I misunderstood that part during my first read, so I would like to add a few clarifications: > > Are you saying that the DC voltage difference is always 0 VDC?

I misunderstood that part during my first read, so I would like to add a few clarifications: > > Are you saying that the DC voltage difference is always 0 VDC? > Measured across the only connection between two otherwise isolated systems, yes. The systems are isolated, so there is no fixed potential between them, their "DC" potential difference is therefore random (unknown quantity of excess/missing electrons) before they are connected, and you can have charge transfer if you connect both, that will balance the extra electrons on both sides. Electrons will flow in a way that the resulting density is uniform, like repulsing magnets. Therefore, there is no potential difference between any point in these two systems (barring local differences -- like a battery--, or at the very least, no potential difference across the connecting cable). An analogy that could or could not help is trough gravitational potential energy: take two water buckets, isolated. The potential across them is the height difference between their water lines. We didn't specify where we left the buckets, so it is random. Now if we connect the bottom of the buckets, the system will find its way to the "ground state", or lowest energy state: the water line in both buckets will align (Communicating vessels), and you therefore have no DC potential across those: connect an extra piece of tubing, nothing will flow. This analogy is quite powerful and can be used for capacitors, diodes, etc. (as most flux/potential analogies). It has its limits, though.