1. Learning outcomes
Appreciate how thermal energy is transferred form a higher temperature region to one of lower temperature
Describe how energy transfer occurs through molecules in solids
Describe density changes in fluids leading to convection
Explain that radiation does not require a material medium and rate of energy transfer is affected by: (i) colour and texture of the surface;
(ii) surface temperature; (iii) surface area
Apply the concept of thermal energy transfer to everyday applications

2. 9.1 Conduction
Conduction is the process by which heat is transmitted through a medium from one particle to another

3. 9.1 Conduction How Conduction Works
Solids: conduction occurs when thermal energy transferred from one particle to another.
When one end of a rod is heated, molecules there gain energy and vibrate faster.
These molecules collide with less energetic neighbors.
Some of their energy is transferred to neighboring molecules which thus gains kinetic energy.

4. 9.1 Conduction
Thermal energy is passed along the rod by the vibrating molecules.
Vibration transfers energy from molecule to molecule until cold end reaches same temperature as hot end
No net movement of molecule during conduction.
Solids are better conductors of heat than liquids and gases as molecules are closer together.
Kinetic energy transferred more quickly.

5. 9.1 Conduction
Conduction takes place through vibrating molecules as well as free electrons in metals.
When heated, free electrons gain energy and move faster.
They move in the spaces between atoms before colliding with some atoms and transferring energy to them.
Much faster than conduction by vibration of molecules in non-metals.
Metals conduct heat much faster then non-metals which have no free electrons.

6. Poor conductors (insulators)
9.1 Conduction
Examples of good and poor heat conductors (insulators)
Good conductors
Poor conductors (insulators)
Gold
Silver
Copper
Iron
Brass
Aluminium
Glass
Stone
Water
Plastics
Wood
Polystyrene foam*
Wool*
fibreglass*
*Materials containing trapped air

7. 9.1 Conduction Applications of Conduction Uses of Good Conductors
Used where thermal energy has to be quickly transmitted
E.g. Pans, kettles, cooking utensils are made of metals like aluminium, copper, steel
Copper pipes for heat exchangers
Heat sinks in electronic equipment

8. 9.1 Conduction Uses of Poor Conductors (Insulators)
E.g. Cloth, plastics, wood
Help keep unwanted heat away
Help prevent heat loss

9. 9.1 Conduction Air is good insulator, reduces conduction of heat
Felt material used as insulators to trap air pockets
Prevent?

10. Thermal energy of a body is the total kinetic energy of the atoms or molecules in the body.
Heat is energy in transit from a high temperature object to a lower temperature object. It is recognised only as it crosses the boundary of a system.
Conduction is the process by which thermal energy is transmitted through a medium from one particle to another.
Conduction in metals is far better than in other solids because other than the vibrating atoms, the free electrons in a metal are also responsible for the conduction of thermal energy.
Liquids and gases are poor thermal conductors.

12. 9.2 Convection
Convection is the process by which heat is transmitted from one place to another by the net movement of heated particles of a gas or liquid.
Convection takes place in liquids and gases where atoms or molecules move from one place to another
Transfer of thermal energy depends on transfer of heated matter.

13. 9.2 Convection Convection in Liquids
Add potassium permanganate crystals into a flask of water
Crystals sink to bottom of the flask
Heat flask gently just below the crystals
Purple-streaked water rise upwards and fan outwards.
Water becomes uniformly purplish after a while.

14. 9.2 Convection Circulation of a liquid called a convection current
Due to heating at the bottom of liquid
When a liquid is heated, it expands, becomes less dense and floats upwards.
Cold and denser liquid moves down to replace the rising liquid masses.
This cold liquid, in turn, gets heated up.

15. 9.2 Convection A Hot Water System based on convection principle
Consists of a boiler, water storage tank and cold tank interconnected by pipes
Convection currents drive hot water from the top of boiler into hot water storage tank.
Cold water from storage tank is drawn down to the boiler where it in turn becomes heated.

16. 9.2 Convection Car Radiator
Car engines are kept cool by a water jacket
When water in the water jacket gets heated, it flows into copper tubes which include many cooling fans.
A fan causes air to flow past the tubes and cool water in them.
Cooled water flows down and back into engine through a hose at the bottom.

17. 9.2 Convection Convection in Gases To show convection currents in air
Fit two glass chimneys to the top of a wooden box with plane glass window
Light a candle below one chimney
Heat from this initiates a convection current
Hot air flows out of this chimney
Cold air flows in through other chimney
Used to ventilate underground mines

18. 9.2 Convection Air-conditioners
Cold air blown in sinks because it’s denser then warm air.
Warm air displaced by cold air rises and is drawn into air-conditioner to be cooled.
Refrigerator
Cold air below freezer sinks
Set up convection currents
Today’s fridge, freezer below??

19. 9.2 Convection Ventilation in a Kitchen Hot air rises through chimney
Sets up a convection current
Cool air drawn into kitchen

20. 9.2 Convection Sea and Land Breezes
In the day, land heats up faster than the sea
Air above land expands and rises
Cool air above sea moves inland
Result in sea breeze

21. 9.2 Convection At night, the land cools faster than the sea
Warm air above sea rises
Cool air above land moves out to take its place
Result in land breeze

22. The process of thermal energy transfer most significant in fluids (liquids and gases) is convection.
Convection is the process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid.
Convection currents are caused by density changes.
Hot fluids rise and cold fluids sink.

24. 9.3 Radiation
Radiation is the transfer of energy by electromagnetic waves
Emission of Radiation
The rate of energy transfer by radiation is affected by:
(a) surface temperature
(b) colour and texture of the surface
(c) surface area

25. 9.3 Radiation Uses of Poor Emitters of Radiation
Shiny surfaces are poor radiators
E.g. Teapots and aluminum foil
Uses of Good Emitters of Radiation
Dull surfaces radiate quickly
E.g. Cooling fins of refrigerators and heat sinks

26. 9.3 Radiation Absorption of Radiation
Dull black surfaces better absorbers than shiny surfaces
Good emitter is also good absorber
Shiny surfaces good reflectors of heat

27. 9.3 Radiation Uses of Good and Poor Absorbers of Radiation
Houses in hot countries painted white to keep them cooler.
Factory roofs coated with aluminium to reduce radiation absorption.
Solar control films allow light to pass through but keep out infrared radiation.
Dark clothes absorb radiation readily.
Solar panels painted black to absorb as much radiation from sun as possible.

28. 9.3 Radiation Vacuum Flask
Reduces transfer of thermal energy by conduction, convection and radiation
Consists of a double-walled glass container with a vacuum in between
Silvered walls on vacuum side
Container supported by foam plastic, a poor conductor

29. 9.3 Radiation
No heat can enter or leave flask by conduction or convection across the vacuum
Inner silvered surface reflects radiation from hot fluids back into flask
Outer silvered surface reflects radiation in external surroundings away from flask
Foam plastic support and plastic cup minimises heat transmitted by conduction through thin glass walls
Plastic cup stops convection and evaporation

30. Radiation is the transfer of energy by infra-red electromagnetic waves.
Dull black surfaces are good emitters and absorbers of radiation.
The greater the surface area of the object, the faster is the rate of thermal transfer from it.
The greater the temperature of an object, the faster is the rate of thermal energy transfer from it.