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JEE Main & Advanced Physics Transmission of Heat Question Bank Critical Thinking

  • question_answer
    The only possibility of heat flow in a thermos flask is through its cork which is 75 cm2 in area and 5 cm thick. Its thermal conductivity is 0.0075 cal/cmsecoC. The outside temperature is 40oC and latent heat of ice is 80 cal g?1. Time taken by 500 g of ice at 0oC in the flask to melt into water at 0oC is [CPMT 1974, 78; MNR 1983]

    A)            2.47 hr

    B)            4.27 hr

    C)            7.42 hr

    D)            4.72 hr

    Correct Answer: A

    Solution :

                       \[mL=\frac{KA\Delta \theta \,\,t}{\Delta x}\] Þ  \[500\times 80=\frac{0.0075\times 75\times (40-0)t}{5}\] Þ t = 8.9 ´ 103 sec = 2.47 hr.


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