In a cold storage, ice melts at the rate of 2 kg per hour when the external temperature is $20^{\circ}…
In a cold storage, ice melts at the rate of 2 kg per hour when the external temperature is $20^{\circ} \mathrm{C}$. The minimum power output of the motor used to drive the refrigerator which just prevents the ice from melting is (latent heat of fusion of ice $=80 \mathrm{cal} \mathrm{g}^{-1}$ )
28.5 W
13.6 W
9.75 W
16.4 W
Solution
The coefficient of performance of refrigerator is
$\operatorname{cop}=\frac{\mathrm{Q}_2}{\mathrm{~W}}=\frac{\mathrm{T}_2}{\mathrm{~T}_1-\mathrm{T}_2} \Rightarrow \frac{\mathrm{~mL}}{\mathrm{~W}}=\frac{273}{20}$
$\Rightarrow \mathrm{W}=\frac{20}{273} \times \frac{2 \times 10^3}{60 \times 60} \times 80 \times 4.2=13.6 \mathrm{~W}$
