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

  • question_answer
    A parallel plate capacitor is charged to a potential difference of 50 V. It is discharged through a resistance. After 1 second, the potential difference between plates becomes 40 V. Then [Roorkee 1999]

    A)            Fraction of stored energy after 1 second is 16/25

    B)            Potential difference between the plates after 2 seconds will be 32 V

    C)            Potential difference between the plates after 2 seconds will be 20 V

    D)            Fraction of stored energy after 1 second is 4/5

    Correct Answer: A , B

    Solution :

     By using \[V={{V}_{0}}{{e}^{-t/CR}}\Rightarrow 40=50\,{{e}^{-1/CR}}\Rightarrow {{e}^{-1/CR}}=4/5\]            Potential difference after 2 sec            \[{V}'={{V}_{0}}{{e}^{-2/CR}}=50{{({{e}^{-1/CR}})}^{2}}=50{{\left( \frac{4}{5} \right)}^{2}}=32\,V\]                           Fraction of energy after 1 sec           \[=\,\]\[\frac{\frac{1}{2}C{{({{V}_{f}})}^{2}}}{\frac{1}{2}C{{({{V}_{i}})}^{2}}}={{\left( \frac{40}{50} \right)}^{2}}=\frac{16}{25}\]

    Solution :

               Fraction of stored energy after 1 second is 16/25 (a, b) By using \[V={{V}_{0}}{{e}^{-t/CR}}\Rightarrow 40=50\,{{e}^{-1/CR}}\Rightarrow {{e}^{-1/CR}}=4/5\]            Potential difference after 2 sec            \[{V}'={{V}_{0}}{{e}^{-2/CR}}=50{{({{e}^{-1/CR}})}^{2}}=50{{\left( \frac{4}{5} \right)}^{2}}=32\,V\]                           Fraction of energy after 1 sec             \[=\,\]\[\frac{\frac{1}{2}C{{({{V}_{f}})}^{2}}}{\frac{1}{2}C{{({{V}_{i}})}^{2}}}={{\left( \frac{40}{50} \right)}^{2}}=\frac{16}{25}\]


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