KVPY2020PhysicsThermal Properties of Matter
The "Kangri" is an earthen pot used to stay warm in Kashmir during the winter months. Assume that the "Kangri" is spherical and of surface area 7 × 10 - 2   m 2 . It contains 300   g of a mixture of coal, wood and leaves with calorific value of 30   kJ   g - 1 (and provides heat with 10 % efficiency). The surface temperature of the 'Kangri' is 60 ° C and the room temperature is 0
Options
- A8
- B10
- C12
- D16
Correct answer
B. 10
Step-by-step solution
d Q d t = e A σ T 0 4 - T s 4 e = 1 ,   A = 7 × 10 - 2 ,   s = 5 . 67 × 10 - 8 T 0 = 333   K ,   T s = 273   K d Q d t = 26 . 75 Watt total energy produced = 10 100 × 30 × 10 3 × 300 ⇒ 9 × 10 5   J ∴ time = 9 × 10 5 26 . 75 × 3600 hrs = 9 . 35   hrs