Heat and Thermodynamics 1 Question 19
19. Earth receives $1400 W / m^{2}$ of solar power. If all the solar energy falling on a lens of area $0.2 m^{2}$ is focussed onto a block of ice of mass $280 g$, the time taken to melt the ice will be … minutes. (Latent heat of fusion of ice $=3.3 \times 10^{5} J / kg$ )
(1997, 2M)
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Solution:
- Heat required to melt the block
$$ =(0.28 kg)\left(3.3 \times 10^{5} J / kg\right)=9.24 \times 10^{4} J $$
Heat received per second $=\left(1400 J / m^{2}\right)\left(0.2 m^{2}\right)=280 J$
$\therefore$ Time taken (in minutes) $=\frac{9.24 \times 10^{4}}{280 \times 60}=5.5$
