BHU PMT BHU PMT (Mains) Solved Paper-2005

  • question_answer
    When a rod is heated but prevented from expanding the stress developed is independent of:

    A)  material of the rod

    B)  rise in temperature

    C)  length of rod

    D)  none of the above

    Correct Answer: C

    Solution :

                     \[{{L}_{t}}={{L}_{0}}(1+\alpha \Delta \theta )\] \[\Delta L={{L}_{t}}-{{L}_{0}}={{L}_{0}}\,\alpha \,\Delta \theta \]                        ...(i) If the same rod of length Lo is subjected to stress along its length, then extension in length can be calculated by Hooke's law. \[Y=\frac{tress}{strain}=\frac{stress}{\Delta L/{{L}_{0}}}\]            \[=\frac{{{L}_{0}}\times stress}{\Delta L}\]           \[\therefore \]  \[\Delta L=\frac{{{L}_{0}}\times stress}{Y}\]                        ?.(ii) If the rod is prevented by expanding, we have                 \[{{L}_{0}}\alpha \,\Delta \theta =\frac{{{L}_{0}}\times stress}{Y}\] \[\therefore \]Stress \[=Y\,\alpha \,\Delta \theta \]        (independent of\[{{L}_{0}}\])


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