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1.2 Transfer Processes
1.2.1 Conduction
1.2.2 Convection
1.2.3 Radiation
Prior Knowledge Linkage
Temperature difference
Heat flow
Change of internal energy
Transfer process
General Introduction
Heat capacity
is transferred by
1.2.1 Conduction (傳導 )
Conduction
Conductors
Heat
Convection Radiation
Insulators
Solids fluids vacuum
Molecular collisionsElectronic motion
General meaning of heat conduction
Concept maps
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
are good conductors of heat while are poor conductors (i.e., good ) of heat
Conduction of heat in solids
Concept maps
Arrange the above materials in descending order of ability to conduct heat.
polystyrene silver glass brick water wood aluminium
Experiments Conduction of heat in solids
Good conductor Good insulatorsilver aluminium glass brick water wood polystyrene
In winter, the tile floor usually feels colder than a carpet floor, do you know why?
Water and air are both conductors of heat.
Conduction of heat in fluids
Concept maps
Experiments Conduction of heat in water and air
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?
Microscopic interpretation of conduction
Concept maps
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 .
hot end
cold end
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.
Conduction in solids, liquids and gases
All metals contain , which are electrons that can move freely within the metals.
Conduction in metals
Concept maps
hot end
cold end
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
Daily applications of conduction
is transferred by
Conduction Radiation
Convection current
1.2.2 Convection (對流 )
Convection
Heat
Solids vacuum fluids
Rising of hot fluids Falling of cold fluids
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 currentConcept maps
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.
Hot air rises
Cold air sinks
Do you know why candle flames always point upwards?
Concept maps
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.
Microscopic interpretation of convectionIn a fluid of uniform temperature, each part of the fluid shows no net movement because it is subjected the same gravity and buoyancy force.
The gravity force on that part of fluid therefore , which results in a net
force on the part and so it .
buoyancy force
Heated region
Gravity force
Gravity force
buoyancy force
Concept maps
Concept maps
Heaters and air conditioners
The running lantern
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 .
Day timesWarm air rises Cooler air
moves towards lands
At night
Warm air rises
Cooler air moves towards sea
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
.
Infra-red radiation
is transferred by
1.2.3 Radiation (輻射 )
Radiation
Heat
Conduction Convection
Electromagnetic radiation
Dull black surface
Green house effect
Solids fluids vacuum
Shiny white surface
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 transfer by radiation
Heat radiation is the process in which heat is transferred from one place to another by means of electromagnetic waves (電磁波 ) called .
How can energy from the Sun arrives the Earth?
Concept maps
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
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.
Experiments Good and bad radiators of heat
What are the factors affecting the rate of emission of radiations?
Concept maps
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 radiationA dull black surface tends to most of the radiation.
The internal energy of an object on absorbing heat radiation.
Reflection of radiation
Experiments Good and bad absorbers of heat
Concept maps
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
Concept maps
The thermal flasksA thermal flask (or thermos) can keep foods hot or cold by reducing heat exchange with the surrounding through conduction, convection and radiation.
Fabrication of thermal flasks
Experiments The thermal flasks