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Wednesday, June 24, 2009 @ 9:31 PM
applications applications of conduction : 1. - good conductors of heat ~ cooking utensils, such as kettles, saucepans, pots Ø usually made from aluminum and steel Ø can conduct heat quickly from fire to food 2. - insulators of heat ~ handles of cooking utensils, such as pans, pots, kettles, iron Ø made from wood or plastic Ø hot utensil can be picked up without scalding our hands 3.  ~ woolen clothes, fur, feathers Ø traps a lot of air, which is an insulator of heat Ø keeps people/ animals warm 4. ~ Styrofoam boxes Ø hot food can remain warm applications of convection : 1.  - cooking appliances, such as electric kettles, ovens, rice cookers Ø heating elements are found at the bottom Ø convection currents would be formed Ø hot air would rise and heat the food further from the heating elements, food at the bottom would be directly heated by the element Ø food gets heated up quickly 2.  - air conditioners Ø installed near the top of the ceiling to facilitate convection currents Ø rotary fan inside an air conditioner releases cool air into the room Ø warm air rises to be cooled , denser cool air sinks Ø cool air then cools us Ø air is also circulated 3. - refrigerator Ø freezers are constructed at the top Ø allows cold air to sink and hot air to rise Ø helps to cool down the contents inside - household water systems Ø water is heated up in the boiler 4.  - hot water system Ø water is heated up in the boiler Ø hot water expands and becomes less dense, hence it rises to the upper part of the storage tank ( in red) Ø cold water from the cistern/cold water supply as well as the cooler water in the storage tank will fall into the boiler Ø hot water will come out from the hot tap Ø the overflow/ expansion pipe is attached to the storage tank in case the hot water expands too much and overflows 5. air and sea breezes  ~ day Ø land heats up faster than the sea Ø hot air (red arrow)above land rises, cold air (blue arrow)above sea sinks Ø sea breeze  ~ night Ø sea is warmer than land Ø hot air (red arrow) above sea rises, cold air (blue arrow)above the land sinks Ø land breeze 7. - hot air balloons Ø they are able to rise, Ø the air inside the balloon is heated up Ø the hot air expands and is less dense than the surrounding air outside Ø hot air rises, also causing the balloon to rise applications of radiation : 1.  - greenhouse Ø greenhouses are used in cold climates to help plants grow better by trapping heat Ø in the day, infrared radiation from the sun passes through the glass roof of the greenhouse Ø warms up the plants Ø the contents starts to get warm and emit infrared radiation Ø the radiation emitted cannot pass through the glass roof Ø it gets trapped and builds up over time, causing the temperature in the greenhouse to increase 2. - kettles, teapots Ø brightly polished, light coloured, smooth surface to reduce heat emission through radiation Ø can keep liquids hot for a longer time 3. - cooling fins Ø at the back of the refrigerator, in a car radiator, on a motor bike engine Ø painted black to radiate heat quickly to the surroundings 5.  - solar panels Ø dark surfaces to absorb heat  how solar panels work ~heat from the sun warms the water in the pipes ~ water is pumped to the hot water tank ~ heat is also transfered from water in the pipe to water in the tank ~ cooler water is circulated backto the solar panel · the sheet of glass traps heat · insulation reduces heat loss to the surroundings 6. - shiny blankets(that marathon runners use) 7. - firefighting suits 8. - houses Ø in hot countries, people paint their houses white to reduce heat absorption from the sun. 9.  - vacuum flask Ø designed to keep liquids hot by minimising heat loss through conduction, convection, radiation and evaporation Ø stopper : made of plastic which is a poor conductor of heat Ø trapped air (above the liquid) : reduces conduction since air is an insulator of heat Ø vacuum (between the double glass walls): conduction and convection through the sides are prevented Ø silvered glass walls: reduces heat loss through radiation, walls reflect radiant heat back into the liquid Saturday, June 13, 2009 @ 11:05 PM
radiation what is radiation? - continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium. - does not require a medium , and can even take place in a vacuum. - all objects emit some radient heat -the sun is a major source of radient heat it emits electromagnetic waves to make us feel warm ---> infrared waves  factors affecting the rate of radiation A. colour and texture of the surface ~ dull, black surfaces are better absorbers and emitters of infrared radiation than shiny, white surfaces  absorption of radiation: dull black surfaces absorb radiation faster(so it will be hotter) than white and bright surfaces. (note the temperature difference on the thermometer)  emission of radiation: dull black surfaces radiate more heat faster than white shiny surfaces. (again, note the temperature difference)  B. surface temperature ~ the higher the temperature, the higher the rate of radiation  ~ the colder the object, the higher the rate of heat absorption  C. surface area ~ the larger the surface area, the higher the rate of radiation Friday, June 12, 2009 @ 11:04 PM
