Do we REALLY need to bring static charges into this, when someone is apparently having trouble grasping voltage?
I think the real question is, how can voltage be explained in a way that's intuitively understandable. There are some examples here of how, and some, markedly, of how
not. (And "technically more precise" is not always the best way to go about it.)
Also, during one of my walks, I think it finally came in a way that makes sense to me.
Voltage really is a game of "relatives".
Hypothetically, I have a brother, a sister, a mother, and a father. If I am the reference point, then this person (relative to me) is my brother, or my sister. If a person is not connected to me through my family, then the question "Relative to me, who are you?" doesn't make sense. However, if my sister married this stranger, then I can say that person is my brother-in-law because he is connected to my sister, who is connected to me.
Numbers work well too.
1 is less than 2, which is less than 3. We can say that 5 is greater than 3 because 5 is greater than 4 which is greater than 3. We can get to 3 from 5 because they are connected through 4. A question like "Is 5 greater than blue?" makes no sense because blue is not connected to any numbers.
But without any reference point, "sister" has no meaning. It even applies to directions. To the left.........to the left of what?
Back to the example, A and B are "connected" because they are two parts of the same cell, but A and D are not. Without physically connecting A to D, asking what is the voltage of D w/respect to A is undefined.
I'm not certain that the part about the relatives and numbers is entirely on the right track, but ultimately, it's safe to think that without a connection, the voltage difference between A and D is undefined.
For the specific case where you connect cells in series, you can think about it in terms of circuits (I believe you said you understand circuits in a practical way)... If A and B are connected, according to Ohm's law, there is no potential difference between them. Uad=R*I, R=0 (we assume an ideal conductor connecting the two points), therefore Uad=0*I=0 Sorry probably scratch that, I don't want to confuse you further.