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