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Sunrays are allowed to fall on a lens of...

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

Text Solution

Verified by Experts

The correct Answer is:
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
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