RAJASTHAN ­ PET Rajasthan PET Solved Paper-2011

  • question_answer
    A parallel plate capacitor of value 1.77 \[\mu \]F is to be designed using a dielectric material (dielectric constant = 200), breakdown strength of\[3\times {{10}^{6}}\] V/m. In order to make such a capacitor which can withstand a potential difference of 20 V across the plates, the separation between the plates d and area A of the plates respectively are

    A)  \[6.6\times {{10}^{-6}}m,{{10}^{3}}{{m}^{2}}\]

    B)  \[6.6\times {{10}^{-5}}m,{{10}^{4}}{{m}^{2}}\]

    C)  \[6.6\times {{10}^{-4}}m,\text{ }{{10}^{5}}{{m}^{2}}\]

    D)  \[6.6\times {{10}^{-6}}m,\text{ }{{10}^{2}}{{m}^{2}}\]

    Correct Answer: A

    Solution :

     We knows, electric field is given by \[E=\frac{V}{d}\] \[{{E}_{\max }}=\frac{V}{{{d}_{\min }}}\] \[{{d}_{\min }}=\frac{V}{{{E}_{\max }}}\] Substitute\[V=20\text{ }V\]and \[{{E}_{\max }}=3\times {{10}^{6}}V\] \[{{d}_{\min }}=\frac{20}{3\times {{10}^{6}}}=6.6\times {{10}^{-6}}m\] We knows, the capacitance is given by \[C=\frac{k{{\varepsilon }_{0}}A}{d}\] \[\frac{A}{d}=\frac{C}{k{{\varepsilon }_{0}}}\] \[A=\frac{C}{k{{\varepsilon }_{0}}}\] Putting \[C=1.77\text{ }\mu F=1.77\times {{10}^{-6}}C,\] \[d=6.6\times {{10}^{-6}},k=200,\] \[{{\varepsilon }_{0}}=8.85\times {{10}^{-12}}{{C}^{2}}{{N}^{-1}}{{m}^{2}}\] \[A=\frac{1.77\times {{10}^{-6}}\times 6.6\times {{10}^{-6}}}{200\times 8.85\times {{10}^{-12}}}={{10}^{3}}{{m}^{2}}\]


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