JEE Main & Advanced Physics Transmission of Heat Question Bank Topic Test - Transmission Of Heat

  • question_answer
    Two rods having thermal conductivities in the ratio of 5 : 3 having equal lengths and equal cross-sectional area are joined in series. If the temperature of the free end of the first rod is \[{{100}^{o}}C\] and free end of the second rod is \[{{20}^{o}}C\]. Then temperature of the junction is

    A) \[~{{70}^{o}}C\]

    B) \[~{{60}^{o}}C\]

    C) \[{{50}^{o}}C\]

    D) \[{{90}^{o}}C\]

    Correct Answer: A

    Solution :

    [a] Temperature of interface \[\theta =\frac{{{k}_{1}}{{\theta }_{1}}+{{k}_{2}}{{\theta }_{2}}}{{{k}_{1}}+{{k}_{2}}}\]
    It is given that \[\frac{{{K}_{1}}}{{{K}_{2}}}=\frac{5}{3}\]
    Let \[{{K}_{1}}=5K\] and \[{{K}_{2}}=3K\]
    \[\Rightarrow \]   \[\theta =\frac{5K\times 100+3K\times 20}{5K+3K}\]
    \[\Rightarrow \]   \[\theta =\frac{560K}{8K}=70{{\,}^{o}}C\]
    \[\therefore \]      \[\theta =70{{\,}^{o}}C\]


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