Year 8 Science: Heat Transfer |
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| Key ideas: • Heat flows from hot to cold • Heat energy makes the particles of a material vibrate further and faster • The temperature of a solid or liquid represents how far the particles vibrate • The temperature of a gas represents how fast the particles are travelling • For gases, more heat energy make the particles move faster • Conduction is heat energy transferred from a material to another in contact with it or through the same material • Materials can be conductors or insulators (or in-between!) • Gases are always bad conductors as the particles don't touch • Convection is the flow of liquids and gases caused by heating • Do not say “heat rises”, but “hot air rises” or “hot liquids rise” • Radiation is heat travelling like light rays • Dark objects absorb better and radiate more heat rays • Light / shiny objects reflect heat rays and do not radiate heat well Key words: • conduction Concept diagram:
Useful notes: Conduction The vibration of particles in a hot object that touches a colder one makes the “colder” particles vibrate faster, so they are hotter. The movement of the “hotter” particles is reduced slightly, so they are cooled. E.g. when you touch a hot radiator, the vibration of the radiator particles is connected to your hand. Your hand is therefore hotter. The radiator gets colder, but because it is a lot bigger than your hand, you can't tell. Also you took your hand off as it was hot! Metals are always good conductors. Gases never are, they are insulators, because they are mostly empty space, so the particles are seldom touching. The particles in liquids are free to move, so the vibrations can't be carried along and they are not good conductors either. Most plastic materials are poor conductors, so they are useful for saucepan handles. Wood is another good material for this sort of job. Convection When you heat fluids (gases and liquids), they expand. If only part has been heated, then this part is less dense than surrounding cooler parts and tries to float above the cooler parts. [This is how a hot-air balloon works, as it is full of air that is lighter than the surrounding air, making it float.] In a single container, the rising currents are called convection currents and they move heat energy as the warm gases or liquids move around. Radiation
If you sit in the sunshine, you can feel the heat of the Sun on your body. You are not touching the Sun, it is 140 million kilometres away, nor is hot air rising from below, as there is no air in space. The heat is being sent directly to you as infra-red rays, which are heat rays - heat travelling like light. This is easy to prove, as going into shade will reduce the heat you receive and you will only feel the air temperature. This way of transferring heat is called radiation. Radiated heat travels in all directions, not just up. Radiation is the only way to transfer heat in a vacuum, but infra-red is reflected by mirrors, so a “vacuum flask” has silvered surfaces inside to reflect heat back in. Snow and sand make a difference, too, as infra-red rays can reflect off these, making the air above them hotter. Infra-red does not sunburn you, however, that is ultra-violet.
Radiation “shines” off hot objects more than cold ones. Black objects radiate better than shiny ones at the same temperature. They also absorb heat rays better than shiny ones. This is why people in hot countries paint their houses white, to reflect light and heat rays. Electric fires have shiny reflectors behind the hot bars for the same reason. Solar collectors have black panels to get as much heat as possible into the water that circulates inside.
In the laboratory, we use the Leslie Cube to demonstrate how differently-coloured surfaces radiate differently Useful websites: This site has photos of the Earth taken in infra-red light: http://observe.arc.nasa.gov/nasa/exhibits/er-2/highsci_3.html
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