CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2010

  • question_answer
    n identical drops, each of capacitance C and charged to a potential V, coalesce to form a bigger drop. Then the ratio of the energy stored in the big drop to that in each small drop is

    A)  \[{{n}^{5/3}}:1\]             

    B)         \[{{n}^{4/3}}:1\]

    C)  \[n:1\]                

    D)         \[{{n}^{3}}:1\]

    E)  \[{{n}^{2/3}}:1\]

    Correct Answer: A

    Solution :

    Volume of big drop\[=n\times \]volume of small drop \[\frac{4}{3}\pi {{R}^{3}}=n\times \frac{4}{3}\pi {{r}^{3}}\]                           \[R={{n}^{1/3}}r\] Capacitance of small drop, \[C=4\pi {{\varepsilon }_{0}}r\] Capacitance of big drop, \[C=4\pi {{\varepsilon }_{0}}R\]                 \[=4\pi {{\varepsilon }_{0}}{{n}^{1/3}}r\]                 \[C={{n}^{1/3}}C\]           The potential of small drop\[V=\frac{q}{C}=\frac{q}{4\pi {{\varepsilon }_{0}}r}\] The potential of big drop \[V=\frac{nq}{(4\pi {{\varepsilon }_{0}}){{n}^{1/3}}r}\] \[V={{n}^{2/3}}V\] \[\therefore \]Energy of small drop\[=\frac{1}{2}C{{V}^{2}}\] Energy of big drop\[=\frac{1}{2}CV{{}^{2}}\]                                 \[=\frac{1}{2}{{n}^{1/3}}C{{({{n}^{2/3}}V)}^{2}}\]                                 \[={{n}^{5/3}}\frac{1}{2}C{{V}^{2}}\] \[\therefore \]  \[\frac{Energ{{y}_{(big\,drop)}}}{Energ{{y}_{(small\,drop)}}}=\frac{{{n}^{5/3}}}{1}\]


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