The plasma membrane of eukaryotic cells functions as what kind of electrical circuit element?

The answer is a capacitor. A capacitor, in electrical circuits,  stores potential energy in an electric field. This is similar to cell membranes that create an electric potential difference by pumping out particular ions (sodium ions) to the outside of the cell hence making the inside of the cell more electronegative relative to the outside. This allows the cell to do work when the ions are suddenly allowed to rush into the cell.

Gravitational force i believe