AFMC AFMC Solved Paper-2001

  • question_answer
    A capacitor C1 is charged by a potential difference v0 as shown in the figure. This charging battery is then removed and the capacitor is connected as into an uncharged capacitor C2. What is the final potential difference V across the combination?

    A) \[{{V}_{0}}\frac{({{C}_{1}}-{{C}_{2}})}{{{C}_{1}}}\]                           

    B) \[{{V}_{0}}\frac{{{C}_{1}}}{({{C}_{1}}-{{C}_{2}})}\]

    C) \[{{V}_{0}}\frac{{{C}_{1}}}{({{C}_{1}}+{{C}_{2}})}\]                          

    D) \[{{V}_{0}}\frac{({{C}_{1}}+{{C}_{2}})}{{{C}_{1}}}\]

    Correct Answer: C

    Solution :

    Key Idea: Total amount of charge in an isolated system remains constant. When a conductor is given a charge, its potential rises in proportion to the charge given. Thus, if a charge Q raises the potential of the conductor by \[{{V}_{o}},\]then \[Q={{C}_{1}}{{V}_{0}}\]where \[{{C}_{1}}\]is capacitance of first capacitor. When the charging battery is removed and capacitor is connected to an uncharged capacitor \[{{C}_{2}},\]this charge \[{{C}_{1}}{{V}_{0}}\]is now shared  across \[{{C}_{1}}\]and \[{{C}_{2}}\]. From law of conservation of charge, we have \[{{C}_{1}}{{V}_{0}}={{C}_{1}}V+{{C}_{2}}V\] \[\Rightarrow \]       \[V={{V}_{0}}\frac{{{C}_{1}}}{{{C}_{1}}+{{C}_{2}}}\]


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