Question

A parallel-plate capacitor is charged and the charging battery is then disconnected. If the plates of the capacitor are moved further apart by means of insulating handles

• A. the charge on capacitor increases
• B. the capacitance increases
• C. the voltage across the plates increases
• D. the electrostatic energy stored in the capacitor increases

Answer 1

Answer: (C), (D)

The correct options are
C the voltage across the plates increases
D the electrostatic energy stored in the capacitor increases

When a capacitor is charged and then disconnect battery, first the capacitance will remain constant and then slowly discharge according to time constant.
The potential between the plates is $V=\frac{qd}{A{ϵ}_0}$, so when increasing the between plate, the voltage will increase.
The energy stored in capacitor is $W=\frac{1}{2}C{V}^2$. Thus, the electrostatic energy stored in the capacitor also increases.