F4: Transmission of HeatTextbook page 128 to 143

Objectives

• At the end of this lesson you will be able to:– State how heat energy flow– State the 3 mechanisms of heat

energy to flow

How does heat move from one place to another?

• When you stir a cup of hot chocolate with a metal spoon, what happens?– After awhile the metal spoon will be hot

• How does the heat move?– From region of high temperature to

region of lower temperature– High temperature (cup of hot chocolate)

and low temperature (metal spoon)

Different ways heat can move through different

medium• Solid

– Conduction

• Liquid and Air– Convection

• No medium (does not require solid, liquid or air)– Radiation

Conduction in Solid

• Define conduction– Movement of heat through a material

without the obvious movement of the material itself

Conduction in Solid

• Example: heating of metal rod– Heat energy moves along the metal

rod– From hot end to the cool end of the rod– The metal atoms gain energy from the

flame and vibrate faster– These atoms hit neighbouring atoms

causing them to vibrate faster – It is like a book is passed from student

to student

Animation – Conductionhttp://www.phy.cuhk.edu.hk/contextual/heat/hea/condu/co

nduction_e.html

Are all conductors the same?

• Example: heating of metal rod– Heat energy moves along the metal

rod– From hot end to the cool end of the rod– The metal atoms gain energy from the

flame and vibrate faster– These atoms hit neighbouring atoms

causing them to vibrate faster – It is like a book is passed from student

to student

Are all conductors the same?

• Some materials conduct heat faster than others

• Metal: good conducter• Poor conductor conduct heat slowly

example, wood, plastic, ceramics, air and glass

• Poor conductor = insulator• Vacuum (empty space) is perfect

insulator. No heat moves through a vacuum by conduction

Uses of conductors

• Electric iron• Metal pan

Uses of insulators

• An ice box• Air has many uses as an insulator,

to use air as insulator, it must be trapped– Oven gloves– Blankets– Jackets

Do inquiry time page 131

• The air trapped between the two layers and prevent heat loss by conduction

• Conductor = metal pan/pot and kettles, rice cooker, electric iron

• Insulators = oven gloves, plastic handles, blankets

Do checkpoint page 131

• Conduction.• Conductor. Metals.• Insulator. Plastics• Electric iron/cooking pots

Convection

• Liquids and gases are poor conductors of heat

• It moves through convection• Convection is the movement of heat

energy from one place to another by the movement of the material itself

• It is like a student walk to the back of the room carrying the book and pass it to another student

Animation

• Convection in liquids

Objectives

• At the end of this lesson you will be able to:– State how convection occur– State the applications of convection

How does convection take place in liquids?

• As water get heated, it rises from bottom of beaker to the top

• Cooler water from beaker sink to take the place of the heated water that has risen

• Convection current

Using particulate model of matter to explain

• As water get heated, molecules move faster and spread apart

• Volume of water increases and density decreases

• Hot water less dense so it will move up

• Cold water more dense it will sink

How does convection take place in gases?

• Same as liquid• Hot air rises and cooler air sinks• Convection current form, similar as

in liquid

Using particulate model of matter to explain

• As gas get heated, molecules move faster and spread apart

• Volume of gas increases and density decreases

• Hot air less dense so it will move up

• Cold air more dense it will sink

Application of convection

• Electric kettle– Heater at the bottom– Hot water rises and cold water sinks– Convection current forms, eventually

all the water becomes warm

Application of convection

• Hot water system– Hot water rises from the boiler– Cold water from the tank sinks into

the boiler– Convection cycle– Supply of hot water

Application of convection

• Air conditioner– Placed at the top of the wall– Cool air sinks– Hot air rises– Convection currents are set up in the

room

Application of convection

• Warming a room– Heater is places as low as possible– Hot air rises– Cold air from the room get heated by

the heater and rises up– Convection currents is form

Sea breeze – During the day

• Hot day, land heats up faster than the sea

• Hot air rises from the land• The hot air replaced by cold,

denser air from the sea• Produce sea breeze

Land breeze – During the night

• At night, land cools quicker than the sea

• Hot air rises from the sea• The hot air replaced by cold,

denser air from the land• Produce land breeze

Do checkpoint pg 136

1. Convection2. Convection current3. Rise. Hot air/water is less dense.4. At the top. Because cold air sinks

and hot air rises, it causes convection current to occur.

5. Hot water system, electric kettle, heater

Objectives

• At the end of this lesson you will be able to:– State how radiation occur– State the application of radiation

How does heat from the sun reach the earth?

• Radiation• Space between Sun and Earth is a

vacuum• Cannot transfer by conduction or

convection• Radiation is the transfer of energy

without the need for a medium• It is like a book can be thrown to the

back of the room

How radiation is absorbed?

• When we stand outdoors, our body absorb radiation from the Sun

• All objects are able to absorb radiation

• Black, dull or rough surface are good absorbers of heat

• Shiny, white or smooth surfaces are poor absorbers of heat

How radiation is emitted?

• All objects give out heat• Black, dull or rough surface are

good radiators of heat• Shiny, white or smooth surfaces

are poor radiators of heat

In conclusion• Shiny, white surfaces reduce the

amount of energy absorbed from the surroundings by reflecting radiation

• It also reduce heat from losing to the surrounding

• Dull and rough surfaces, absorb heat from the surrounding better

• It also lose heat to the surrounding faster

Do checkpoint 139

Application of Absorbers and Radiators of heat

• Refrigerator– Surface is black– So it can radiate heat faster to the

surrounding

Application of Absorbers and Radiators of heat

• Hot water containers– Shiny kettle is poor radiators– So heat remain in the container and

thus water remain hot longer

Application of Absorbers and Radiators of heat

• Petrol tanks– Shiny and white – So it will reflect heat from the Sun– Prevent petrol from heating up– Thus the tanks remain cool

Do checkpoint 140