JEE Main & Advanced Physics Transmission of Heat Question Bank Topic Test - Transmission of Heat (3-5-21)

  • question_answer
    A body takes 5 minute to cool from \[80{}^\circ C\] to \[50{}^\circ C\]. How much time it will take to cool from \[60{}^\circ C\] to \[30{}^\circ C\], if room temperature is \[20{}^\circ C\].

    A) 40 minute

    B) 9 minute

    C) 30 minute

    D) 20 minute

    Correct Answer: B

    Solution :

    According to Newton’s law of cooling
    in first case,\[\frac{80-50}{5}=K\left[ \frac{80+50}{2}-20 \right]\]            …..(i)
    in second case,\[\frac{60-30}{t}=k\left[ \frac{60+30}{2}-20 \right]\]            (ii)
    Dividing equation (i) by (ii) we get, \[\frac{t}{2}=\frac{45}{25}\]\[\Rightarrow \]\[t=9\]min.

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