Transcript Document
Energy Menu • Transfer of Energy • Conduction • Convection • Radiation • Heat Transfer in Homes The Transfer of Energy Heat flows from a warm place to a cold place This hot cup of tea will cool down because its heat energy is being released into the surroundings. These cold bottles will warm up because they are taking in heat energy from the warmer surroundings. Return to Menu Conduction Conduction Direction of energy travel Heat If one end of a metal bar is heated as above, the other end eventually becomes too hot to touch. Thermal energy (heat) is transferred from the hot end to the colder end as the faster particles pass on their energy. This process is called CONDUCTION Why does this robin ‘fluff’ up its feathers in the winter? To trap air between its feathers. The air is a good insulator which prevents heat transfer by conduction. The air can’t move so it also reduces heat transfer by convection. The robin remains warmer as a result. Return to Menu Convection Convection Hot air rises as it becomes less dense A Convection Current Colder air falls as it becomes more dense Gases (& liquids) are poor thermal conductors, but if they are free to circulate, they can carry the thermal energy from one place to another very quickly. Hot air balloons use Convection……. The balloonist heats the air inside the balloon. As a result the air becomes less dense and rises. The balloon moves upwards. Birds and Hang Gliders use the convection currents in the air to give them lift Coal mines were ventilated using convection – HOW? Fresh, cold air falls into shaft Hot air rises as it is lighter Mine is ventilated with fresh air and miners don’t suffocate. Is there a need for a pump with natural convection? Cold water falls Result: Water circulates without a pump! Heated water rises How does a boiler work? HOT Heater COLD The hot water pipe is at the top of the boiler. The hottest water is here because it is less dense & has risen to the top. The cold water must go in at the bottom because it is more dense. Do we get convection in microgravity? Because of the effects of gravity and convection, the candle flame on Earth always points ‘up’. It also burns with a sooty yellow flame. In a weightless environment – there is no ‘up’ & ‘down’. The candle flame burns with a hotter blue flame and with a distinct circular shape Return to Menu Radiation What is Radiation? Thermal Radiation is given out by any hot object. The hotter the object – the more radiation it gives out. On Earth we are warmed by the Sun. Its energy travels to us in the form of electromagnetic waves that travel through ‘Empty Space’ (it is the only method of Heat transfer that can do this). Thermal radiation is invisible to our eyes. We can detect thermal radiation with our bodies (we feel this as heat!) Infra red cameras can also detect heat radiation. This is of special use to: The Police (when chasing suspected criminals at night) & to the Fire Brigade to rescue people. Radiation Matt Black Silver Some colours are better at absorbing thermal radiation than others because they do so at a faster rate. Which colour cylinder will heat up the quickest? Petrol tankers are often painted with a silver paint. WHY? To reflect the thermal radiation. By doing this the petrol tanker won’t heat up too quickly and explode! White or silvery surfaces are poor absorbers of thermal radiation because they reflect most of the thermal radiation away. This is why houses are often painted white to keep them cool inside, in hot countries like Spain Keeping Heat Radiation OUT: Spacesuits Cooking Foil on the turkey Keeping Heat Radiation IN: Shiny metal kettles Survival blankets Emitters & Absorbers Some surfaces are better at emitting (sending out) thermal radiation than others. For example, a black saucepan cools down more quickly than a similar silver one because it emits heat radiation at a faster rate. How does the Vacuum Flask work? It has an insulated stopper to reduce conduction & convection A gap between the walls of the container with air removed to reduce conduction & convection. Walls with silvery surfaces to reduce thermal radiation A polar bear is white to reduce the heat loss by radiation. (White is a poor emitter of heat radiation). Its body is covered by fur (which has ‘trapped air’ between each hair) to reduce heat loss by conduction & convection. Return to Menu Heat Transfer In Our Homes How do our homes waste heat energy? This is a picture taken with an infra red camera. It shows some places where heat is wasted from a house 35 0C This is a picture of the roof area. Can you see where most of the heat is wasted? 20 0C How can we reduce heat loss from our homes? Loft insulation Cavity wall insulation reduces heat loss through walls Carpets & underlay for floors Double glazing for windows Draught proofing for doors & windows Why do we use LOFT INSULATION? It reduces the heat waste by convection (hot air is less dense and rises). It also reduces heat wasted by conduction (air is trapped between the fibres. The air is a good insulator) Return to Menu