MGIMS WARDHA MGIMS WARDHA Solved Paper-2015

  • question_answer
    Capacitance of a parallel plate capacitor becomes \[\frac{3}{2}\]times its original value if a dielectric slab of thickness\[\frac{d}{2}\]is inserted between the plates, where d is the separation between the plates. The dielectric constant of the slab is

    A)  4                                            

    B)  2

    C)  3                                            

    D)  1

    Correct Answer: C

    Solution :

                    \[{{C}_{air}}=\frac{{{\varepsilon }_{0}}A}{d}\]With dielectric slab inserted between the plates, the capacitance                 \[C=\frac{{{\varepsilon }_{0}}A}{\left( d-t+\frac{t}{k} \right)}\] Here,                     \[C=\frac{3}{2}C\] \[\Rightarrow \]               \[\frac{{{\varepsilon }_{0}}A}{d-t+\frac{t}{k}}=\frac{3}{2}\frac{{{\varepsilon }_{0}}A}{d}\] \[\Rightarrow \]               \[\frac{1}{d\left( 1-\frac{1}{2}+\frac{1}{2k} \right)}=\frac{3}{2d}\] \[\Rightarrow \]               \[\frac{1}{2}+\frac{1}{2k}=\frac{2}{3}\] \[\Rightarrow \]               \[\frac{1}{2k}=\frac{2}{3}-\frac{1}{2}=\frac{1}{6}\] \[\Rightarrow \]               \[k=3\]


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