WB JEE Medical WB JEE Medical Solved Paper-2010

  • question_answer
    \[n\]identical droplets are charged to V volt each. If they coalesce to form a single drop,  then its potential will be

    A)  \[{{n}^{2/3}}V\]                              

    B)  \[{{n}^{1/3}}V\]

    C)  \[nV\]                                 

    D)  \[V/n\]

    Correct Answer: A

    Solution :

                     Let the radius of each droplet be r units. So, the volume of each droplet is equal to \[\frac{4}{3}\pi {{r}^{3}}.\] Thus, n droplets have the total volume equal to \[n\left( \frac{4}{3}\pi {{r}^{3}} \right).\] Since, the number of the drop would be equal to the total volume of the droplets hence, \[\Rightarrow \]               \[{{R}^{3}}=n{{r}^{3}}\] \[\Rightarrow \]               \[R={{n}^{1/3}}r\]                                            ?(i) The capacitance of each droplet is equal to, \[{{C}_{d}}=4\pi {{\varepsilon }_{0}}r\]and thus the charge of each droplet would equal \[{{q}_{d}}={{C}_{d}}{{V}_{d}}=4\pi {{\varepsilon }_{0}}r{{V}_{d}}\]                         ?(ii) The capacitance of the bigger drop would be equal to \[C=4\pi {{\varepsilon }_{0}}R.\] The potential of the bigger drop would be equal to V(say). Hence, \[V=\frac{n{{q}_{d}}}{C}=\frac{n(4\pi {{\varepsilon }_{0}}r{{V}_{d}})}{4\pi {{\varepsilon }_{0}}{{n}^{1/3}}r}={{n}^{2/3}}{{V}_{d}}\]


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