Solution to 2008 Problem 93

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.

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