A parallel plate capacitor whose capacitance C is $14 \mathrm{pF}$ is charged by a battery to a potential difference $\mathrm{V}=12 \mathrm{~V}$ between its plates. The charging battery is now disconnected and a porcelin plate with $\mathrm{k}=7$ is inserted between the plates, then the plate would oscillate back and forth between the plates with a constant mechanical energy of ___________$\mathrm{pJ}$.
(Assume no friction)
$\mathrm{U}_{\mathrm{i}}=\frac{1}{2} \times 14 \times 12 \times 12 \mathrm{pJ} \quad\left(\because \mathrm{U}=\frac{1}{2} \mathrm{CV}^{2}\right)$
$=1008 \mathrm{pJ}$
$\mathrm{U}_{\mathrm{f}}=\frac{1008}{7} \mathrm{pJ}=144 \mathrm{pJ} \quad\left(\because \mathrm{C}_{\mathrm{m}}=\mathrm{kC}_{0}\right)$
Mechanical energy $=\Delta \mathrm{U}$
$=1008-144$
$=864 \mathrm{pJ}$