This is a newbie question.
It is clear that if you put two (or more) DC voltage sources in series like this:
And you measure total voltage between A and D you get the sum of the two voltages. I also realized that if you flip one of the sources, its voltage is to be subtracted from the total. So if I flip V1, I get a total voltage of 7,5 from A to D.
The point is, in short, is that I cannot understand why.
Voltage is the potential difference between two points. As far as I have understood; when you take a single cell into account, that voltage is given by the fact that there is a difference in charge between one pole and the other of the cell. So, when you close a circuit between A and B (or A and C whatsoever being nothing in the way from B and C), then electrical charge searching for its balance finds a way to, and it start flowing from one point to the other, according to the resistance it may encounter. Same goes for Battery #2 of course, between C (or B) and D.
But when you put the batteries in series, what is going on between B and C if you close the circuit between A and D (with eventually any resistance in the middle)? Why is the overall potential difference increased (or decreased, according to the polarity?). How are electrons flowing between all the points? Why?