JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2007

  • question_answer
        The thermo-emf of a thermocouple varies with the temperature\[\theta \]of the hot junction as\[E=a\theta +b{{\theta }^{2}}\]in volts where the ratio a/b is\[700{}^\circ C\]. If the cold junction is kept at\[0{}^\circ C,\] then the neutral temperature is

    A)  \[700{}^\circ C\]

    B)  \[350{}^\circ C\]

    C)  \[1400{}^\circ C\]

    D)  no neutral temperature is possible for this thermocouple

    Correct Answer: D

    Solution :

                                   \[E=a\theta +b{{\theta }^{2}}\]               (given) For neutral temperature \[({{\theta }_{n}}),\frac{dE}{d\theta }=0\] \[\Rightarrow \]               \[a+2b{{\theta }_{n}}=0\] \[\Rightarrow \]               \[{{\theta }_{n}}=-\frac{a}{2b}\] \[\therefore \]  \[{{\theta }_{n}}=-\frac{700}{2}\]             \[\left( \because \frac{a}{b}=700{}^\circ C \right)\]                 \[=-350{}^\circ C<0{}^\circ C\] But neutral temperature can never be negative (less than zero) i.e.,\[{{\theta }_{n}}<|0{}^\circ C\]. Hence, no neutral temperature is possible for this thermocouple;.


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