1 2 transfer processes
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1.2Transfer Processes. 1.2.1Conduction. 1.2.2Convection. 1.2.3Radiation. Prior Knowledge Linkage. Temperature difference. Heat flow. Heat capacity. Transfer process. Change of internal energy. General Introduction. is transferred by. Radiation. Convection. 1.2.1Conduction ( 傳導).

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1.2Transfer Processes

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1 2 transfer processes

1.2Transfer Processes

1.2.1Conduction

1.2.2Convection

1.2.3Radiation


Prior knowledge linkage

Prior Knowledge Linkage

Temperature difference

Heat flow

Heat capacity

Transfer process

Change of internal energy

General Introduction


1 2 1 conduction

is transferred by

Radiation

Convection

1.2.1Conduction (傳導)

Heat

Conduction

vacuum

Solids

fluids

Conductors

Insulators

Electronic motion

Molecular collisions


General meaning of heat conduction

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


Conduction of heat in solids

silveraluminiumglassbrickwaterwoodpolystyrene

Good insulator

Good conductor

Conduction of heat in solids

Experiments

Conduction of heat in solids

polystyrenesilverglassbrickwaterwoodaluminium

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


Conduction of heat in fluids

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


Microscopic interpretation of conduction

Extension

hot end

cold end

Microscopic 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.

Concept maps


Daily applications of conduction

Extension

hot end

cold end

Conduction in metals

Daily 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

Concept maps


1 2 2 convection

is transferred by

Radiation

Conduction

1.2.2Convection (對流)

Heat

Convection

fluids

Solids

vacuum

Falling of cold fluids

Rising of hot fluids

Convection current


General meaning of convection

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


Convection current

Hot air rises

Cold air sinks

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


Microscopic interpretation of convection

Extension

buoyancy force

Gravity force

buoyancy force

Heated region

Gravity force

Microscopic 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 .

Concept maps


Daily applications of convection

Day times

At night

Warm air rises

Cooler air moves towards sea

Warm air rises

Cooler air moves towards lands

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


1 2 3 radiation

is transferred by

Convection

Conduction

1.2.3Radiation (輻射)

Heat

Radiation

Solids

fluids

vacuum

Infra-red radiation

Electromagnetic radiation

Green house

effect

Dull black surface

Shiny white surface


General meaning of heat radiation

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 (電磁波) called .

How can energy from the Sun arrives the Earth?

Heat transfer by radiation

Concept maps


Properties of electromagnetic waves

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

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


Absorption of radiation

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


Good and bad absorbers and radiators

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


The thermal flasks

Plastic stopper to reduce heat loss by.

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.

Experiments

The thermal flasks

Fabrication of thermal flasks

Concept maps


The greenhouse effect

3.The low energy radiations can no longer pass through the glass but is repeatedly inside the house.

  • The sun emits radiations that are energetic enough to get through the glass.

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.

Experiments

The greenhouse effect

Concept maps


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