free space?
(Radii of the Earth and the Sun are Re = 6.38×106 m, Rs = 7×108 m.)
Average consumption of US= 1.3x108 Watts 42

12. Summary Conduction - contact Convection - fluid motion Radiation
Ch 14 Problems
1, 11, 19, 29, 35, 39, 41, 49, 51, 59, 63 50