NEET Physics Transmission of Heat NEET PYQ-Transmission Of Heat

  • question_answer
    The radiant energy from the sun, incident normally at the surface of earth is \[20\text{ }kcal/{{m}^{2}}\,\min \]. What would have been the radiant energy, incident normally on the earth, if the sun had a temperature, twice of the present one?                                                                                                                                                       [AIPMT 1998]

    A)  \[160\text{ }kcal/{{m}^{2}}\min \]

    B)       \[40\text{ }kcal/{{m}^{2}}\min \]

    C)  \[320\text{ }kcal/{{m}^{2}}\min \]

    D)       \[80\text{ }kcal/{{m}^{2}}\min \]

    Correct Answer: C

    Solution :

    According to Stefan's law, the rate at which an object radiates energy is proportional to the fourth power of its absolute temperature i.e.,
                            \[E=\sigma {{T}^{4}}\,or\,E\,\propto \,{{T}^{4}}\]
                or         \[\frac{{{E}_{1}}}{{{E}_{2}}}={{\left( \frac{{{T}_{1}}}{{{T}_{2}}} \right)}^{4}}\]
                Here,  \[{{T}_{1}}=T,\,{{T}_{2}}=2T,\,{{E}_{1}}=20\,kcal/{{m}^{2}}\,\min \]
                \[\therefore \]      \[\frac{20}{{{E}_{2}}}={{\left( \frac{T}{2T} \right)}^{2}}\]
                or         \[\frac{20}{{{E}_{2}}}=\frac{1}{16}\]
                \[\therefore \]      \[{{E}_{2}}=20\times 16\]
                            \[=320\,kcal/{{m}^{3}}\,\min \]


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