JEE Main & Advanced JEE Main Paper (Held On 10-Jan-2019 Evening)

  • question_answer
    Four equal point charges Q each are placed in the xy plane at\[\left( 0,\text{ }2 \right),\text{ }\left( 4,\text{ }2 \right),\text{ }\left( 4,\,-2 \right)\text{ }and\text{ }\left( 0,-\,2 \right)\]. The work required to put a fifth charge Q at the origin of the coordinate system will be- [JEE Main Online Paper (Held On 10-Jan-2019 Evening]          

    A) \[\frac{{{Q}^{2}}}{4\pi {{\varepsilon }_{0}}}\]                        

    B) \[\frac{{{Q}^{2}}}{2\sqrt{2}\pi {{\varepsilon }_{0}}}\]

    C) \[\frac{{{Q}^{2}}}{4\pi {{\varepsilon }_{0}}}\left( 1+\frac{1}{\sqrt{3}} \right)\]

    D)                  \[\frac{{{Q}^{2}}}{4\pi {{\varepsilon }_{0}}}\left( 1+\frac{1}{\sqrt{5}} \right)\]

    Correct Answer: D

    Solution :

    \[W=q\Delta V=q\,\left[ {{V}_{0}}-{{V}_{\infty }} \right]=q{{V}_{0}}\] Potential at origin \[=\text{ }{{V}_{0}}\] \[{{V}_{0}}\,=\,2.\frac{kQ}{2}+2.\frac{kQ}{2\sqrt{5}}\] \[=\,\,kQ\left( 1+\frac{1}{\sqrt{5}} \right)\] \[\therefore \] Word done \[=\text{ }kqQ\left( 1+\frac{1}{\sqrt{5}} \right)\] \[=\,\,\frac{{{Q}^{2}}}{4\pi {{\varepsilon }_{0}}}\,\left( 1+\frac{1}{\sqrt{5}} \right)\]


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