First slide
Heat transfer-Radiation

A metal cylinder of 5 kg is heated electrically by a 20 W heater in a room 20oC. The cylinder temperature rises uniformly to 30oC in 5 minute and finally becomes constant at 45oC. Assuming that the rate of heat loss is proportional to the excess temperature over the surroundings. 


Troom =20Cmcylinder =5kg            P=20W

Since temperature becomes constant at 45°C

Power lost at 45°C = 20 W

From Newton's law of cooling

 dQdtT=45C=20=K(4520) K=45

Since temperature rises uniformly to 30°C thus, at 25°C,


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