BHU PMT BHU PMT (Mains) Solved Paper-2005

  • question_answer
    A parallel plate capacitor is connected to a battery. The plates are pulled apart with a uniform speed v. If\[x\]is the separation between the plates, then the time rate of change of the electrostatic energy of the condenser is proportional to:

    A)  \[{{x}^{2}}\]                                     

    B)   \[x\]

    C)  \[\frac{1}{x}\]                                  

    D)   \[\frac{1}{{{x}^{2}}}\]

    Correct Answer: D

    Solution :

                     \[U=\frac{1}{2}C{{V}^{2}}=\frac{1}{2}\frac{{{\varepsilon }_{0}}A}{d}{{V}^{2}}\] At any instant, let the separation between plates be\[x\]. So,                          \[U=\frac{1}{2}\frac{{{\varepsilon }_{0}}A}{x}{{V}^{2}}\] \[\therefore \]  \[\frac{dU}{dt}=\frac{1}{2}{{\varepsilon }_{0}}A{{V}^{2}}(-1)\frac{1}{{{x}^{2}}}=\frac{dx}{dt}\]                 \[=-\frac{1}{2}\frac{{{\varepsilon }_{0}}A{{V}^{2}}}{{{x}^{2}}}(v)\] i.e., potential energy decreases as\[(1/{{x}^{2}})\].


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