CONVECTION AND RADIATION

Tuesday, September 2, 2008

CONVECTION
Convection is the transfer of thermal energy by means of currents in a fluid(liquid or gas). Convection currents occur only in fluids such as liquids and gases but not in solids, this is because convection involves the bulk movement of the fluids which carry thermal energy with them. For an example, boiling of water. How the water nearer the flame gets heated up and is moved up by the currents to allow cool water to sink and get heated up. And so the cycle goes on...

RADIATION
Radiation is the continual emmision of infarered waves from the surface of all bodies, transmitted without the aid of a medium. Unlike conduction and convection, radiation does not require a medium for energy transfer, which means it can take place in vacuum. For example how thermal energy from the Sun reaches Earth by the process of radiation. The sun emits EM waves , that makes us feel warm are called infrared and thermal energy from the Sun are called radiant heat. In fact, all objects emit some radiant heat, the hotter the object, the greater the amount of radiant heat emitted.

Also, all objects and surfaces obsorbs infrared radiation and the absorbtion will cause the temperature rise! And of course, radiation is emitted by all objects and surfaces, and thus causing the drop in temperature.

Generally, a good emitter of radiant heat, is also a goos absorber of radiant heat, and vice versa.

There are 3 factors that attects the rate of infrared radiation
1. Colour and texture of the surface
-Dull, black surfaces are better absorbers of infrared radiation than shiny, white surfaces.
2. Surface temperature
-The higher the temperature of the surface of the object relative to the surronding temperature, the higher the rate of infrared radiation.
3. Surface area
-Object with the larger surface area will emit infrared radiation at a higher rate.

Posted by xiaowei aunty at 11:18 PM