Physics. Mr Rishi Gopie THERMAL ENERGY TRANSFER

Transcription

1 Mr Rishi Gopie THERMAL ENERGY TRANSFER

2 Thermal Energy Transfer Thermal energy transferred from one system/region to another due to a temperature difference is called heat and there are three main ways in which thermal energy/heat can be transferred: conduction, convection, and radiation. Conduction (solids) Conduction is the transfer of thermal energy/heat through a material medium from a region of high temperature to a region of lower temperature, without movement of the material medium as a whole. Conduction occurs primarily in solids and to a lesser extent in liquids. It is almost negligible in gases. Good thermal conductors have high thermal conductivities and examples include all metals (including liquid metals such as mercury) and certain non- metals such as graphite ( a form of carbon) and silicon. Poor thermal conductors are good thermal insulators and have low thermal conductivities. Examples include most non- metals such as plastic, cork, wood, felt, glass, gases (including air), and water. Consider and experiment to compare the thermal conductivities of different metals. 2

3 Consider an experiment to show that water is a pour thermal conductor: 3

4 Poor thermal conductors (i.e. good thermal insulators) are used extensively to insulate thermal systems and so minimize both heat loss and heat gain. Examples of materials used in this way include: felt, fiberglass and wool, all of which contain trapped, unconnected pockets of air. The same is for hollow clay/ concrete blocks and expanded polystyrene. Convection Convection is the transfer of thermal energy/heat through a material medium from a region of high temperature to a region of lower temperature by the movement of the medium itself, which carries the thermal energy/heat with it. Convection occurs only in fluids (i.e. liquids and gases) through the following sequences of occurrences: i) a region of the fluid is heated and it becomes hotter (i.e. its temperature rises) ii) that region of the fluids expands (i.e. the volume increases) while its mass remains the same. iii) As a result the density of that region decreases (since ρ = m/v and v increases while m is constant) iv) That region of the fluid rises, displacing fluid above it and being replaced itself by cooler denser fluid from the sides. 4

5 v) As the hot fluid rises it carries thermal energy/heat with it to the upper regions of the fluid. vi) The series of occurrences is repeated through the formation of convection currents. The formation and action of convection current are related to many common phenomena, including the formation of land and sea breezes. 5

6 water has a much higher specific heat capacity than does soil and so absorbs about the same quantity of heat but rises in temperature less than does soil during the day. Also during the night, the water loses about the same quantity of heat but falls in temperature less than does soil. Radiation Radiation is the process of thermal energy/heat transfer through the emission of certain electromagnetic waves known as infra red (I.R.) radiation. The thermal energy/heat transferred in this way is referred to as radiant heat. The hotter an object the shorter the wavelength of the I.R. waves it emits and the cooler the object the longer the wavelength of the I.R. waves it emits. Radiation does not require a material medium (as do convection and conduction). Radiant heat can be transferred across a vacuum- for instance radiant heat from the sun reaches the earth through the vacuum of space. 6

7 A set of thermocouples (with blackened junction to enhance the absorption of radiant heat) joined in series and known as a thermocouple can be used to detect radiation. Dull, rough, dark surfaces tend to be good absorbers and good radiators (i.e. emitters) of radiant heat but poor reflectors of radiant heat; while smooth, bright, shiny surfaces tend to be poor absorbers and poor radiators/emitters of radiant heat, but good reflectors of radiant heat. Some of the applications of the ideas/principles involved in the thermal energy/heat transfer include: 1) External walls of buildings are painted white or in light; pastel colors. Roof are bright (e.g. galvanized sheets.) this is to reduce radiant heat absorption and improve radiant heat reflection- to help in keeping the interior of buildings cooler. 2) White clothes are worn in hot, tropical countries during the playing of certain outdoor sports such as cricket and lawn tennis. 3) Silver foil is folded around the heating elements/burners of stoves- so as to reflect radiant heat (that would normally go downward) upward onto the bottom of the container on the burner/element. 4) Heating elements in storage heaters and kettles, etc. are placed at the bottom so that the water (or other liquid being heated) can be heated through out its volume by convection. 5) The glass of which a greenhouse is made is able to transmit incident radiant heat/i.r. radiation from the sun but radiant heat reflected from and emitted by objects within the greenhouse (such as soil, plants, etc.) which is of longer wavelength, is not transmitted by glass. So some of the radiant heat becomes trapped within the greenhouse- thus warming its interior. This warming or greenhouse effect also occurs on a global scale with certain gases, such as CO2, in the atmosphere transmitting the incident radiant heat from the sun to the earth but not reflected/emitted radiant heat from the earth`s surface. The result is global warming which can have serious adverse effects on the world s climate. 6) The vacuum flask is a container design to minimize heat gain from its contents and heat loss from its contents. 7

8 7) Solar water heaters (and solar stills) are designed to maximize the use of radiant heat gained by the sun in heating water: 8) Solar heat (i.e. radiant heat from the sun) is used to dry certain crops by radiation and absorption (on top of galvanized sheets). 9) Lagging is technique employed to thermally insulate a system so as to minimize thermal energy loss or gain. 8