A battery is used to charge a parallel-plate capacitor, after which it is disconnected.Then the plates are pulled apart to twice their original separation.This process will double the:
A)capacitance
B)surface charge density on each plate
C)stored energy
D)electric field between the two places
E)charge on each plate

Question

Quizplus · Accepted Answer

When the separation between the plates of a capacitor is increased (while disconnected from the battery), the capacitance decreases. However, since the charge remains constant (no path for it to go elsewhere), the stored energy in the capacitor increases. The energy stored in a capacitor is given by $U = \frac{1}{2} \frac{Q^2}{C}$, where $Q$ is the charge and $C$ is the capacitance. As $C$ decreases (due to increased separation), $U$ increases. Therefore, doubling the separation does not double the capacitance, surface charge density, electric field, or charge on each plate, but it does affect the stored energy, making it increase.

A Battery Is Used to Charge a Parallel-Plate Capacitor, After