Question

Rays from the sun are focused by a lens of diameter 5 cm on to a block of ice and 10 g of ice is melted in 20 min. Therefore the heat from the sun reaching the earth per min per square centimeter is (Latent heat of ice L = 80 cal $g^{- 1}$)

• A. 2.04 cal
• B. 0.51 cal
• C. 4.08 cal
• D. 3.02 cal

Answer 1

Answer: (A)

2.04 cal

Answer 2

If a calorie of heat is received from the sun per $cm^{2}$per minute, the heat collected by the lens of radius $2.5cm$in 20 minutes is

$Q_{1} = s \times A \times t = s \times \pi \times (2.5)^{2} \times (20) = (392.5)scal$Heat required to melt 10g ice is

$Q_{2} = mL = 10 \times 80 = 800cal$

According to given problem $Q_{1} = Q_{2}$

$\therefore 392.5s = 800ors = 2.04calcm^{- 2}\min^{- 1}{}$