A parallel plate capacitor whose capacitance C is 14 pF is charged by a battery to a potential difference V = 12 V between its plates. The charging battery is now disconnected and a porcelin plate with k = 7 is inserted between the plates, then the plate would oscillate back and forth between the plates with a constant mechanical energy of ________pJ. (Assume no friction)
Option: 1 864
Option: 2 34
Option: 3 28
Option: 4 76

Answers (1)

Using \ \mathrm{U}=\frac{1}{2} \mathrm{CV}^{2} and 

\because \mathrm{C}_{\mathrm{m}}=\mathrm{kC}_{0}

 

\begin{array}{l} \mathrm{U}_{\mathrm{i}}=\frac{1}{2} \times 14 \times 12 \times 12 \mathrm{pJ} =1008 \mathrm{pJ} \\ \end{array}

\mathrm{U}_{\mathrm{f}}=\frac{1008}{7} \mathrm{pJ}=144 \mathrm{pJ} \\ \Rightarrow \text { Mechanical energy }=\Delta \mathrm{U} =1008-144 =864 \mathrm{pJ}

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