BHU PMT BHU PMT (Screening) Solved Paper-2011

  • question_answer
    Capacitance of a capacitor becomes\[\frac{4}{3}\]times its original value if a dielectric slab of thickness\[t=\frac{d}{2}\]is   inserted   between   the   plates (d = separation between the plates). The dielectric constant of the slab is

    A)  6                                            

    B)  8

    C)  4                                            

    D)  2

    Correct Answer: D

    Solution :

                     Original capacity \[{{C}_{0}}=\frac{{{\varepsilon }_{0}}A}{d}\] On introducing dielectric slab of thickness \[\frac{d}{2}\] the capacity becomes                 \[C=\frac{{{\varepsilon }_{0}}A}{\left( d-\frac{d}{2} \right)+\frac{d}{2K}}=\frac{{{\varepsilon }_{0}}A}{\frac{d}{2}\left( 1+\frac{1}{K} \right)}\] As           \[C=\frac{4}{3}{{C}_{0}}\] \[\therefore \]  \[\frac{{{\varepsilon }_{0}}A}{\frac{d}{2}\left( 1+\frac{1}{K} \right)}=\frac{4}{3}\frac{{{\varepsilon }_{0}}A}{d}\] Or           \[1+\frac{1}{K}=\frac{3}{2}\]or \[K=2\]


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