Heat transfer
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Heat can be transferred through three main processes: conduction, convection and radiation. Radiation involves the transfer of heat energy through electromagnetic waves called infrared radiation. Infrared radiation is able to travel through empty space, which is how heat from the Sun travels to the Earth. All objects emit infrared radiation, with the amount emitted depending on the object's surface properties - matte black surfaces are the best emitters and absorbers of infrared radiation.
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Heat transfer
- 1. © Boardworks Ltd 20061 of 32
- 2. © Boardworks Ltd 20062 of 32
- 3. © Boardworks Ltd 20063 of 32 How is heat transferred? Where and how is heat transfer taking place at this seaside?
- 4. © Boardworks Ltd 20064 of 32 Heat energy only flows when there is a temperature difference from a warmer area to a cooler area. Why does heat transfer happen? Heat is a type of energy called thermal energy. 1. conduction During heat transfer, thermal energy always moves in the same direction: HOT COLD 2. convection 3. radiation Heat can be transferred (moved) by three main processes:
- 5. © Boardworks Ltd 20065 of 32 Why do objects get hotter or colder? Temperature is a measure of how hot an object is. Why does an ice lolly melt on a warm tongue? There is a temperature difference between the tongue and the lolly, so heat energy flows from the warm tongue into the cold ice lolly. How might climate change cause the polar ice caps to melt? Heat transfer only takes place when there is a temperature difference. The heat energy flows from a warmer area to a cooler area. This heat transfer means that the ice lolly melts as it gets warmer, and the warm part of the tongue touching it gets cooler.
- 6. © Boardworks Ltd 20066 of 32 Heat The flow of thermal energy from one object to another. Heat always flows from warmer to cooler objects. Ice gets warmer while hand gets cooler Cup gets cooler while hand gets warmer
- 7. © Boardworks Ltd 20067 of 32
- 8. © Boardworks Ltd 20068 of 32 How are the particles arranged in a solid, a liquid and a gas? What is conduction? solid liquid gas Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction. What type of solids are the best conductors? Conduction is the method of heat transfer in solids but not liquids and gases. Why?
- 9. © Boardworks Ltd 20069 of 32 How do non-metals conduct heat?
- 10. © Boardworks Ltd 200610 of 32 Metals are good conductors of heat. 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. How do metals conduct heat? heat This means that the free electrons move faster and transfer the energy through the metal. This makes heat transfer in metals very efficient. Insulators do not have free electrons and so they do not conduct heat as well as metals.
- 11. © Boardworks Ltd 200611 of 32
- 12. © Boardworks Ltd 200612 of 32 What happens a fluid is heated? Liquids and gases can both flow and behave in similar ways, so they are called fluids. 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 What happens to the particles in a fluid when it is heated?
- 13. © Boardworks Ltd 200613 of 32 What is convection? Warmer regions of a fluid are less dense than cooler regions of the same fluid. called a convection current. The warmer regions will rise because they are less dense. The cooler regions will sink as they are more dense. This is how heat transfer takes place in fluids and is called convection. The steady flow between the warm and cool sections of a fluid, such as air or water, is
- 14. © Boardworks Ltd 200614 of 32 How does convection in a liquid occur?
- 15. © Boardworks Ltd 200615 of 32 How does convection in a gas occur?
- 16. © Boardworks Ltd 200616 of 32 Why is the freezer compartment at the top of a fridge? The freezer compartment is at the top of a fridge because cool air sinks. This warmer air rises and so a convection current is set up inside the fridge, which helps to keep the fridge cool. Why is convection important in fridges? 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.
- 17. © Boardworks Ltd 200617 of 32 Conduction and convection – summary
- 18. © Boardworks Ltd 200618 of 32
- 19. © Boardworks Ltd 200619 of 32 infrared waves The Earth is warmed by heat energy from the Sun. There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection. ? The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space. How does heat travel through space? How does this heat energy travel from the Sun to the Earth?
- 20. © Boardworks Ltd 200620 of 32 Heat can move by travelling as infrared waves. 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. What are infrared waves? Infrared waves heat objects that absorb them and are also known as thermal radiation. These are electromagnetic waves, like light waves, but with a longer wavelength.
