1.2 Transfer Processes. 1.2.1 Conduction. 1.2.2 Convection. 1.2.3 Radiation. Prior Knowledge Linkage. Temperature difference. Heat flow. Heat capacity. Transfer process. Change of internal energy. General Introduction. is transferred by. Radiation. Convection. 1.2.1 Conduction ( 傳導).
Change of internal energy
Convection1.2.1 Conduction (傳導)
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
Good conductorConduction of heat in solids
Conduction of heat in solids
polystyrene silver glass brick water wood aluminium
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?
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?
cold endMicroscopic interpretation of conduction
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.
Conduction in metalsDaily applications of conduction
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
Conduction1.2.2 Convection (對流)
Falling of cold fluids
Rising of hot fluids
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
Heat transfer in water by convection
Convection in air
Cold air sinksConvection 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?
Gravity forceMicroscopic interpretation of convection
In a fluid of uniform temperature, each part of the fluid shows no net movement because it is subjected the same gravity andbuoyancy 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 .
Warm air rises
Cooler air moves towards sea
Warm air rises
Cooler air moves towards landsDaily 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
Conduction1.2.3 Radiation (輻射)
Dull black surface
Shiny white surface
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 (電磁波) called .
How can energy from the Sun arrives the Earth?
Heat transfer by radiation
Electromagnetic waves are sometimes called EM radiations. Visible lights are members of electromagnetic waves. They all travel at metres per second in lines.
All radiations undergo reflection.
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?
Good and bad radiators of heat
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.
Good and bad absorbers of heat
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?
Silvery glass wall to reduce heat loss by .
Vacuum layer to reduce heat loss by.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.
The thermal flasks
Fabrication of thermal flasks
3. The low energy radiations can no longer pass through the glass but is repeatedly inside the house.
4. Radiations are thus inside the house and warm up the plants inside.
2. Radiations after passing though the glass become .The greenhouse effect
A green house has a glass roof and glass walls constructed to keep plants warm.
The greenhouse effect