0222 conduction-convection-radiation-1203645646948318-3

physics

1. Conduction, Convection, and
Radiation
Heat Energy Transfer

2. Methods of Heat Energy
Transfer
•Conduction is the transfer of heat energy by
–Between particles of objects in direct contact
•Convection is the transfer of heat energy by
–the movement of fluids(gas or liquid)
–convection currents due to hot fluid rising and
cold fluid sinking
•Radiation is the transfer of heat energy by
–electromagnetic waves
–does not involve the movement of matter

3. A Conduction
transfers energy
as heat along the
wire and into the
hand
B. Embers swirl
upward in the
convection currents
that are created by
the warmed air
above the fire which
rises
C. Electromagnetic
waves emitted by
the hot campfire
transfer energy by
radiation

4. Heat Transfer by Conduction
•Conduction is the transfer of thermal
energy without any net movement of the
material itself.
•When a metal poker is put in a hot fire,
the exposed end of the poker soon
becomes hot as well, even though it is not
directly in contact with the source of
heat. We say that heat has been
conducted from the hot end to the cold
end.

5. Heat conduction in many materials can be visualized
as the result of molecular collisions. As one end of
the object is heated, the molecules there move
faster and faster. As they collide with their slower-
moving neighbors, they transfer some of their energy
to these molecules whose speeds thus increase.
These in turn transfer some of their energy by
collision with molecules farther along the
object. Thus the energy of
thermal motion is transferred by
molecular collision along the object.
Good thermal conductors such as
silver, copper, aluminum, and gold
are also good electrical conductors.

6. Heat Transfer by Convection
•Convection is the process of
heat transfer through the
mass motion or flow of some
fluid, such as air or water.
•When a pot of water is
heated, convection currents
are set up as the heated
water at the bottom of the
pot rises because of its
reduced density and is
replaced by cooler water
from above.

7. Heat Transfer by
Convection
•Although liquids and gases are generally
not very good conductors of heat, they can
transfer heat quite rapidly by convection.
Convection is the process whereby heat is
transferred by the mass movement of
molecules from one place to another.
Whereas conduction involves molecules
(and/or electrons) moving only over small
distances and colliding, convection involves
the movement of molecules over large
distances.

8. Perhaps the first thing that most people say is "heat
rises". While not wrong, what you should say is "hot
air rises" or "hot water rises".
Anything fluid - that is gases or liquids - will tend to
change density with changes in temperature.
For example, if heated, air decreases in density. The
surrounding air is cooler and denser. This makes it
heavier, so it falls beneath the hot air, forcing it
upwards.

9. Heat Transfer by
Radiation
Radiation is a more rapid transfer of
thermal energy in the form of electromagnetic
radiation accomplished by a process that
requires neither contact nor mass flow.
A hot object also loses heat energy by
radiation. This radiation is similar to light
and can pass through empty space. The
warmth you fell when you warm yourself by
a fire is due to this radiation. If the
object is hot enough, some of the radiation
is visible and can indeed be seen.

10. What type of heat transfer is shown in the
following pictures?
A.
B.
C.
D.
Convection
Radiation
Radiation
Conduction

11. Conductors and Insulators
•Conductors readily transfer heat
energy.
–Gases are the poorest conductors
–Solids are the best conductors
•Insulators are poor conductors of heat
energy.
–Fiberglass batting is better than air alone
because it traps air.