Here's the scenario: I need two plates that start off in proximity (a few mm apart), and I would like to vary the charge in these plates to achieve a repulsion force. Ideally it would be varied by a voltage source, so that I have fine control over the repulsion. But as it happens, it is not a simple matter of connecting the two plates to the same potential of a voltage supply. The scenario would work if I wanted to achieve attraction, by connecting one plate to positive and the other to ground. But in my case, no charging would occur. So how could I charge two plates to the same charge and vary it at will, without fancy manoeuvres like taking it far away, charging by capacitance, disconnecting from voltage, and bringing it closer again?