BHU PMT BHU PMT (Mains) Solved Paper-2007

  • question_answer
    Two organ pipes both closed at one end, have lengths\[l\]and\[l+\Delta l\]. If the velocity of sound in air is v, then the number of beats per second is

    A)  \[\frac{v}{4l}\]                                 

    B)  \[\frac{v}{2l}\]

    C)   \[\frac{v}{4{{l}^{2}}}\Delta l\]                  

    D)  \[\frac{v}{2{{l}^{2}}}\Delta l\]

    Correct Answer: C

    Solution :

                     Fundamental frequency of closed organ pipe \[n=\frac{v}{4l}\] Here,      \[{{n}_{1}}=\frac{v}{4l}\] and        \[{{n}_{2}}=\frac{v}{4(l+\Delta l)}\] Number of beats per second\[={{n}_{1}}-{{n}_{2}}\]                 \[=\frac{v}{4l}-\frac{v}{4(l+\Delta l)}\]                 \[=\frac{v}{4}\left[ \frac{l+\Delta l-l}{l(l+\Delta l)} \right]\]                 \[=\frac{v}{4}\frac{\Delta l}{{{l}^{2}}+l\Delta l}\] Neglecting the term\[l\Delta l\]. Number of beats per second \[=\frac{v}{4{{l}^{2}}}\Delta l\]


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