CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2008

  • question_answer
    Two identical conducting spheres carrying different charges attract each other with a force F when placed in air medium at a distance d apart. The spheres are brought into contact and then taken to their original positions. Now the two spheres repel each other with a force whose magnitude is equal to that of the initial attractive force-The ratio between initial charges on the spheres is

    A)  \[-(3+\sqrt{8})\]only

    B)  \[-3+\sqrt{8}\]only

    C)  \[-(3+\sqrt{8})\,or\ (-3+\sqrt{8})\]

    D)  \[+\sqrt{3}\]

    E)  \[-\sqrt{8}\]

    Correct Answer: C

    Solution :

    Since the two charges (spheres) attract, they will be opposite in sign, ie,\[{{q}_{1}}\]and\[-{{q}_{2}}\]. Force,      \[F=\frac{1}{4\pi {{\varepsilon }_{0}}}\,\frac{{{q}_{1}}\times \,-{{q}_{2}}}{{{d}^{2}}}\] After touching, charge on each will be\[\frac{{{q}_{1}}-{{q}_{2}}}{2}\]. New force, \[F=\frac{1}{4\pi {{\varepsilon }_{0}}}{{\left( \frac{{{q}_{1}}-{{q}_{2}}}{2} \right)}^{2}}\times \frac{1}{{{d}^{2}}}\] Given    \[|F|=|F|\] On solving by quadratic equations, we get \[\frac{{{q}_{1}}}{{{q}_{2}}}=-(3+\sqrt{8})\]or\[(-3+\sqrt{8})\]


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