Saturday, June 20, 2009

Part 9- Application of Radiation

Poor Radiators

Food is kept warm by a shiny aluminium foil.

A shiny metal teapot is a poor radiator.

Vacuum Flasks

A vacuum flask (thermos flask) is designed to keep liquids hot by minimizing heat loss in four possible ways, namely conduction, convection, radiation and evaporation.
  1. The plastic cap helps to prevent evaporation and convection.
  2. The vacuum above the liquid is a poor conductor of heat, thus conduction through the vacuum is reduced.
  3. The inner silvered surfaces reflects radiation from the hot fluids back into the flask.
  4. The outer silvered surfaces reflects radiation from the external surroundings away from the flask.
  5. The foam plastic support helps to reduce heat loss by conduction(plastic is a poor conductor) through the thin walls of the flask.

A Greenhouse i used in cold climates to help plants grow better by trapping heat. Infrared radiation from the sun passes through the glass roof of the greenhouse. This warms up the plants in the greenhouse. As they get warmer, the plants start to emit infrared radiation.

The infrared radiation emitted is unable to pass through the glass roof. Therefore the infrared radiation gets trapped and over time, the amount of infrared radiation builds up. This will cause the temperature of the greenhouse to increase.

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