1. Chapter 13
The Transfer of Heat

2. 13.1 Convection
The process in which heat is carried from place to place by the bulk movement of a fluid.
Examples: heating a pot of water, heating (or cooling) a home, ground warming neighboring air
Birds use “thermals” to soar through the air. Convection principles are being used to cool everything from athletes to automobile engines.

3. 13.2 Conduction
The process whereby heat is transferred directly through a material, with any bulk motion of the material playing no role in the transfer.
Movement of particles causes collisions with other particles. Collisions increase KE of all particles involved, thus increasing temperature of material.
Metals are great conductors because the “sea of electrons” allows quick, easy transfer of energy.

4. Factors Affecting Conduction
Q is proportional to the time conduction takes place. (More time = more heat flow)
Q is proportional to the temperature difference. (Larger ∆T = more heat flow)
Q is proportional to cross-sectional area (Larger area = more heat flow)
Q is inversely proportional to length. (Greater lengths conduct less heat)

5. Equation for Conduction of Heat
SI unit of thermal conductivity: J/(smC°)
Why does a down coat keep me warm?
Q is heat conducted
t is time through material
L is material length
A is cross-sectional area
∆T is temperature difference
k is thermal conductivity (a numerical constant describing the ability of the material itself to conduct heat due to its chemical structure)

6. 13.3 Radiation
Radiation is the process in which energy is transferred by means of electromagnetic waves.
These waves do not require a medium.
ALL BODIES continuously radiate energy in the form of electromagnetic waves.
Objects on Earth radiate infrared waves.

7. Why am I so uncomfortable wearing black in the summer?
Interesting Tidbits
The term blackbody is used to describe an object that absorbs all the electromagnetic waves falling on it.
All objects emit and absorb electromagnetic waves simultaneously.
A good absorber is also a good emitter.
Why am I so uncomfortable wearing black in the summer?

8. Factors Affecting Radiation
Q increases with more time.
Q increases with surface area.
Q increases with the 4th power of Kelvin Temperature. (Increasing temp. dramatically increases heat transfer)

9. The Stefan-Boltzmann Law of Radiation
σ is the Stefan-Boltzmann constant and is equal to 5.67 x J/(sm2 K4 )
e is the emissivity. This number will be between 0 and 1. It describes the ratio of the energy the object actually radiates to the energy it would if it were a perfect emitter. This value depends on the condition of the surface.

10. Sample Problem
The supergiant star Betelgeuse has a surface temperature of about 2900K and emits a radiant power of approximately 4 x 1030 W. Assuming that Betelgeuse is a perfect emitter (e =1) and spherical, find its radius.
Q/t is the power emitted so A is equal to …
And A is equal to
Rearranged
Finally…