Haryana PMT Haryana PMT Solved Paper-2005

  • question_answer
    Two identical mercury drops each of radius r are charged to the same potential V. If the mercury drops coalesce to form a big drop of radius R, then potential of the combined drop will be :

    A)  \[{{\left( 2 \right)}^{3/2}}V\]                                    

    B)  \[{{\left( 2 \right)}^{2/3}}V\]

    C)  \[{{\left( 3 \right)}^{2/3}}V\]                                    

    D)  \[{{\left( 3 \right)}^{1/2}}V\]

    Correct Answer: C

    Solution :

                    Potential on small drop \[V=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{q}{r}\](charge on small drop = q, radius of small drop = r) \[\because \] Volume of 2 small drops = volume of one big drop \[\Rightarrow \]               \[2\times \frac{4}{3}\pi {{r}^{3}}=\frac{4}{3}\pi {{R}^{3}}\] or \[R={{(2)}^{1/3}}r\] Potential on big drop \[V=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{(2q)}{R}=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{2q}{{{(2)}^{1/3}}r}\] \[V=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{q}{r}{{(2)}^{1-\frac{1}{3}}}=V{{(2)}^{2/3}}\]


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