PHYSICS PROJECT (:

Radiation
Definition:
Radiation is the continual emission of infrered waves from surface of all bodies, transmitted without aid of medium.

• does not require medium
• all objects emit some radiant heat
• the hotter an object, the greater amount of radiant heat emitted

Absorption of infrared radiation
Infrared radiation is absorbed by all objects and surfaces, which cause a temperature rise.

Emission of infrared radiation
Infrared radiation is emitted by all objects and surfaces, which cause a temperature fall.

Factors affecting rate of infrared radiation

• Colour, texture of surface
• Surface temperature
• Surface area

Colour, texture of surface
Dull coloured, rough surfaces are better absorber and emitter
Shiny, white coloured, smooth surfaces are poorer absorber and emitter



Surface temperature
"The higher the temperature of surface of object relative to surrounding temperature, the higher the rate of infrared radiation."

Meaning of statement: The larger the temperature difference between surface and surroundings, the higher the rate of radiation of object.

Rate of emission

As seen from the picture above, the cup on the left side has a temperature difference with the surroundings of 50°C, while the cup on the right side has a temperature difference with surroundings of 10°C. The cup on the left side has a larger temperature difference, hence it will emit heat faster than the cup on the right.

Rate of absorption

As seen from the picture above, cup on the left side has a temperature difference with surroundings of 20°C, while cup on the right side has a temperature difference with surroundings of 10°C. The cup on the left side has a larger temperature difference, hence the cup on the left will absorb heat faster than the cup on right.

Graph

The high temperature at A shows a steep gradient, which proves that the rate of temperature decrease is high, which means the object is emitting heat, the rate of infrared radiation is high.

The lower temperature at B shows a gentle gradient, which proves that the rate of temperature decrease is low, which means the object is emitting heat at a slower speed than before, the rate of infrared radiation is low as the surface temperature gets nearer to room temperature.

Surface area
The larger the surface area of object, the higher the rate of radiation (emission, absorption).

Both cups have the same mass and material. The larger in size cup radiate heat faster than the smaller cup as it has a larger surface area, it will emit and absorb heat faster than the smaller cup.

Applications of radiation
Cooking utensils (eg. pots)

Cooking utensils are usually sliver in colour, polished and smooth, hence it is a bad absorber and emitter of heat. This helps to keep food cold when taken out of fridge, as it does not absorb much heat. This also helps to keep food warm, as it also does not emit much heat.

Cooling fins

The cooling fins are painted black as black colour is a good emitter of heat. It is able to radiate heat to the surroundings quickly and allow the equipment to be cooled.

Greenhouse

Solar radiation from the Sun enters greenhouse through the glass roof. This warms up the plants and soil, which cause them to start to emit infrared radiation. This infrared radiation emitted by plants is different from the solar radiation, hence it is unable to pass through the roof and gets trapped inside. The infrared radiation trapped inside gets built up and cause a temperature increase. This trapped heat helps plants grow better in cold climates.