Answer
The energy stored in the capacitor increases by 50%
Work Step by Step
We can write an expression for the energy stored in the capacitor when the separation distance is $d$:
$E_1 = \frac{Q^2}{2C}$
$E_1 = \frac{Q^2}{2~(\epsilon_0~A/d)}$
$E_1 = \frac{Q^2~d}{2~\epsilon_0~A}$
We can write an expression for the energy stored in the capacitor when the separation distance is $1.5~d$:
$E_2 = \frac{Q^2}{2C}$
$E_2 = \frac{Q^2}{2~(\epsilon_0~A/1.5d)}$
$E_2 = \frac{Q^2~(1.5~d)}{2~\epsilon_0~A}$
$E_2 = 1.5\times \frac{Q^2~d}{2~\epsilon_0~A}$
$E_2 = 1.5\times E_1$
The energy stored in the capacitor increases by 50%
