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© Boardworks Ltd 2005 1 of 31 KS4 Physics Heat Transfer.

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Presentation on theme: "© Boardworks Ltd 2005 1 of 31 KS4 Physics Heat Transfer."— Presentation transcript:

1 © Boardworks Ltd of 31 KS4 Physics Heat Transfer

2 © Boardworks Ltd of 31 Heat Transfer Conduction Insulators Thermal radiation Summary activities Convection Contents

3 © Boardworks Ltd of 31 Heat energy only flows when there is a temperature difference from a w______ area to a c______ area. armer ooler Heat transfer Heat is a type of energy called thermal energy. Heat can be transferred (moved) by three main processes: 1. conduction 2. convection 3. radiation During heat transfer, thermal energy always moves in the same direction: HOT COLD

4 © Boardworks Ltd of 31 How are the particles arranged in a solid, a liquid and a gas? What is conduction? Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction. Conduction is the method of heat transfer in solids but not liquids and gases. Why? What type of solids are the best conductors? solid liquid gas

5 © Boardworks Ltd of 31 Graphite is a non-metal that is a good conductor of heat. If you heat one end of a graphite rod, the heat travels to the other end. As the rod is heated, the atoms vibrate. These vibrations make the adjacent atoms vibrate, and so on. This is how the heat energy travels along the rod. Conduction in non-metals

6 © Boardworks Ltd of 31 The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms. When a metal is heated, the free electrons gain kinetic energy. This means that the free electrons move faster and transfer the energy throughout the metal. This makes heat transfer in metals very efficient. Conduction in metals Insulators do not have these free electrons, which is why they do not conduct heat as well as metals. heat

7 © Boardworks Ltd of 31 Heat Transfer Conduction Insulators Thermal radiation Summary activities Convection Contents

8 © Boardworks Ltd of 31 The metal feels cold and the wood feels warm. Metal is a good conductor and conducts the heat away from your hands, so it feels cold. Wood is not a good conductor and does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal. Touch a piece of metal and a piece of wood at the same temperature. Which material feels warmer and why? A temperature problem

9 © Boardworks Ltd of 31 Insulators Air becomes a very effective insulator when it is trapped and stopped from moving. This is how your clothes keep you warm – air is trapped between the fibres and so acts as an insulator. Some materials are very poor conductors of heat. These are called insulators. Examples of materials that are insulators include plastics, wood, ceramics and air. Other insulating materials, including polystyrene and loft insulation, use trapped air because it is so effective.

10 © Boardworks Ltd of 31 Side-view of double glazing insulating layer of air Double glazing keeps a house warmer because there is a layer of a__ between the panes of glass. Air is a poor c________, so it acts like an i_______,. The trapped air r_______ heat loss by c_________ from a house. ir onductor nsulator educes onduction Double glazing How does double glazing keep a house warmer?

11 © Boardworks Ltd of 31 Charlie the Chef and his frozen chicken! Charlie forgot to take the chicken out of the freezer last night! Will his plan to defrost the chicken in time for lunch work?

12 © Boardworks Ltd of 31 How does it insulate?

13 © Boardworks Ltd of 31 Heat Transfer Conduction Insulators Thermal radiation Summary activities Convection Contents

14 © Boardworks Ltd of 31 Heating fluids Liquids and gases can behave in similar ways and so are called fluids. What happens to the particles in a fluid when it is heated? The heated fluid particles gain energy, so they move about more and spread out. The same number of particles now take up more space so the fluid has become less dense. heat less dense fluid

15 © Boardworks Ltd of 31 Cooler regions of a fluid are more dense than warmer regions of the same fluid. The cooler regions will sink as they have the greatest mass per unit volume. The warmer regions will rise as they have a lower density. In effect, they float on top of the denser, cooler regions. This is how heat transfer takes place in fluids and is called convection. What is convection?

16 © Boardworks Ltd of 31 Convection current hot water cools cool water sinks hot water rises cool water warms This is called a convection current.

17 © Boardworks Ltd of 31 Why is it windy at the seaside?

18 © Boardworks Ltd of 31 Why is the freezer compartment at the top of a fridge? freezer compartment The freezer compartment is at the top because cool air sinks. The freezer cools the air at the top and this cold air cools the food on the way down. It is warmer at the bottom of the fridge. This warmer air rises and so a convection current is set up. Convection in a fridge

19 © Boardworks Ltd of 31 Why are boilers placed beneath hot water tanks in people’s homes? Hot water rises. When the boiler heats the water, and the hot water rises, the water tank is filled with hot water. Why does hot air rise and cold air sink? Cool air is more dense than warm air. The cool air ‘falls through’ the warm air and so the warm air rises to the top. Convection questions

20 © Boardworks Ltd of 31 Conduction and convection – summary

21 © Boardworks Ltd of 31 Heat Transfer Conduction Insulators Thermal radiation Summary activities Convection Contents

22 © Boardworks Ltd of 31 Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth? There are no particles between the Sun and the Earth so the heat cannot travel by conduction or by convection. ? infrared radiation The heat travels to Earth by infrared waves. They are similar to light waves and are able to travel through empty space. The third type of heat transfer

23 © Boardworks Ltd of 31 Heat can move by travelling as infrared waves. These are electromagnetic waves, like light waves, but with a longer wavelength. This means that infrared waves act like light waves: They can travel through a vacuum. They travel at the same speed as light – 300,000,000 m/s. They can be reflected and absorbed. Infrared waves Infrared waves heat objects that absorb them and so can be called thermal radiation.

24 © Boardworks Ltd of 31 Emitting thermal radiation All objects emit (give out) some thermal radiation. Some surfaces are better at emitting thermal radiation than others. Matt black surfaces are the best emitters of radiation. Shiny surfaces are the worst emitters of radiation. Which type of kettle would cool down faster: a black kettle or a shiny metallic kettle? white silver matt black best emitter worst emitter

25 © Boardworks Ltd of 31 Absorbing thermal radiation Infrared waves heat objects that absorb (take in) them. Some surfaces are better at absorbing thermal radiation than others – good emitters are also good absorbers. Matt black surfaces are the best absorbers of radiation. Shiny surfaces are the worst emitters because they reflect most of the radiation away. Why are solar panels that are used for heating water covered in a black outer layer? best emitter worst emitter best absorber worst absorber white silver matt black

26 © Boardworks Ltd of 31 Radiation questions Why are houses painted white in hot countries? White reflects heat radiation and so keeps the house cooler. Why are shiny foil blankets wrapped around marathon runners at the end of a race? The shiny metal reflects the heat radiation from the runner back in to their body, which stops the runner from getting cold.

27 © Boardworks Ltd of 31 Infrared radiation – true or false?

28 © Boardworks Ltd of 31 Heat Transfer Conduction Insulators Thermal radiation Summary activities Convection Contents

29 © Boardworks Ltd of 31 Glossary absorber – A material that takes in thermal radiation. conduction – The heat transfer mechanism in solids. conductor – A material that lets heat flow through it. convection – The heat transfer mechanism in fluids, which occurs because hot fluids are less dense than cold fluids. emitter – A material that gives out thermal radiation. free electrons – Electrons in a metal that are free to move throughout the metal. heat transfer – The flow of heat energy from a hotter area to a colder area. insulator – A material that does not let heat flow through it and so is a very poor conductor of heat. radiation – Heat energy transferred by infrared waves. This method of heat transfer does not need particles.

30 © Boardworks Ltd of 31 Anagrams

31 © Boardworks Ltd of 31 Multiple-choice quiz


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