JEE Main & Advanced Physics Electrostatics & Capacitance Question Bank Critical Thinking

  • question_answer
    In an isolated parallel plate capacitor of capacitance C, the four surface have charges \[{{Q}_{1}}\], \[{{Q}_{2}}\], \[{{Q}_{3}}\] and \[{{Q}_{4}}\] as shown. The potential difference between the plates is [IIT-JEE 1999]

    A)            \[\frac{{{Q}_{1}}+{{Q}_{2}}+{{Q}_{3}}+{{Q}_{4}}}{2C}\]

    B)   \[\frac{{{Q}_{2}}+{{Q}_{3}}}{2C}\]

    C)  \[\frac{{{Q}_{2}}-{{Q}_{3}}}{2C}\]

    D)            \[\frac{{{Q}_{1}}+{{Q}_{4}}}{2C}\]

    Correct Answer: C

    Solution :

               Plane conducting surfaces facing each other must have equal and opposite charge densities. Here as the plate areas are equal, \[{{Q}_{2}}=-{{Q}_{3}}\]. The charge on a capacitor means the charge on the inner surface of the positive plate (here it is \[{{Q}_{2}}\]) Potential difference between the plates \[=\frac{charge}{capacitance}\]\[=\frac{{{Q}_{2}}}{C}=\frac{2{{Q}_{2}}}{2C}\]


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