The ''Kangi'' is and earthen pot used to stay warm in Kashmir during the winter monts. Assume that the " Kangri" is shericla and of surface are `7 xx 10^(10-2) m^2` . It contains 300 g of mixture of coal. Wood and leaves with calorific value of 30 kj/g (and provides heat with 10 % efficiency.) The surface temperature of the "Kangri" is `60^@C` and the room temperature is `0^@C`. Then, a reasonable estime for the duration t( in hours) that the "kangri" heat will last is (take the "kangri" to be a black body).

Options

A. 8
B. 10
C. 12
D. 16

Answer

B

Explanation

`(dQ)/(d t) = eA sigma [T_(0)^(4) - T_(s)^(4)]`
` e= 1 , A = 7 xx 10^(-2) , s = 5.67 xx 10^(-8)`
` T_0 = 333 K ,T_s = 273 K`
` (dQ)/( dt) = 26.75 ` watt
total energy produced `= (10)/(100) xx 30 xx 10^3 xx 300`
`implies 9 xx 10^5 J`
` therefore ` time ` = ( 9xx 10^5 )/(26.75 xx 3600 ) hrs = 9.35 hrs `