Download The Transfer of Heat

Chapter 13
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.

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.

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)

Equation for Conduction of Heat

SI unit of thermal conductivity: J/(smC°)
(kAT )t
Q
L
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)
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.

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?
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)

The Stefan-Boltzmann Law of
Radiation
σ is the Stefan-Boltzmann constant and is
equal to 5.67 x 10-8 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.

Q  eT At
4
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 …
A  4r 2
And A is equal to
A
r
4
Rearranged
Q
A t 4
eT
Finally…
r
Q
t
4eT 4