CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2009

  • question_answer
    Two conducting spheres A and B of radius a and b respectively are at the same potential. The ratio of the surface charge densities of A and B is

    A)  \[\frac{b}{a}\] 

    B)         \[\frac{a}{b}\]

    C)  \[\frac{{{a}^{2}}}{{{b}^{2}}}\]                   

    D)         \[\frac{{{b}^{2}}}{{{a}^{2}}}\]

    E)  \[\frac{(a+b)}{(ab)}\]

    Correct Answer: A

    Solution :

                    Given electric potential of spheres are same ie, \[{{V}_{A}}={{V}_{B}}\] \[\frac{1}{4\pi {{\varepsilon }_{0}}}.\frac{{{Q}_{1}}}{a}=\frac{1}{4\pi {{\varepsilon }_{0}}}.\frac{{{Q}_{2}}}{b}\] \[\frac{{{Q}_{1}}}{{{Q}_{2}}}=\frac{a}{b}\]                            ?? (i)    Surface charge density                 \[\sigma =\frac{Q}{4\pi {{r}^{2}}}\] \[\Rightarrow \]               \[\frac{{{\sigma }_{1}}}{{{\sigma }_{2}}}=\frac{{{Q}_{1}}}{{{Q}_{2}}}\times \frac{{{b}^{2}}}{{{a}^{2}}}\]                 \[=\frac{a}{b}\times \frac{{{b}^{2}}}{{{a}^{2}}}\]                 \[=\frac{b}{a}\]


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