Question

Sunrays are allowed to fall on a lens of diameter 20 cm. They are then brought to focus on a calorimeter containing 20 g of ice. If the absorption by the lens is neglible, the time required to melt all the ice is
(solar constant =`1.9 cal min^(-1) cm^(-2) and L = 80 calg^(-1)`)
A. 6.4 min
B. 3.2 min
C. 7.2 min
D. 2.7 min

Answer 1

Correct Answer - D
Heat required to melt whole ice is
Q = mL = `20 xx 80`=1600 cal
Energy incident per minute is
`P=1.9 xx (pi)/4 xx d^(2)`
`=1.9 xx(pi)/4 xx (20)^(2)`=597cal `min^(-1)`
Therefore, time required to melt whole ice is `t= Q/P = 2.7 min