JEE Main & Advanced Physics Electrostatics & Capacitance Question Bank Capacitance

  • question_answer
    The plates of parallel plate capacitor are charged upto\[100\ V\]. A\[2mm\] thick plate is inserted between the plates. Then to maintain the same potential difference, the distance between the plates is increased by\[1.6\ mm\]. The dielectric constant of the plate is                       [MP PMT 1991]

    A)                    5                                        

    B)            1.25

    C)                    4                                        

    D)            2.5

    Correct Answer: A

    Solution :

                       In air the potential difference between the plates                    \[{{V}_{air}}=\frac{\sigma }{{{\varepsilon }_{0}}}.d\]        ..... (i) In the presence of partially filled medium potential difference between the plates \[{{V}_{m}}=\frac{\sigma }{{{\varepsilon }_{0}}}(d-t+\frac{t}{K})\]               ..... (ii) Potential difference between the plates with dielectric medium and increased distance is \[{{V}_{m}}'=\frac{\sigma }{{{\varepsilon }_{0}}}\left\{ (d+d')-t+\frac{t}{K} \right\}\]   ..... (iii) According to question \[{{V}_{air}}={{V}_{m}}'\] which gives \[K=\frac{t}{t-d'}\] Hence \[K=\frac{2}{2-1.6}=5\]


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