Wednesday 9 December 2009

Transfer process

Conduction: It is the process that heat is transferred through a body from a region of higher temperature to region of lower temperature. Relative conductivity of heat of different materials: Metal (Ag>Cu>Al>Fe)>glass>plastic (rubber)>wood>cotton>polystyrene Usually liquid and gas is poor conductor of heat (good insulator of heat). Molecular motion about conduction: We know that molecules with higher K.E. vibrate more vigorously. Intermolecular force bonds the molecules together. Therefore when one end of the substance is hotter than another side, molecules with higher K.E. set their neighbouring molecules to vibrate more vigorously. Then the energy is transferred to those neighbouring molecules. Within metallic bond, free (mobile) electrons and ions exist. Those electrons move freely within that piece of metal. When they receive energy, their K.E. increase and quickly transfer to other electrons or ions, then energy is transferred quickly. Therefore metals are good conductors of heat, and non-metals are usually poor conductors of heat. Factors affecting rate of conduction There’re four factor: conductivity of heat of the material; temperature difference between two ends; length between two ends and cross-section of the body. Experiment about conduct heat of different materials Soak a piece of filter with Cobalt(II) chloride solution, place four rods (copper, brass, aluminium and glass) with the same length and cross-sectional area on the paper. Heat one end of the rods by Bunsen flame. Compare the relative conductivity of heat by looking the colour of the filter paper. Applications on conduction Good conductors of heat for heating: Cookware (Cu exterior) and metal skewer. Good conductors of heat for cooling: heat sink (Cu, Al) in CPU and radiator in a car. Good insulators of heat for keeping warm: fur on animals, jackets, polystyrene food containers, plastic handles for cookware and refrigerator. Conduction: It take place in a fluid (liquid/gas). It’s a process which heat is transferred by the movement of the fluid from one to each other. When a fluid is heated, the molecules inside it moves (vibrate) faster. As a result fluid expands and become less dense. Then it has a lower density than other surrounding fluid so that it’ll rises. Cooler fluid flows down because it has a higher density. As a result, convection current is formed: Hotter fluid rises and cooler fluid sinks. The convection current taught us that air conditioners should be set in a higher place of a room so that cooler air sinks. Also radiators should be set in a lower place such that warm air flows up. In the environment sea breezes and land breezes also shows the convection current. Sea breezes in fine and warm weathers: Air in the land is heated faster and it rises, as a result, air in the sea is heated in a slow way (water has a high specific heat cap.) so that it sinks. Then gas pressure in the land decreases and sea breezes come from the sea. Land breezes in the night: air in the sea cool down slowly so that it is comparatively hotter, it rises while air in the land sinks, As a result, land breezes go to the sea. Radiation is a process in which heat is transferred from one place to another by means of electromagnetic waves (mainly IR, for hotter object it emits visible lights as well.) It can take place in a vacuum since electromagnetic waves (light) can pass through vacuum. Object with temperature above absolute zero emits radiation and hotter objects emit stronger radiation. Objects were heated when they receive (absorb) radiation and objects were cooled through emitting radiation. (It’ll emit in all temperature higher than absolute zero, despite the surrounding temperature.) Dull black surface are good absorbers (since they don’t reflect most of the light) and good emitter of radiation while shiny silver surface is a poor absorbers (they reflect most of the light) and poor emitters of light. Applications on radiation: Silvery surface: oil tanks, car sunshields, white-painted house, kettle, aluminium foil. Black surface: Radiator and solar power panel. Missing table: Comparing where the transfer process take places (not a big problem) Notes available. It's integrated into one pile of notes for Unit 1-4. The topic "Heat and gaese" is finished. (except "kinetic theory")

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