The only possibility of heat flow in a thermos flask is through its cork which is $75 cm^2$ in area and $5 cm$ thick. Its thermal conductivity is $0.0075 cal/cmsec^oC$. The outside temperature is$ 40^oC$ and latent heat of ice is $80 cal g^{-1}$. Time taken by $500 g$ of ice at $0^oC$ in the flask to melt into water at $0^oC$ is ....... $hr$
Diffcult
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(a) $mL = \frac{{KA\Delta \theta \,\,t}}{{\Delta x}}$
==> $500 \times 80 = \frac{{0.0075 \times 75 \times (40 - 0)t}}{5}$
==>$ t = 8.9 \times 10^3 sec = 2.47 hr.$
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