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Wednesday, September 3, 2008
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CHAPTER 8: TRANSFER OF THERMAL ENERGY
THERMAL ENERGY ALWAYS FLOWS FROM A REGION OF HIGHER TEMPERATURE TO A REGION OF A LOWER TEMPERATURE.
THERMAL ENERGY IS TRANSFERRED BY THREE PROCESSES, NAMELY:
- conduction
- convection
- radiation
CONDUCTION
Conduction is the transfer of energy through a medium
without the medium moving
This diagram shows how heat travels from a high temperature
to a low temperature during conduction
the process of conduction in liquid and gases is inefficient.
the particles in them are further apart, compared to those in solids.
thus, they wont bump into each other that often.
in coonclusion, air is a poor conductor of heat compared to water,
and water is a poor conductor of heat compared to most solids.
Applications of good conductors of heat
-cooking utensils
-soldering iron rods
-heat exchangers
Application of bad conductors of heat
-handle of appliances and utensils
-Table mats
-Sawdust
-Wooden ladles
-Woollen clothes
-Fibre glass, felt and expanded polystryene foam
CONVECTION
Convection is the transfer of thermal energy by means of currents in liquids or gases
Convection currents only occur in liquid and gases but not solids.
Its is because convcection involves the bulk
movements of the fluid which carry thermal energy with them.
Common applications of convection
- electric kettles
-household hot water systems
-Air conditioners
-Refrigerators
Radiation
Radiation is the continual emission of infrared waves from the surface of all bodies,transmitted without the aid of medium.
radiation can take place in vacuum
infrared radiation is absorbed by all objects and surfaces.
The absorption of radiant heat causes the temperature to rise.
Dull,black surfaces absorb infrared radiation faster compared to shiny,white surfaces
Dull,black surfaces are better emitters of infrared radiation than shiny,white surfaces
A GOOD EMITTER OF RADIANT HEAT IS ALSO A
GOOD ABSORBER OF RADIANT HEAT
Factors affecting the rate of infrared radiation
-Colour and texture of the surface
black surfaces are better emitters of infrared radiation than shiny or white surfaces
-Surface temperature
the higher the temperature of the surface of the object, the higher the rate of infrared radiation
- Surface area
an object with a larger surface area will emit infrared radiation at a higher rate
Common applications of radiation
-teapots
-the greenhouse
-vacuum flasks
