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Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 Learning outcomes Appreciate how thermal energy is transferred.

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Presentation on theme: "Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 Learning outcomes Appreciate how thermal energy is transferred."— Presentation transcript:

1 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r Conduction Conduction is the process by which heat is transmitted through a medium from one particle to another

3 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r Conduction Examples of good and poor heat conductors (insulators) Good conductorsPoor conductors (insulators) Gold Silver Copper Iron Brass Aluminium Glass Stone Water Plastics Wood Polystyrene foam* Wool* fibreglass* *Materials containing trapped air

7 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r Conduction Uses of Poor Conductors (Insulators) E.g. Cloth, plastics, wood Help keep unwanted heat away Help prevent heat loss

9 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 Air is good insulator, reduces conduction of heat Felt material used as insulators to trap air pockets 9.1 Conduction Prevent?

10 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 1.Thermal energy of a body is the total kinetic energy of the atoms or molecules in the body. 2.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. 3.Conduction is the process by which thermal energy is transmitted through a medium from one particle to another. 4.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. 5.Liquids and gases are poor thermal conductors.

11 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

12 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 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. 9.2 Convection

16 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r Convection Air-conditioners Cold air blown in sinks because its 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 Todays fridge, freezer below??

19 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r Convection Ventilation in a Kitchen Hot air rises through chimney Sets up a convection current Cool air drawn into kitchen

20 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 1.The process of thermal energy transfer most significant in fluids (liquids and gases) is convection. 2.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. 3.Convection currents are caused by density changes. 4.Hot fluids rise and cold fluids sink.

23 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

24 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 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 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9 1.Radiation is the transfer of energy by infra-red electromagnetic waves. 2.Dull black surfaces are good emitters and absorbers of radiation. 3.The greater the surface area of the object, the faster is the rate of thermal transfer from it. 4.The greater the temperature of an object, the faster is the rate of thermal energy transfer from it.

31 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

32 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

33 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

34 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

35 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

36 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

37 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9

38 Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9


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