Energy Heat Transfer 1 of 31 Boardworks Ltd

Energy Heat Transfer 1 of 31 © Boardworks Ltd 2005

Contents Heat Transfer Conduction Insulators Convection Thermal radiation Summary activities 2 of 31 © Boardworks Ltd 2005

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 Heat energy only flows when there is a temperature ooler area. difference from a w______ armer area to a c______ 3 of 31 © Boardworks Ltd 2005

What is conduction? How are the particles arranged in a solid, a liquid and a gas? solid liquid gas 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? 4 of 31 © Boardworks Ltd 2005

Conduction in non-metals Graphite is a non-metal that is a good conductor of heat. Heat one end of a graphite rod and 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. 5 of 31 © Boardworks Ltd 2005

Conduction in metals 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. heat This makes heat transfer in metals very efficient. Insulators do not have these free electrons which is why they do not conduct heat as well as metals. 6 of 31 © Boardworks Ltd 2005

Contents Heat Transfer Conduction Insulators Convection Thermal radiation Summary activities 7 of 31 © Boardworks Ltd 2005

A temperature problem Touch a piece of metal and a piece of wood at the same temperature. Which material feels warmer and why? 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. 8 of 31 © Boardworks Ltd 2005

Insulators Some materials are very poor conductors of heat. These are called insulators. Examples of materials that are insulators include plastics, wood, ceramics and air. 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. Other insulating materials, including polystyrene and loft insulation, use trapped air because it is so effective. 9 of 31 © Boardworks Ltd 2005

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? 10 of 31 © Boardworks Ltd 2005

Double glazing How does double glazing keep a house warmer? Double glazing keeps a house warmer because there is a layer of ir between the panes of glass. a__ onductor so it acts Air is a poor c____, nsulator. like an i_______, educes heat loss The trapped air r_______ onduction from a house. by c_____ Side-view of double glazing 11 of 31 insulating layer of air © Boardworks Ltd 2005

Contents Heat Transfer Conduction Insulators Convection Thermal radiation Summary activities 12 of 31 © Boardworks Ltd 2005

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? heat less dense fluid 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. 13 of 31 © Boardworks Ltd 2005

What is convection? 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. 14 of 31 © Boardworks Ltd 2005

Convection current hot water cools hot water rises cool water sinks cool water warms This is called a convection current. 15 of 31 © Boardworks Ltd 2005

Why is it windy at the seaside? 16 of 31 © Boardworks Ltd 2005

Convection in a fridge 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. 17 of 31 It is warmer at the bottom of the fridge. This warmer air rises and so a convection current is set up. © Boardworks Ltd 2005

Convection questions 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. 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. 18 of 31 © Boardworks Ltd 2005

Conduction and convection – summary 19 of 31 © Boardworks Ltd 2005

Contents Heat Transfer Conduction Insulators Convection Thermal radiation Summary activities 20 of 31 © Boardworks Ltd 2005

The third type of heat transfer Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth? ? infrared radiation There are no particles between the Sun and the Earth so the heat cannot travel by conduction or by convection. 21 of 31 The heat travels to Earth by infrared waves. They are similar to light waves and are able to travel through empty space. © Boardworks Ltd 2005

Infrared waves 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: l They can travel through a vacuum. l They travel at the same speed as light – 300, 000 m/s. l They can be reflected and absorbed. Infrared waves heat objects that absorb them and so can be called thermal radiation. 22 of 31 © Boardworks Ltd 2005

Emitting thermal radiation All objects emit (give out) some thermal radiation. Some surfaces are better at emitting thermal radiation than others. worst emitter best emitter matt black white silver 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? 23 of 31 © Boardworks Ltd 2005

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. worst emitter best emitter matt black best absorber white silver worst absorber 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? 24 of 31 © Boardworks Ltd 2005

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. 25 of 31 © Boardworks Ltd 2005

Infrared radiation – true or false? 26 of 31 © Boardworks Ltd 2005

Contents Heat Transfer Conduction Insulators Convection Thermal radiation Summary activities 27 of 31 © Boardworks Ltd 2005

Glossary l absorber – A material that takes in thermal radiation. l conduction – The heat transfer mechanism in solids. l conductor – A material that lets heat flow through it. l convection – The heat transfer mechanism in fluids which l l l 28 of 31 occurs because hot fluids are less dense than cold fluids. emitter – A material that gives out thermal radiation. free electrons – Electrons in a metal which 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. © Boardworks Ltd 2005