convection what is convection? - transfer of thermal energy through currents in a liquid or gas (convection currents) how does convection work? - convection current occurs only in fluids (liquids and gases) but not solids why? ~ convection involves the bulk movement of fluids which carry the thermal energy with them ~ in solids, thermal energy is transferred through vibrations, not through bulk movement of the particles - a convection current is the movement of the fluid caused by the change in density in various parts of the liquid ~ less dense- rise, more dense- sink  red arrows show the hotter region rising, blue arrows show the cooler regions sinking  cooler gas/ liquid ---> denser ---> sink hotter gas/ liquid ---> less dense ---> rise 1. convection in liquids 2.bmp) this diagram shows what will be observed when a beaker filled with water and some potassium permanganate crystals at the bottom. what will be observed: ~ circulation of purple stream of water, which represents convection currents Ø when the water is heated, it expands and is then less dense than the surrounding water Ø it starts to rise Ø the cooler parts of the water, which is denser, sinks. experiment~ - red is cold dye, green is hot dye - the red cooler liquid will sink because it is denser the results~ 2. convection in gases .bmp) this diagram shows what will be observed when a glass box with 2 chimneys has a candle inside the box under one chimney, and a burning paper above the other chimney. what will be observed: ~ smoke from the paper flow down from one chimney towards the candle and rises up from the other chimney at the other end. Ø air above the candle is heated and expands , it is less dense than the surrounding air Ø rises out of the chimney Ø the cooler surrounding air is denser, it sinks through the chimney to replace the less dense air. · the burning candle is used as a heat source to generate convection currents · paper is burnt to produce smoke to help us observe the movement of convection current experiment the paper spiral will twirl when placed under a lighted candle because: the candle heats up the air above it, the hot air becomes less dense and will rise Sunday, May 31, 2009 @ 11:27 PM
conduction what is conduction? - transfer of thermal energy through a medium, without it moving - conduction is the process of thermal energy transfer without any flow of the material medium how does conduction work? - can occur in solids, liquids and gases, but it works better and is more efficient in solids. why? ~ in liquids and gases, according to the kinetic model of matter, their molecules are spaced further apart than solids. ~ there would be lesser collisions in liquids and gases ~ so the transfer of kinetic energy from faster moving molecules (molecules of the hotter end of the liquid/gas) to the slower moving neighbouring molecules is slower ~ in solids, their molecules are closely packed so more collisions, kinetic energy is transferred ~ heat conducts through solids the fastest; liquids and gases are poor conductors of heat.  - 2 types of conduction: molecular vibration and free electron diffusion ~ molecular vibration Ø when heat is supplied to one end, the molecules at the hot end vibrate vigorously Ø they collide with neighbouring molecules, making them gain kinetic energy and vibrate as well Ø kinetic energy at the hot end is transferred to the neighbouring molecules Ø quite a slow way  molecules at the hot end vibrate and pass the kinetic energy to the other molecules at the cooler end ~ free electron diffusion Ø this type of conduction only takes place in metals as only metals have free electrons Ø when the metal is heated, the free electrons gain kinetic energy and move faster Ø the electrons then diffuse into the cooler parts of the metal, colliding with the molecules in the cooler part of the metal Ø much faster mechanism ~ comparing the 2 types Molecular vibration: occurs in solids(metals and non-metals), slow process Free electron diffusion: Occurs only in metals, faster process this explains why metals are more capable of transferring thermal energy more quickly: 1. metals have 2 methods of conduction occurring at the same time 2. in metals, free electron diffusion is the main mechanism, which is faster 1. conduction in solids  this diagram shows an experiment of conduction through a copper rod and a glass rod to compare their rate of conduction what will be observed: ~ wax melts faster along the copper rod than the wax on the glass rod Ø copper is a conductor of heat Ø glass is an insulator Ø also shows that metals transfer thermal energy more quickly than non metals because of the presence of free electrons in metals experiment~ - 2 spoons, one plastic and the other metal is put into a cup of very hot water - equal amouts of butter is smeared on the spoons at the same level - the butter on the metal spoon melted faster than the butter on the plastic spoons - metals are better conductors of heat than non metal - heat travelled through the solid through conduction 2. conduction in liquids  this diagram shows an experiment of conduction in water what will be observed: ~ the water at the upper end of the test tube boils quickly ~ the ice melts very slowly Ø thermal energy transfer through conduction from the water at the top to the bottom is very slow Ø it shows that water is a poor conductor of heat experiment when a matchstick is held away from a candle flame, it will not burst into flames this is because: - air between the matchstick and the flame is an insulator of heat in conclusion... - materials that conduct heat easily/are readily good conductors of heat.(eg. metals like copper, silver, steel, iron) - materials that do not conduct heat easily are insulators and poor conductors of heat.(eg. non-metals like glass, plastic, wood, brick, wool, air, water) - metals are conduct heat faster because they have 2 methods of conduction of which free electron diffusion is faster @ 11:20 PM
thermal energy transfer of thermal energy - thermal energy is transferred only when there is a difference in temperature. - thermal energy flows from a region of higher temperature to a region of lower temperature. - thermal energy is transferred by 3 processes: conduction, convection and radiation   |
hello, welcome to my physics blog:) please comment, thanks!:) physics txbk chap 10: transfer of thermal energy oh all the blogger videos are done by clarissa and me:)haha profile
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