KVPY 2020-21 SB SX Exam Previous Papers

Question : 100 of 120

Marks: +1, -0

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 (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 estimate for the duration t (in hours) that the 'kangri' heat will last is (take the 'kangri' to be a black body) :

Validate

Solution:

{{dQ}}/{{dt}}={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}

{{dQ}}/{{dt}}=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}

Go to Question:

Prev Question

Next Question