- 21. © Boardworks Ltd 200621 of 32 Investigating thermal emission
- 22. © Boardworks Ltd 200622 of 32 Emitting thermal radiation All objects emit (give out) some thermal radiation. Matt black surfaces are the best emitters of radiation. white silver matt black best emitter worst emitter Certain surfaces are better at emitting thermal radiation than others. 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. © Boardworks Ltd 200623 of 32 Investigating thermal absorption
- 24. © Boardworks Ltd 200624 of 32 Absorbing thermal radiation Infrared waves heat objects that absorb (take in) them. best emitter worst emitter best absorber worst absorber white silver matt black Matt black surfaces are the best absorbers of radiation. Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers. 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?
- 25. © Boardworks Ltd 200625 of 32 Infrared radiation – true or false?
Editor's Notes
- Teacher notes This illustration contains several examples of heat transfer, including: Barbecue grill cooking burgers and sausages (conduction, radiation). The car bonnet is so hot that as a sideline to the barbecue, someone is cooking food on it (conduction). Picnic boxes labelled “cold” and “hot” contain items that have been kept cool and war, respectively, to prevent heat transfer taking place. The twins on the left are wearing identical clothing except that one is wearing a white t-shirt and keeping cool, while the other is wearing a black t-shirt and looking much hotter (radiation). The twin in the black t-shirt is trying to keep himself cool with a portable fan (convection). The fluttering sails on the boat out at sea and the fluttering flag on the flagpole show that it is windy at the seaside (convection). The ice-creams are melted very quickly by the heat of the Sun, much to the annoyance of the child near the ice-cream van (radiation).
- Teacher notes The importance of these visual representations of the arrangement of particles in solids, liquids and gases should be stressed. Students could be asked to discuss the limitations of these models, which include: the models are static whereas the particles are actually moving the models are two-dimensional whereas the particles are actually 3D dimensional. Students could also be asked to discuss the good points of the models and why they are useful.
- Teacher notes This animation shows how heat is conducted through a non-metal on a particle level. Whilst viewing the animation it should be emphasised that this process is relatively slow, which is why conduction in non-metals is slow compared to conduction in metals.
- Teacher notes Please note that this diagram is not drawn to scale. It should be stressed to students that an electron is tiny compared to a metal ion, especially the solid nucleus of a metal ion. This means that the electrons are able to pass between the metal ions.
- Teacher notes The diagram illustrates how temperature affects the density of particles in a fluid. When using the diagram it should be highlighted that the particles near the top are further apart and the particles near the bottom are closer together. The widely-space particles at the top of the diagram represent a warmer region of the fluid. They are floating on top of a cooler region of air (represented by the particles spaced closely together) because the warm region of the fluid is less dense. The dotted boxes have been included to help students visualise the density of the fluid. Students could be asked to count the number of particles in each box to compare the density of the different regions. It should be noted that the density of the regions of the fluid in this diagram have been exaggerated to increase the contrast and make the regions clearer to see.
- Teacher notes This three-stage animation uses the example of boiling water in an electric kettle to demonstrate how convection occurs in a liquid. Whilst showing the animation it could be highlighted that the hot water floats up through the cold water because the hot water is less dense. The denser cold water falls below the hot water, pushing the hot water up.
- Teacher notes This three-stage animation uses the example of seaside breezes to demonstrate how convection occurs in a a gas. Red arrows are used to represent the movement of warm air and blue arrows are used to represent the movement of cooler air. Suitable prompts include: Stage 1: Why is the hot air rising? Stage 2: What happens to the warmer air as it moves out to sea? Stage 3: Why has the convection current changed direction?
- Teacher notes This completing sentence activity provides the opportunity for some informal assessment of students’ understanding of conduction and convection.
- Teacher notes This virtual experiment uses a Leslie’s cube to investigate the emissions of heat radiation from different surfaces. It could be use as a precursor to running the practical in the lab or as a revision exercise. When using this activity, it should be made clear that the water inside the cube and the initial temperature of each side is the same in each experiment. The thermopile measures the amount of heat radiation emitted by the surface, which is then recorded in the table.
- Teacher notes This virtual experiment investigates the ability of different surfaces to absorb heat. It could be use as a precursor to running the practical in the lab or as a revision exercise. To extend the activity, students could be asked to consider which variables need to be controlled to make this experiment fair and accurate.
- Teacher notes This true-or-false activity could be used a plenary or revision exercise to check students’ understanding of infrared radiation. Students could be given coloured traffic light cards (red=false, green=true) to vote on the statements shown. To stretch students, they could be asked to explain their voting.