$ = \frac{{{Q^2}}}{{2c}}\left[ {1 - \frac{1}{k}} \right]$
$ = \frac{1}{2} \times 12 \times 100\,pJ\left( {1 - \frac{1}{{6.5}}} \right)$
$ = \frac{{12 \times 100 \times 11}}{{2 \times 13}}\,{\text{pJ}}$
$ = 507.69\,{\text{pJ}}$




Reason : The dipoles of a polar dielectric are randomly oriented.

$K(x) = K_0 + \lambda x$ ( $\lambda =$ constant)
The capacitance $C,$ of the capacitor, would be related to its vacuum capacitance $C_0$ for the relation