JCECE Engineering JCECE Engineering Solved Paper-2007

  • question_answer
    \[27\]small drops each having charge \[q\] and radius \[r\] coalesce to from big drop. How many times charge and capacitance will become?

    A) \[3,\,\,27\]                         

    B)  \[27,\,\,3\]

    C)  \[27,\,\,27\]                     

    D)  \[3,\,\,3\]

    Correct Answer: B

    Solution :

    Key Idea: In coalescing into a single drop charge remains conserved. Also volume before and after coalescing remains same. Let \[R\] and r be the radii of bigger and each smaller drop respectively. In coalescing into a single drop, charge remains conserved. Hence, charge on bigger drop                 \[=27\times \]charge on smaller drop \[ie,\]    \[q'=27q\] Now, before and after coalescing, volume remains same. That is,                 \[\frac{4}{3}\pi {{R}^{3}}=27\times \frac{4}{3}\pi {{r}^{3}}\] \[\therefore \]         \[R=3r\] Hence, capacitance of bigger drop                 \[C'=4\pi {{\varepsilon }_{0}}R\]                      \[=4\pi {{\varepsilon }_{0}}R(3r)\]                     \[=3(4\pi {{\varepsilon }_{0}}r)\]                     \[=3C\]


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