Battery and Separable Capacitor 5C10.21
The separable parallel-plate capacitor and the battery are in the Electrostatics Cabinet, A4.
The Electrometer is in the Electrostatics Cabinet, B1.
Connect the parallel-plate capacitor to the Electrometer. Charge the capacitor to 9V using the battery. Vary the spacing between the plates. The voltage on plates goes up as the plate spacing goes up.
If lecturing to a large class, project a camera view of the Electrometer on a screen.
Discussion: Q=CV. The battery charges the plates to 9V, however, once the battery is removed, the charge Q on the plates is fixed. As the plates are separated, the capacitance C decreases. Therefore the voltage V must increase. Since CV is fixed, the stored energy (1/2 CV^2) increases as the plates are separated. It takes work to separate the plates.