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.
Since temperature becomes constant at 45°C
Power lost at 45°C = 20 W
From Newton's law of cooling
Since temperature rises uniformly to 30°C thus, at 25°C,