Sunday, February 21, 2010

Convection and Radiation

Convection is another process of thermal energy transfer. It is the transfer of thermal energy by means of currents in a fluid (liquids or gases).
How does Convectino work?
When water at the bottom of a falsk is heated, it expands. This expanded water is less dense than the surrounding water and therefore starts to rise. In doing so, the cooler regions of the water in the upper part of the flask, being denser, sink. This movement of the liquid due to a difference in its density sets up a convection current.
Convection currents occur only in fluids such as liquids and gases but not solids. This is because convection involves the bulk movements of the fluids which carry thermal energy with them. For solids, the thermal energy is transferred from one particle to another through vibrations, without any bulk movement of the particles themselves.


Radiation
Radiation is the continual emission of infrared waves from the surface of all bodies, transimitted without the aid of a medium.
Unlike conduction and convection, radiation does nto require a medium for energy transfer. This means that radiation can take place in a vacuum. For example, thermal eernegy from the sun reaches the Earth by the processof radiation. Conduction or convection is not possilbe because of the vacuum between the Sun and the Earth.
The sun emits electromagnetic waves. A part of this family of electromagnettic waves emitted by the sun makes us feel warm. This group of electromagnetic waves is called infrared waves. Thermal energy from infrared waves is called radiant heat. In fact, all objects emit some radiant heat. The hotter the object, the greater the amount of radiant heat absorbed.

Factors affecting rate of infrared radiation:
1. Colour and texture of the surface:
Black absorbers of infrared radiation than shiny, white surfaces.
2. Surface temperature:
The rate of infrared radiation also depends on the surface temperature. The higher the temperatureof the sufrface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
3. Surface Area
The third factor whci haffects the rate of radiation emission and absorption is the surface area of the object. If we can compare two objects of the same mass and material, but with different surface areas, the object with the larger surface area will emit infrared radiation at a higher rate.