Heat Transfer

Complete the sentences below. a) Thermal radiation is energy transfer by __________________________________________. b) All bodies emit ________________________________________. c) The hotter a body is, the more thermal radiation _______________. d) Dark ________ surfaces are better absorbers and emitters of thermal radiation than __________ shiny surfaces. e) Conduction in a metal is due to _______ electrons transferring energy inside the metal. f) Non-metals are ________ conductors because they do not contain free electrons. g) Materials such as fibre glass are _________ insulators because they contain pockets of trapped _________. h) Convection takes place only in liquids and __________. i) Heating a liquid or a _______ makes it _________ dense. j) Convection is due to a hot liquid or gas _________. k) A radiator has a _________ surface area so it can lose heat easily. l) ________ objects lose heat more easily than _________ objects do. m) Heat loss from a building can be reduced using: i) Aluminium foil behind______________________. ii) Cavity wall _________________________. iii) Double _________________________. iv) Loft _________________________.

In the next 14 questions at least one answer will be correct but there may be more than one correct answer. Circle all you think are correct.

Heat transfer The transfer of thermal energy through space from the Sun to the Earth is by: A Conduction B Convection C Condensation D Radiation (1) 2 Infra-red radiation is absorbed, reflected and emitted to different extents by different surfaces. Which of the following statements best applies to dark, matt surfaces? A They are good emitters of infra-red radiation. B They are good reflectors of infra-red radiation. C They are poor absorbers of infra-red radiation. D They are poor emitters of infra-red radiation. (1) The diagram shows water being heated in a saucepan on a hotplate. Match words from the list with the numbers 1 to 4 in the sentences. A conduction B convection C radiation D insulator Heat energy is transferred through the base of the saucepan by ……1…… Heat energy is transferred through the water in the saucepan by ……2…… Some energy is transferred from the hotplate to the air by ……3…… The handle of the saucepan is made from wood because wood is a good ……4……. (4)

The diagram shows some ways of reducing energy loss from a house. The table shows the cost of fitting, and the annual savings on energy bills, for different methods of reducing energy loss from a house. (a) Which method pays for itself in the shortest time? A cavity wall insulation B double glazing C draught proofing D loft insulation (1) (b) Which method reduces energy loss by the greatest amount? (c) The time it takes for the saving on energy bills to equal the cost of installing the insulation is called the pay-back time. What is the pay-back time for loft insulation? A 1 year B 2 years C 3 years D 5 years (1) (d) Fitting double glazing reduces energy loss by … A conduction B convection C evaporation D radiation (1) Method of reducing energy loss Cost of fitting Annual saving Cavity wall insulation £800 £100 Double glazing £4 000 £80 Draught proofing £60 Loft insulation £300

A cook at a barbecue has baked some potatoes A cook at a barbecue has baked some potatoes. He takes them off the barbecue. (a) The hot potatoes radiate thermal energy. The amount of thermal energy radiated by a potato depends upon the nature of its surface. Name two other variables that affect the amount of thermal energy radiated by a potato. ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………. (2) (b) The cook wraps the hot potatoes in clean, shiny foil. Explain why. (c) Explain what the difference would be if the cook wrapped the potatoes in black, dull foil. Suggest why: (i) the outside of the potatoes cooks before the inside. ……………………………………………………………………………………………………………………………………………………………………………………………………………………… (ii) some cooks put metal skewers through the potatoes before they put them on the barbecue. …………………………………………………………………………………………………………………………………………………………………………………………………………………………(2)

a) Match words, A, B, C and D, with the numbers 1– 4 in the sentences. 6) The drawing shows part of a bedroom. The bedroom is heated by an electric fire. There is a loft space above the bedroom ceiling. Above the bedroom ceiling is a layer of fibreglass. When the electric fire is switched off, the bedroom cools down. a) Match words, A, B, C and D, with the numbers 1– 4 in the sentences. A conduction B convection C insulation D radiation The bedroom cools down because heat is transferred through the floor, the ceiling and the walls by . . . 1 . . . . Heat is transferred from the outside of the walls by . . . 2 . . . . Heat is transferred through the air in the loft by . . . 3 . . . . Fibreglass reduces heat transfer through the ceiling because it has good . . . 4 . . . properties.

7). Many homes have hot-water tanks 7) Many homes have hot-water tanks. An electric heater is used to heat the water. Some heat (thermal energy) is lost through the metal walls to the surroundings. 7A The energy spreads through the water by . . . 1 heated water contracting and falling. 2 heated water expanding and rising. 3 heat rising. 4 the movement of free electrons. 7B The energy is transferred through the metal walls by . . . 1 heated water expanding and rising. 2 infra red waves passing through the metal. 3 the atoms gaining energy and moving faster through the metal. 7C The metal walls of the tank transfer energy to the surroundings by . . . 1 heated air contracting and falling. 2 infra red waves passing through the air. 3 metal atoms gaining energy and escaping into the air. 4 the movement of free electrons 7D The air in contact with the outside of the metal walls . . . 1 contracts and falls due to decreased density. 2 contracts and falls due to increased density. 3 expands and rises due to decreased density. 4 expands and rises due to increased density.

8) A student investigated the question: Does the type of surface affect how an object loses or gains heat? She carried out two experiments. In her first experiment, she filled two cans with boiling water. One can had a dull, black surface. The other can had a shiny, silver surface. Later, she measured the temperature of the water in each can. These are her results. 8A The student wanted her experiment to be a fair test. Which row of the table is most likely to give a fair test? 8B If her results are reliable, the experiment shows that, compared with a shiny, silver surface, a dull, black surface is a . . . 1 better absorber of thermal radiation. 2 better conductor of thermal radiation. 3 better emitter of thermal radiation. 4 better reflector of thermal radiation.

In her second experiment, the student put cold water in each can In her second experiment, the student put cold water in each can. She put a heater (which is a source of thermal radiation) between the cans. She switched the heater on and, ten minutes later, she measured the temperature of the water in each can. These are the results of her second experiment. 8C If her results are reliable, the second experiment shows that, compared with a shiny, silver surface, a dull, black surface is a . . . 1 better absorber of thermal radiation. 2 better conductor of thermal radiation. 3 better emitter of thermal radiation. 4 better reflector of thermal radiation. 8D What was the average (mean) rate of the rise in temperature of the water in the can with a dull, black surface? 1 0.9 °C per minute 2 2.1 °C per minute 3 9 °C per minute 4 21 °C per minute