JEE Main & Advanced Physics Electrostatics & Capacitance Question Bank Critical Thinking

  • question_answer
    A charge \[+q\] is fixed at each of the points \[x={{x}_{0}},\,x=3{{x}_{0}},\,x=5{{x}_{0}}\]..... infinite, on the \[x-\]axis and a charge \[-q\]  is fixed at each of the points \[x=2{{x}_{0}},\,x=4{{x}_{0}},x=6{{x}_{0}}\],..... infinite. Here \[{{x}_{0}}\] is a positive constant. Take the electric potential at a point due to a charge \[Q\] at a distance \[r\] from it to be \[Q/(4\pi {{\varepsilon }_{0}}r)\]. Then, the potential at the origin due to the above system of charges is                                                                         [IIT 1998]

    A)                    0                                        

    B)            \[\frac{q}{8\pi {{\varepsilon }_{0}}{{x}_{0}}\ln 2}\]

    C)                    \[\infty \]                       

    D)            \[\frac{q\ln 2}{4\pi {{\varepsilon }_{0}}{{x}_{0}}}\]

    Correct Answer: D

    Solution :

                       \[V=\frac{q}{4\pi {{\varepsilon }_{0}}{{x}_{0}}}\left[ 1+\frac{1}{3}+\frac{1}{5}+... \right]-\frac{q}{4\pi {{\varepsilon }_{0}}{{x}_{0}}}\left[ \frac{1}{2}+\frac{1}{4}+\frac{1}{6}+... \right]\] \[=\frac{q}{4\pi {{\varepsilon }_{0}}{{x}_{0}}}\left[ 1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+.... \right]=\frac{q}{4\pi {{\varepsilon }_{0}}{{x}_{0}}}{{\log }_{e}}2\]


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