The energy stored in a capacitor with capacitance $C$ is
$\begin{align*}\frac{1}{2}C V^2\end{align*}$
where $V$ is the voltage difference between the two plates. The effect of adding a dielectric with dielectric constant $\kappa$ is to multiply the capacitance by $\kappa$ . So, given that the voltage is fixed, the stored energy is increased by a factor of $\kappa$ , becoming $\kappa U_0$ . The electric field between the plates is $V_0/d$ regardless of whether there is a dielectric between the plates or not. Therefore, answer (B) is correct.

Solution to 2008 Problem 93