|
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 |
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
alyssa:)
3A1
comments
affiliates
linklink link link link This layout is done by balloon.s with the help from here. archives
May 2009
June 2009
|
.bmp){kind=link}