1.2Transfer Processes

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1.2 Transfer Processes
1.2.1 Conduction
1.2.2 Convection
1.2.3 Radiation
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Prior Knowledge Linkage
Temperature difference
Heat flow
Heat capacity
Transfer process
Change of internal energy
General Introduction
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1.2.1 Conduction (??)
Heat
Conduction
vacuum
Solids
fluids
Conductors
Insulators
Molecular collisions
Electronic motion
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General meaning of heat conduction
Many cooking utensils are made of metal with
plastic handles. Do you know why?
Conduction is a process in which heat flows
through a from places of temperature to
places of temperature. It takes place fast in
but not in .
Conduction along metal rods
Concept maps
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Conduction of heat in solids
Experiments
Conduction of heat in solids
polystyrene silver glass brick water wood aluminiu
m
Arrange the above materials in descending order
of ability to conduct heat.
are good conductors of heat while are poor
conductors (i.e., good ) of heat
In winter, the tile floor usually feels colder
than a carpet floor, do you know why?
Concept maps
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Conduction of heat in fluids
Experiments
Conduction of heat in water and air
Water and air are both conductors of heat.
If air is a good conductor of heat, would we feel
colder or hotter in a room of temperature 25 oC?
Can conduction occur in vacuum?
Concept maps
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Microscopic interpretation of conduction
Extension
Molecules in the hot region of a body move more
and than those in the cold region. The
forces between molecules in turn set the
neighboring molecules also vibrate . The
process continue and ( )
energy is transferred along the body as a result
of molecular .
Conduction in solids, liquids and gases
Liquids and gases are conductors of heat
than solids because the intermolecular forces in
them are than those in solids, and so
molecular collisions do not occurs as as
those in solids for fast energy transfer.
Concept maps
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Daily applications of conduction
Extension
Conduction in metals
All metals contain , which are electrons that
can move freely within the metals.
Metals possesses which can carry energy
directly from the hot end to the cold end.
Therefore, most metals are conductors of heat.
Good conductors of heat
Good insulators of heat
Concept maps
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1.2.2 Convection (??)
Heat
Convection
fluids
Solids
vacuum
Falling of cold fluids
Rising of hot fluids
Convection current
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General meaning of convection
Air-conditioners are usually installed high in a
room. Do you known why?
Convection is the process in which heat is
transferred by the movement of hot fluid
(liquid / gas) due to a temperature difference
along a direction.
Convection in air
Experiments
Heat transfer in water by convection
Experiments
Convection in air
Experiments
Convection current
Concept maps
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Convection current
Convection occurs only in , not in
nor .
When a fluid is heated from , that part of the
fluid and thus becomes . That
part of fluid therefore and is replaced by
the fluid coming down from above, forming a
flowing in the fluid.
Do you know why candle flames always point
upwards?
Concept maps
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Microscopic interpretation of convection
Extension
In a fluid of uniform temperature, each part of
the fluid shows no net movement because it is
subjected the same gravity and buoyancy force.
When a small region of the fluid near the bottom
is heated, the molecules there move and
collides with each other more .
The spaces between molecules there then ,
making the fluid there becoming dense.
The gravity force on that part of fluid therefore
, which results in a net force on the part
and so it .
Concept maps
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Daily applications of convection
Formation of sea and land breezes
During day times, lands have temperature than
sea (why?). Warmer air over the land while
cooler air from the sea flows in to take its
place forming the .
During the night, lands have temperature than
sea (why?). Warm air over the sea while
cooler air from the land flows in to take its
place forming the .
The running lantern
Heaters and air conditioners
Concept maps
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1.2.3 Radiation (??)
Heat
Radiation
Solids
fluids
vacuum
Infra-red radiation
Electromagnetic radiation
Green house effect
Dull black surface
Shiny white surface
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General meaning of heat radiation
When we put our hand near a lamp, our hand feel
warm quickly. Is energy transferred from the
lamp to our hand by conduction or convection?
Heat radiation is the process in which heat is
transferred from one place to another by means of
electromagnetic waves (???) called .
How can energy from the Sun arrives the Earth?
Heat transfer by radiation
Concept maps
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Properties of electromagnetic waves
Electromagnetic waves are sometimes called EM
radiations. Visible lights are members of
electromagnetic waves. They all travel at
metres per second in lines.

• Other members of electromagnetic radiations are
• radio waves,
• microwaves,
• ultra-violet,
• x-rays,
• gamma rays.

All radiations undergo reflection.
Concept maps
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Emission of radiation
All objects of temperature above oC
(the absolute zero) emit infra-red (heat)
radiation in direction.
The hotter an object is, the the heat radiation
it emits.
The internal energy of an object on
emitting heat radiation.
What are the factors affecting the rate of
emission of radiations?
Experiments
Good and bad radiators of heat
Concept maps
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Absorption of radiation
When radiation falls on an object, it is partly
, partly and partly .
A shiny silvery surface tends to most of the
radiation. A transparent surface tends to most
of the radiation A dull black surface tends to
most of the radiation.
Reflection of radiation
The internal energy of an object on
absorbing heat radiation.
Experiments
Good and bad absorbers of heat
Concept maps
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Good and bad absorbers and radiators
Dull black surfaces are absorbers and
radiators of radiation. Shiny silvery surfaces
are absorbers and radiators of
radiation.
Why is a kettle usually made with shiny surface?
Concept maps
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The thermal flasks
A thermal flask (or thermos) can keep foods hot
or cold by reducing heat exchange with the
surrounding through conduction, convection and
radiation.
Experiments
The thermal flasks
Fabrication of thermal flasks
Concept maps
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The greenhouse effect
A green house has a glass roof and glass walls
constructed to keep plants warm.
Experiments
The greenhouse effect
Concept maps