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1.2 Transfer Processes Conduction Convection

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Presentation on theme: "1.2 Transfer Processes Conduction Convection"— Presentation transcript:

1 1.2 Transfer Processes 1.2.1 Conduction 1.2.2 Convection
1.2.3 Radiation

2 Prior Knowledge Linkage
Temperature difference Heat flow Heat capacity Transfer process Change of internal energy General Introduction

3 1.2.1 Conduction (傳導) Heat is transferred by Convection Radiation
Solids fluids vacuum Conductors Insulators Electronic motion Molecular collisions

4 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

5 Conduction of heat in solids
Experiments Conduction of heat in solids polystyrene silver glass brick water wood aluminium Arrange the above materials in descending order of ability to conduct heat. Good conductor Good insulator silver aluminium glass brick water wood polystyrene 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

6 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

7 Microscopic interpretation of conduction
Extension 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 hot end cold end 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

8 Daily applications of conduction
Extension Conduction in metals All metals contain , which are electrons that can move freely within the metals. 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. Daily applications of conduction Good conductors of heat Good insulators of heat Concept maps

9 1.2.2 Convection (對流) Heat is transferred by Conduction Radiation
Solids vacuum fluids Rising of hot fluids Falling of cold fluids Convection current

10 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

11 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

12 Microscopic interpretation of convection
Extension 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 and buoyancy force. buoyancy force Heated region Gravity 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

13 Daily applications of convection
Formation of sea and land breezes Day times Warm air rises Cooler air moves towards lands At night Warm air rises Cooler air moves towards sea 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

14 Electromagnetic radiation
Heat is transferred by Conduction Convection Radiation Solids fluids vacuum Infra-red radiation Electromagnetic radiation Green house effect Dull black surface Shiny white surface

15 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

16 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

17 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

18 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

19 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

20 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. Plastic stopper to reduce heat loss by . Silvery glass wall to reduce heat loss by Vacuum layer to reduce heat loss by . Experiments The thermal flasks Fabrication of thermal flasks Concept maps

21 The greenhouse effect A green house has a glass roof and glass walls constructed to keep plants warm. The sun emits radiations that are energetic enough to get through the glass. 2. Radiations after passing though the glass become 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. Experiments The greenhouse effect Concept maps

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