Basic Knowledge of Radiation

Physics

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Black surfaces are good absorbers of heat

Take two aluminium containers. One painted with black and the other with original colours. Fill them half with water and keep them under the sun. Each one should have a lid and thermometer.Note the temperatures in both the thermometers. Initially both of them will show the same temperature. Later, the thermometer inserted in the blacked can will show higher temperature. The difference between the readings will increase as time advances. This shows that the black surface absorbs more heat and reflects less as compared with the other can.

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Applications of black and white surfaces

Following are some applications of the principle that black surfaces are better abosrbers and radiators than polished surfaces.
  1. The bottom part of cooking utensils is blackened to absorb the heat from the flame more quickly. The upper part of the utensil and the outer surface are kept polished and shining to reduce heat loss due to radiation.
  2. White clothes are preferred for use during summer as they are poor absorbers of heat.
  3. A fire fighting suit is bright and shiny, so that it does not take in a lot of heat energy and the fire fighter is kept cool.
  4. Coffee and tea stay hot longer in a shiny pot than in a blackened one
  5. A light coloured building stays cooler in summer because it reflects most of the heat radiation from the sun. It is also a poor emitter of heat and stays warm in winter.

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Important Questions

The temperature of equal masses of three different liquids and are and respectively. The temperature when and are mixed is and when and are mixed it is . What should be the temperature when and are mixed?

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A black body at a temperature of radiates heat energy at the rate . At a temperature of , the rate of heat radiated per unit area in will be:

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A body cools from C to C in minutes. Calculate the time it takes to cool from C to C. The temperature of the surroundings is C.

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A spherical black body with a radius of 12cm radiates 450W power at 500K. If the radius were halved and the temperature doubled, the power radiated in watts would be :

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State Kirchhoff's law of radiation and prove it theoretically.

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Certain quantity of water cools from to in the first minutes and to in the next minutes. The temperature of the surroundings is ;

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The plots of intensity versus wavelength for three black bodies at temperatures  and  respectively are shown in fig. Their temperatures are such that:

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A pan filled with hot food cools from to in minutes. When the room temperature is . How long will it cool from to ?

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Two spheres of the same material have radii and and temperature and respectively. The energy radiated per second by the first sphere is :

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A body cools down from to in Minutes when the temperature of surrounding is . The temperature of the body after next minutes will be:

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