JEE Main & Advanced JEE Main Paper Phase-I (Held on 08-1-2020 Evening)

  • question_answer
    Consider two charged metallic spheres \[{{S}_{1}}\] and \[{{S}_{2}}\] of radii \[{{R}_{1}}\] and\[{{R}_{2}}\], respectively. The electric fields \[{{E}_{1}}\] (on \[{{S}_{1}}\]) and \[{{E}_{2}}\] (on \[{{S}_{2}}\]) on their surfaces are such that \[{{E}_{1}}\]/\[{{E}_{2}}\] = \[{{R}_{1}}\]/\[{{R}_{2}}\]. Then the ratio \[{{V}_{1}}\](on \[{{S}_{1}}\])/\[{{V}_{2}}\](on \[{{S}_{2}}\]) of the electrostatic potentials on each sphere is [JEE MAIN Held on 08-01-2020 Evening]

    A) \[{{\left( {{R}_{1}}\text{/}{{R}_{2}} \right)}^{2}}\]

    B) \[\left( {{R}_{2}}\text{/}{{R}_{1}} \right)\]

    C) \[{{\left( \frac{{{R}_{1}}}{{{R}_{2}}} \right)}^{3}}\]  

    D) \[{{R}_{1}}/{{R}_{2}}\]

    Correct Answer: A

    Solution :

    Let \[{{\rho }_{1}}\] and \[{{\rho }_{2}}\] are the charge densities of two sphere. Then, \[{{E}_{1}}=\frac{{{\rho }_{1}}{{R}_{1}}}{3{{\varepsilon }_{0}}}\,\,and\,\,{{E}_{0}}=\frac{{{\rho }_{2}}{{R}_{2}}}{3{{\varepsilon }_{0}}}\] \[\because \,\,\,\,\,\,\,\frac{{{E}_{1}}}{{{E}_{2}}}=\frac{{{\rho }_{1}}{{R}_{1}}}{{{\rho }_{2}}{{R}_{2}}}=\frac{{{R}_{1}}}{{{R}_{2}}}\] \[\therefore \,\,\,\,\,\,\,{{\rho }_{1}}={{\rho }_{2}}=\rho \] So, \[{{V}_{1}}=\frac{\rho R_{1}^{2}}{3{{\varepsilon }_{0}}}\] and \[{{V}_{2}}=\frac{\rho R_{2}^{2}}{3{{\varepsilon }_{0}}}\] \[\therefore \,\,\,\,\,\,\,\,\,\frac{{{V}_{1}}}{{{V}_{2}}}={{\left( \frac{{{R}_{1}}}{{{R}_{2}}} \right)}^{2}}\]


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