JEE Main & Advanced Physics Electrostatics & Capacitance Question Bank Charge and Coulombs Law

  • question_answer
    A total charge Q is broken in two parts \[{{Q}_{1}}\] and \[{{Q}_{2}}\] and they are placed at a distance \[R\] from each other. The maximum force of repulsion between them will occur, when [MP PET 1990]           

    A)                    \[{{Q}_{2}}=\frac{Q}{R},\ {{Q}_{1}}=Q-\frac{Q}{R}\]     

    B)                    \[{{Q}_{2}}=\frac{Q}{4},\ {{Q}_{1}}=Q-\frac{2Q}{3}\]

    C)                    \[{{Q}_{2}}=\frac{Q}{4},\ {{Q}_{1}}=\frac{3Q}{4}\]        

    D)                    \[{{Q}_{1}}=\frac{Q}{2},\ {{Q}_{2}}=\frac{Q}{2}\]

    Correct Answer: C

    Solution :

                       \[{{Q}_{1}}+{{Q}_{2}}=Q\]    ..... (i)    and \[F=k\frac{{{Q}_{1}}{{Q}_{2}}}{{{r}^{2}}}\]  .....(ii) From (i) and (ii) \[F=\frac{k{{Q}_{1}}(Q-{{Q}_{1}})}{{{r}^{2}}}\] For F to be maximum \[\frac{dF}{d{{Q}_{1}}}=0\]Þ \[{{Q}_{1}}={{Q}_{2}}=\frac{Q}{2}\]


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