Question

The rays of sun are focussed on a piece of ice through a lens of diameter $5$ cm, as a result of which $10$ grams of ice melts in $10$ min. The amount of heat received from Sun is (per unit area per min)

• A. 4 $cal/c{m}^{2}min$
• B. 40 $cal/c{m}^{2}min$
• C. 4 $J/c{m}^{2}min$
• D. 400 $J/c{m}^{2}min$

Answer 1

Answer: (A)

4 $cal/c{m}^{2}min$

The latent heat of fusion is $80cal/g$.

So heat received per unit times is $10g\times 80cal{g}^{-1}/10min=80cal/min$

The are is $\pi r^2=\pi \times (2.5{)}^{2}c{m}^2\approx 20c{m}^2$

So, amountof heat per unit area per unit time is $80/20=4cal/c{m}^{2}min$